Climate Program Office (CPO) Home Page

All Definitions

Abrupt Climate Change⁽¹⁴⁾

A shift in climate (e.g., temperature or precipitation) that occurs faster than the rate of change in the mechanism causing the change. The shift between glacial and interglacial stages, the shift between warm and cool periods during the last glacial time (Dansgaard-Oeschger cycles), and the Arctic oscillation are all considered abrupt climate changes. The shift in climate can occur over months to thousands of years and can occur in the absence of any known external cause.

Absorption⁽¹⁾
	The process in which radiant energy is retained by a substance. A further process always results from absorption, that is, the irreversible conversion of the absorbed radiation into some other form of energy within and according to the nature of the absorbing medium. The absorbing medium itself may emit radiation, but only after energy conversion has occurred.

Advanced Very High Resolution Radiometer (AVHRR)⁽¹⁾
	A five-channel scanning instrument that quantitatively measures electromagnetic radiation, flown on NOAA environmental satellites. AVHRR remotely determines cloud cover and surface temperature. Visible and infrared detectors observe vegetation, clouds, lakes, shorelines, snow, and ice.

Aerosol⁽¹⁾
	Particles of liquid or solid dispersed as a suspension in gas.

Albedo⁽²⁾
	The percentage of solar radiation that is reflected relative to the total incoming radiation.

Anomaly⁽³⁾
	The deviation of a measurable unit, (e.g., temperature or precipitation) in a given region over a specified period from the long-term average, often the thirty-year mean, for the same region.

Anthropogenic⁽²⁾
	Made by people or resulting from human activities.

Arctic Oscillation (AO)⁽⁶⁾

The Arctic Oscillation is a pattern in which atmospheric pressure at polar and middle latitudes fluctuates between negative and positive phases. The negative phase brings higher-than-normal pressure over the polar region and lower-than-normal pressure at about 45 degrees north latitude. The negative phase allows cold air to plunge into the Midwestern United States and western Europe, and storms bring rain to the Mediterranean. The positive phase brings the opposite conditions, steering ocean storms farther north and bringing wetter weather to Alaska, Scotland and Scandinavia and drier conditions to areas such as California, Spain and the Middle East. In recent years research has shown, the Arctic Oscillation has been mostly in its positive phase. Some researchers argue that the North Atlantic Oscillation is in fact part of the AO.

Arid⁽²⁾
	Lacking moisture, especially having insufficient rainfall to support trees or woody plants.

Atmosphere⁽²⁾

The whole mass of gases surrounding the earth or other celestial bodies. Today's atmosphere is made up primarily of nitrogen (78%), free oxygen (21%) and greenhouse gases which can capture solar radiation: water vapor, which ranges from less than 1% in arid regions to over 3% in moist areas, carbon dioxide (0.035%) and methane (0.00018%). In the past the composition of the Earth's atmosphere has varied.

Atmospheric Circulation Model⁽³⁾
	A mathematical model for quantitatively describing, simulating, and analyzing the structure of the circulation in the atmosphere and the underlying causes. Sometimes referred to as Atmospheric General Circulation Models or AGCMs (See GCMs).

Biogeochemical Cycle⁽⁴⁾
	Natural processes that recycle nutrients in various chemical forms from the environment, to organisms, and then back to the environment. Examples are the carbon, oxygen, nitrogen, phosphorus, and hydrologic cycles.

Biosphere⁽¹⁾
	Part of the Earth system in which life can exist, between the outer portion of the geosphere and the inner portion of the atmosphere.

Calcium carbonate (CaCO₃₎⁽²⁾
	A molecule consisting of calcium, carbon and oxygen that are secreted by corals, forming their skeleton; it also secreted by mollusks (clams, oysters, etc.), forming their protective shells.

Calcium concentrations⁽²⁾
	Concentration (units mass/mass) of the calcium ion (Ca+), often found in layers of ice, and derived from atmospheric transport of dust.

Carbon cycle⁽¹⁾

All parts (reservoirs) and fluxes of carbon. The cycle is usually thought of as four main reservoirs of carbon interconnected by pathways of exchange. The reservoirs are the atmosphere, terrestrial biosphere (usually includes freshwater systems), oceans, and sediments (includes fossil fuels). The annual movements of carbon, the carbon exchanges between reservoirs, occur because of various chemical, physical, geological, and biological processes. The ocean contains the largest pool of carbon near the surface of the Earth, but most of that pool is not involved with rapid exchange with the atmosphere.

Carbon dioxide⁽¹⁾

A minor but very important component of the atmosphere, carbon dioxide traps infrared radiation. Atmospheric CO₂ has increased about 25 percent since the early 1800s, with an estimated increase of 10 percent since 1958 (burning fossil fuels is the leading cause of increased CO₂, deforestation the second major cause). The increased amounts of CO₂ in the atmosphere enhance the greenhouse effect, blocking heat from escaping into space and contributing to the warming of Earth's lower atmosphere.

Carbon sequestration⁽¹⁾
	The uptake and storage of carbon. Trees and plants, for example, absorb carbon dioxide, release the oxygen and store the carbon. Fossil fuels were at one time biomass and continue to store the carbon until burned.

Celsius⁽¹⁾

Temperature scale proposed by Swedish astronomer Anders Celsius in 1742. A mixture of ice and water is zero on the scale; boiling water is designated as 100 degrees. A degree is defined as one hundredth of the difference between the two reference points, resulting in the original term, "centigrade" (100th part). To convert Celsius to Fahrenheit: multiply the Celsius temperature by 1.8 and add 32 degrees. F = 9/5 C + 32. To convert Fahrenheit to Celsius: subtract 32 degrees from the Fahrenheit temperature and divide the quantity by 1.8. C = (F -32) / 1.8.

Chlorofluorocarbons (CFCs)⁽³⁾
	Manufactured substances used as coolants and computer-chip cleaners. When these products break down they destroy stratospheric ozone, creating the Antarctic Ozone Hole in the Southern Hemisphere spring (Northern Hemisphere fall). While no longer in use, their long lifetime will lead to a very slow removal from the atmosphere.

Circulation⁽³⁾
	The flow, or movement, of a fluid (e.g., water or air) in or through a given area or volume.

Climate⁽³⁾
	The average of weather over at least a 30-year period. Note that the climate taken over different periods of time (30 years, 1000 years) may be different. The old saying is climate is what we expect and weather is what we get.

Climate Change⁽³⁾
	A non-random change in climate that is measured over several decades or longer. For example, when less energy reaches the earth, temperature decreases and the area covered by snow increases. Consequently less energy is available at the surface, and temperature further decreases.

Climate Diagnostics Center (CDC)⁽³⁾
	The mission of NOAA's Climate Diagnostics Center is to identify the nature and causes for climate variations on time scales ranging from a month to centuries.

Climate Model⁽³⁾
	Mathematical model for quantitatively describing, simulating, and analyzing the interactions between the atmospheres and underlying surface (e.g., ocean, land, and ice). OR A quantitative way of representing the interactions of the atmosphere, oceans, land surface, and ice. Models can range from relatively simple to quite comprehensive. Also see General Circulation Model.

Climate Outlook⁽³⁾
	A climate outlook gives probabilities that conditions, averaged over a specified period, will be below normal, normal, or above normal.

Climate Prediction Center (CPC)⁽³⁾
	A branch of the National Oceanographic and Atmospheric Agency (NOAA) whose mission is to assess and forecast the impacts of short-term climate variability. The CPC produces official U.S. climate forecasts.

Climate System⁽³⁾
	The system consisting of the atmosphere (gases), hydrosphere (water), lithosphere (solid rocky part of the Earth), and biosphere (living) that determine the EarthÕs climate.

Climate Variability⁽⁶⁾
	The range of values the climate at a particular location can take over time. Example: Although the average September to December rainfall in Entebbe from 1902 to 1992 was 438mm, the actual amounts each year were somewhere in between 200mm and 1000mm, which is a large range of values.

Climatic feedback mechanisms⁽²⁾

A feedback is an enhancement (positive feedback) or a damping (negative feedback) of an initial change, in this case in the climate system. For example, when less energy reaches the earth, temperature decreases and the area covered by snow increases. The albedo of the planet decreases, reflecting more energy towards the atmosphere. Consequently less energy is available at the surface, and temperature further decreases. The whole "cycle" from the initial cooling to the further cooling is a feedback. It is a positive feedback in this example.

Climatological Outlook⁽³⁾
	An outlook based upon climatological statistics for a region, abbreviated as CL on seasonal outlook maps. CL indicates that the climate outlook has an equal chance of being above normal, normal, or below normal.

Climatology⁽³⁾
	(1) The description and scientific study of climate. (2) A quantitative description of climate showing the characteristic values of climate variables over a region.

Composite⁽³⁾
	An average that is calculated according to specific criteria. For example, one might want a composite for the rainfall at a given location for all years where the temperature was much above average.

Condensation⁽³⁾
	The physical process by which water vapor in the atmosphere changes to liquid in the form of dew, fog or cloud; the opposite of evaporation.

Convection⁽³⁾

Transfer of heat by fluid motion between two areas with different temperatures. In meteorology, the rising and descending air motion caused by heat. Atmospheric convection is almost always turbulent and is the dominant vertical transport process over tropical oceans and during sunny days over continents. The terms "convection" and "thunderstorms" are often use interchangeably, although thunderstorms are only one form of convection. In the ocean, convection is prominent in regions of high heat loss to the atmosphere and is the main mechanism for deep-water formation.

Cooling Degree Days⁽³⁾
	A form of degree-day used to estimate energy requirements for air conditioning or refrigeration.

Coral bleaching⁽²⁾
	An environmental stress indicator for coral; if conditions for corals are not optimum, the corals will expel the algae that live among the living polyps, therefore giving the colony a bleached appearance.

Coupled Model⁽³⁾
	In the context of climate modeling this usually refers to a numerical model which simulates both atmospheric and oceanic motions and temperatures and which takes into account the effects of each component on the other. (also referred to as coupled atmosphere-ocean model)

Cyclone⁽³⁾

In general use the term cyclone is applied to any storm, especially violent, small-scale circulations such as tornados, waterspouts, and dust devils. In meteorology, the term refers to a type of atmospheric disturbance centered on a low-pressure center that often results in stormy weather. In common practice the term cyclone, and low, are used interchangeably and are frequently referred to as storms. In the Northern Hemisphere the air rapidly circulates counterclockwise and in the Southern Hemisphere clockwise. Tropical cyclones with sustained winds above 73 miles per hour are known as hurricanes in the North Atlantic Ocean, Caribbean Sea, Gulf of Mexico and the Eastern North Pacific (east of the date line) and cyclones in the Indian Ocean. They are known as typhoons in other areas or the world. Both mid-latitude and tropical storms serve an important function in transferring warmth away from the tropics to the poles.

Data⁽¹⁾
	A collection of facts, concepts or instructions in a formalized manner suitable for communication or processing by human beings or by computer.

Data set⁽¹⁾
	A logically meaningful grouping or collection of similar or related data. Data having mostly similar characteristics (source or class of source, processing level and algorithms, etc.)","d

Decadal⁽³⁾
	A consecutive ten-year period.

Desertification⁽⁴⁾

The progressive destruction or degradation of existing vegetative cover to form desert. This can occur due to overgrazing, deforestation, drought, and the burning of extensive areas. Once formed, deserts can only support a sparse range of vegetation. Climatic effects associated with this phenomenon include increased albedo, reduced atmospheric humidity, and greater atmospheric dust (aerosol) loading.

Dew Point⁽³⁾
	The point at which the air at a certain temperature contains all the moisture possible without precipitation occurring. When the dew point is 65°F, one begins to feel the humidity. The higher the temperature associated with the dew point, the more uncomfortable one feels.

Dobson Unit⁽³⁾
	Unit used to measure the abundance of ozone in the atmosphere. One Dobson unit is the equivalent of 2.69 x 1016 molecules of ozone/cm².

Doppler radar⁽³⁾
	Radar that measures speed and direction of a moving object, such as water or ice particles, birds, and insects.

Downwelling⁽¹⁾
	The process of accumulation and sinking of warm surface waters along a coastline. A change of air flow of the atmosphere can result in the sinking or downwelling of warm surface water. The resulting reduced nutrient supply near the surface affects the ocean productivity and meteorological conditions of the coastal regions in the downwelling area.

Drought⁽³⁾
	Drought is a deficiency of moisture that results in adverse impacts on people, animals, or vegetation over a sizeable area. NOAA together with its partners provides short- and long-term Drought Assessments.

Drought Assessments⁽³⁾

At the end of each month, CPC issues a long-term seasonal drought assessment. On Thursdays of each week, the CPC together with NOAA National Climatic Data Center, the United States Department of Agriculture, and the National Drought Mitigation Center in Lincoln, Nebraska, issues a weekly drought assessment called the United States Drought Monitor. These assessments review national drought conditions and indicate potential impacts for various economic sectors, such as agriculture and forestry.

Earth Radiation Budget Experiment (ERBE)⁽¹⁾
	An experiment to obtain data to study the average radiation budget of the Earth and determine the energy transport gradient from the equator to the poles. Three satellites were flown in different orbits to obtain the data: the Earth Radiation Budget Satellite, ERBS (launched in October 1984), NOAA-9 (launched in December 1984), and NOAA-10 (launched in September 1986).

Earth system⁽¹⁾
	The Earth regarded as a unified system of interacting components, including geosphere (land), atmosphere (air), hydrosphere (water and ice), and biosphere (life).

Earth system science⁽¹⁾
	An integrated approach to the study of the Earth that stresses investigations of the interactions among the Earth's components in order to explain Earth dynamics, evolution, and global change.

Eccentricity⁽²⁾
	The amount that the earth's revolution deviates from a circular path; the variation of an ellipse from a circle, where a circle has an eccentricity of 0.

Eddy⁽⁶⁾
	A small volume of air (or any fluid) that behaves differently from the larger flow in which it exists.

Effective emissivity⁽⁶⁾
	A correction factor, dependent on the radiational characteristics of the earth-atmosphere system, that permits application of black body radiation laws to the earth-atmosphere system

El Niño⁽²⁾
	The appearance of unusually warm waters in the eastern Pacific; termed the "Christ child," because of the time of year it effects the South American coastline.

Electromagnetic radiation⁽¹⁾
	Energy propagated as time-varying electric and magnetic fields. These two fields are inextricably linked as a single entity since time-varying electric fields produce time-varying magnetic fields and vice versa. Light and radar are examples of electromagnetic radiation differing only in their wavelengths (or frequency). Electric and magnetic fields propagate through space at the speed of light.

Emissivity⁽⁶⁾
	The fractional amount of radiation emitted by a given object or substance in comparison to the amount emitted by a perfect emitter.

Emittance⁽⁶⁾
	The rate at which a black body radiates energy across all wave-lengths.

Energy budget⁽¹⁾
	A quantitative description of the energy exchange for a physical or ecological system. The budget includes terms for radiation, conduction, convection, latent heat, and for sources and sinks of energy.

Enhanced Greenhouse Effect⁽⁵⁾
	The increase in the natural greenhouse effect resulting from increases in atmospheric concentrations of GHGs due to emissions from human activities.

Entrainment⁽⁶⁾
	The mixing of environmental air into a preexisting air current or cloud so that the environmental air becomes part of the current or cloud.

Environmental lapse rate⁽⁶⁾
	The rate of decrease of temperature with elevation. It is most often measured with a radiosonde.

Equilibrium vapor pressure⁽⁶⁾
	The necessary vapor pressure around liquid water that allows the water to remain in equilibrium with its environment. Also called saturation vapor pressure.

Equinox⁽⁶⁾
	The time when the sun crosses the plane of the earth's equator occurring about March 21 and September 22.

Evaporation⁽³⁾
	The physical process by which a liquid or solid is changed to a gas; the opposite of condensation.

Evaporation fog⁽⁶⁾
	Fog produced when sufficient water vapor is added to the air by evaporation. The two common types are steam fog, which forms when cold air moves over warm water, and frontal fog, which forms as warm raindrops evaporate in a cool air mass.

Evapotranspiration⁽¹⁾

The sum of evaporation and plant transpiration. Potential evapotranspiration is the amount of water that could be evaporated or transpired at a given temperature and humidity, if there was plenty of water available. Actual evapotranspiration can not be any greater than precipitation, and will usually be less because some water will run off in rivers and flow to the oceans. If potential evapotranspiration is greater than actual precipitation, then soils are extremely dry during at least a major part of the year.

Excessive Heat Outlook⁽³⁾
	This CPC product, a combination of temperature and humidity over a certain number of days, is designed to provide an indication of areas of the country where people and animals may need to take precautions against the heat during May to November.

Exosphere⁽⁶⁾
	The outermost portion of the atmosphere.

Extratropical⁽³⁾
	In meteorology, the area north of the Tropic of Cancer and the area south of the Tropic of Capricorn. In other words, the area outside the tropics.

Fahrenheit⁽¹⁾
	Temperature scale designed by the German scientist Gabriel Fahrenheit in 1709, based upon water freezing at 32°F and water boiling at 212°F under standard atmospheric pressure. Compare with centigrade.

Fall Freeze date⁽⁶⁾
	The date of occurrence in the fall of the first minimum at or below a temperature threshold.

Fall streaks⁽⁶⁾
	Falling ice crystals that evaporate before reaching the ground.

Flux⁽¹⁾
	the measure of the flow of some quantity per unit area per unit time.

Foehn⁽⁶⁾
	See Chinook.

Fog⁽⁶⁾
	A cloud with its base at the earth's surface. It reduces visibility to below 1 km.

Forced convection⁽⁶⁾
	On a small scale, a form of mechanical stirring taking place when twisting eddies of air are able to mix.

Forcing Mechanism⁽⁴⁾
	A process that alters the energy balance of the climate system, i.e. changes the relative balance between incoming solar radiation and outgoing infrared radiation from Earth. Such mechanisms include changes in solar irradiance, volcanic eruptions, and enhancement of the natural greenhouse effect by emission of carbon dioxide. See radiation, infrared radiation, radiative forcing.

Forecasts⁽³⁾
	A weather forecast, or prediction, is an estimation based on special knowledge of the future state of the atmosphere with respect to temperature, precipitation, and wind. Weather forecasts are now routinely provided for up to 14 days in advance and outlooks for seasonal and longer timescales. (Synonymous with predictions and outlooks).

Free convection⁽⁶⁾
	Convection triggered by intense solar heating of the earth's surface.

Freeze⁽⁶⁾
	A condition occurring over a widespread area when the surface air temperature remains below freezing for a sufficient time to damage certain agricultural crops. A freeze most often occurs as cold air is advected into a region, causing freezing conditions to exist in a deep layer of surface air. Also called advection frost.

Freeze free season⁽⁶⁾
	The number of days between the last spring freeze date and the first fall freeze date.

Freezing rain and freezing drizzle⁽⁶⁾
	Rain or drizzle that falls in liquid form and then freezes upon striking a cold object or ground. Both can produce a coating of ice on objects which is called glaze.

Front⁽⁶⁾
	The transition zone between two distinct air masses.

Frontal fog⁽⁶⁾
	See Evaporation fog.

Frost (also called hoarfrost)⁽⁶⁾
	A covering of ice produced by deposition (sublimation) on exposed surfaces when the air temperature falls below the frost point (the dew point is below freezing).

Frost point⁽⁶⁾
	See Dew point.

Frozen dew⁽⁶⁾
	The transformation of liquid dew into tiny beads of ice when the air temperature drops below freezing.

Funnel cloud⁽⁶⁾
	A rotating conelike cloud that extends down-ward from the base of a thunderstorm. When it reaches the surface it is called a tornado.

Gaia hypothesis⁽¹⁾
	The hypothesis that the Earth's atmosphere, biosphere, and its living organisms behave as a single system striving to maintain a stability that is conductive to the existence of life.

General Circulation Model (GCM)⁽¹⁾

A global, three-dimensional computer model of the climate system which can be used to simulate human-induced climate change. GCMs are highly complex and they represent the effects of such factors as reflective and absorptive properties of atmospheric water vapor, greenhouse gas concentrations, clouds, annual and daily solar heating, ocean temperatures and ice boundaries. The most recent GCMs include global representations of the atmosphere, oceans, and land surface.

Geologic time scale⁽²⁾
	Relative time scale based onstratigraphic position and correlation, and many different types of chronologic evidence. Geologic time is broken down into eons, eras, periods and epochs.

Geostrophic wind⁽⁶⁾
	A theoretical horizontal wind blowing in a straight path, parallel to the isobars or contours, at a constant speed. The geostrophic wind results when the Coriolis force exactly balances the horizontal pressure gradient force.

Glaciated cloud⁽⁶⁾
	A cloud or portion of a cloud where only ice crystals exist.

Glaciation⁽⁶⁾
	The conversion of all the supercooled liquid water in a cloud into ice crystals, thus reducing the growth rate of ice crystals and hail.

Glacier⁽²⁾
	River of ice that under pressure can deform and flow plastically.

Glaze⁽⁶⁾
	A coating of ice on objects formed when supercooled rain freezes on contact. A storm that produces glaze is called an icing storm.

Global carbon budget⁽¹⁾
	The balance of the exchanges (incomes and losses) of carbon between the carbon reservoirs or between one specific loop (e.g., atmosphere biosphere) of the carbon cycle. An examination of the carbon budget of a pool or reservoir can provide information about whether the pool or reservoir is functioning as a source or sink for CO₂.

Global climate change⁽⁶⁾

The long-term fluctuations in temperature, precipitation, wind, and all other aspects of the Earth's climate. External processes, such as solar-irradiance variations, variations of the Earth's orbital parameters (eccentricity, precession, and inclination), lithosphere motions, and volcanic activity, are factors in climatic variation. Internal variations of the climate system also produce fluctuations of sufficient magnitude and variability to explain observed climate change through the feedback processes interrelating the components of the climate system.

Global Climate Models⁽⁸⁾
	Sophisticated computer models of the atmosphere and oceans that attempt to include all the processes known to affect climate.

Global measurement⁽¹⁾

All of the activities required to specify a global variable, such as ozone. These activities range from data acquisition to the generation of a data-analysis product, and include estimates of the uncertainties in that product. A global measurement often will consist of a combination of observations from a spacecraft instrument (required for global coverage) and measurements in situ (needed to provide reference points for long-term accuracy).

Global positioning system (GPS)⁽¹⁾
	A system consisting of 25 satellites in 6 orbital planes at 20,000 km altitude with 12 hr periods, used to provide highly precise position, velocity and time information to users anywhere on Earth or in its neighborhood at any time.

Global variables⁽¹⁾
	Functions of space and time that describe the large-scale state and evolution of the Earth system. The Earth system's geosphere, hydrosphere, atmosphere, and biosphere and their components are, or potentially are, global variables.

Global Warming⁽³⁾
	Certain natural and human-produced gases prevent the sun's energy from escaping back to space leading to an overall rise in the temperature of the Earth's atmosphere.

Glory⁽⁶⁾
	Colored rings that appear around the shadow of an object.

Graupel⁽⁶⁾
	See Snow pellets

Green flash⁽⁶⁾
	A small, green color that occasionally appears on the upper part of the sun as it rises or sets.

Greenhouse Effect⁽³⁾

The atmosphere allows solar radiation to reach the earth relatively easily. The atmosphere absorbs the infrared radiation emitted by the Earth's surface and radiates it back to the Earth in much the same way a greenhouse traps heat as the sun's rays pass through the glass, and the heat generated does not pass back through the glass. The "greenhouse effect" causes the surface of the Earth to be much warmer that it would be without the atmosphere 60°F). Without the greenhouse effect, life as we know it might not exist on Earth.

Greenhouse Gas⁽³⁾
	Certain gases, such as water vapor, carbon dioxide, and methane, that more effectively trap heat affecting the Earth's surface temperature.

Greenwich Mean Time (GMT)⁽³⁾

Explain when we would use this relative to satellite images. GMT is the global standard for time that was established in 1884 when delegates from 27 nations met in Washington, DC for the Meridian Conference and agreed on a system basically the same as that now in use. Civilian designations are typically three letter abbreviations (e.g. EST) for most time zones. Military designations use each letter of the alphabet (except 'J') and are known by their phonetic equivalent. For example, Greenwich Mean Time (civilian) or Z = Zulu (military and aviation). Universal Time Coordinated (UTC) has been substituted for GMT.

Ground fog⁽⁶⁾
	See Radiation fog.

Growing degree-day⁽⁶⁾
	A form of the degree-day used as a guide for crop planting and for estimating crop maturity dates.

Growing season⁽⁶⁾
	The number of days between the last spring freeze date and the first fall freeze date.

Gulf stream⁽¹⁾
	A warm, swift ocean current that flows along the coast of the Eastern United States and makes Ireland, Great Britain, and the Scandinavian countries warmer than they would be otherwise.

Haboob⁽⁶⁾
	A dust or sandstorm that forms as cold downdrafts from a thunderstorm turbulently lift dust and sand into the air.

Hail⁽⁶⁾
	Solid precipitation in the form of chunks or balls of ice with diameters greater than 5 mm. The stones fall from cumulonimbus clouds.

Hailstones⁽⁶⁾
	Transparent or partially opaque particles of ice that range in size from that of a pea to that of golf balls.

Hair hygrometer⁽⁶⁾
	An instrument used to monitor relative humidity by measuring the changes in the length of human hair that accompany humidity variations.

Halos⁽⁶⁾
	Rings or arcs that encircle the sun or moon when seen through an ice crystal cloud or a sky filled with falling ice crystals. Halos are produced by refraction of light.

Haze⁽⁶⁾
	Fine dry or wet dust or salt particles dispersed through a portion of the atmosphere. Individually these are not visible but cumulatively they will diminish visibility.

Heat⁽⁶⁾
	A form of energy transferred between systems by virtue of their temperature differences.

Heat balance⁽¹⁾
	The equilibrium existing between the radiation received and emitted by a planetary system.

Heat capacity⁽⁶⁾
	The ratio of the heat absorbed (or released) by a system to the corresponding temperature rise (or fall).

Heat index ((HI))⁽⁶⁾
	An index that combines air temperature and relative humidity to determine an apparent temperature-how hot it actually feels.

Heat lightning⁽⁶⁾
	Distant lightning that illuminates the sky but is too far away for its thunder to be heard.

Heat of fusion⁽⁶⁾
	Heat released when water changes phase from liquid to solid; 80 calories per gram.

Heat of melting⁽⁶⁾
	Heat required to change the phase of water from solid to liquid; 80 calories per gram.

Heating degree-day⁽⁶⁾
	A form of the degree-day used as an index for fuel consumption. Needed on days when average air temperature falls below 69°F (18°C); computed by subtracting the day's average temperature from 65°F.

Heiligenschein⁽⁶⁾
	A faint white ring surrounding the shadow of an observer's head on a dew-covered lawn.

Heterosphere⁽⁶⁾
	The atmosphere above 80 km (50 mi) where gases are stratified, with concentrations of the heavier gases decreasing more rapidly with altitude than concentrations of the lighter gases.

High inversion fog⁽⁶⁾
	A fog that lifts above the surface but does not completely dissipate because of a strong inversion (usually subsidence) that exists above the fog layer.

Highland climate⁽⁶⁾
	Complex pattern of climate conditions associated with mountains. Highland climates are characterized by large differences that occur over short distances.

Historical Flood⁽⁶⁾
	Flood events documented by human observation but recorded prior to the development of systematic streamflow measurements.

Hoarfrost⁽⁶⁾
	Fernlike crystals of ice that form by deposition of water vapor on twigs, tree branches, and other vegetation.

Homosphere⁽⁶⁾
	The atmosphere up to 80 km (50 mi) in which the proportionality of principal gaseous constituents, such as oxygen and nitrogen, is constant.

Humid continental climate⁽⁶⁾
	A relatively severe climate characteristic of broad continents in the middle latitudes between approximately 40-50° north latitude. This climate is not found in the southern hemisphere, where the middle latitudes are dominated by the oceans.

Humid Subtropical Climate⁽⁶⁾
	A climate generally located on the eastern side of a continent and characterized by hot, sultry summers and cool winters.

Hurricane⁽³⁾
	See Cyclone.

Hydrograph⁽⁶⁾
	An instrument that provides a continuous trace of relative humidity with time.

Hydrologic cycle⁽⁵⁾

The process of evaporation, vertical and horizontal transport of vapor, condensation, precipitation, and the flow of water from continents to oceans. It is a major factor in determining climate through its influence on surface vegetation, the clouds, snow and ice, and soil moisture. The hydrologic cycle is responsible for 25-30% of the mid-latitudes' heat transport from the equatorial to polar regions. See The Water Cycle.

Hydrology⁽³⁾
	The scientific study of precipitation, evaporation, distribution, and effects of water on the Earth's surface, in the soil and rocks, and in the atmosphere.

Hygrometer⁽⁶⁾
	An instrument designed to measure the air's water vapor content. The sensing part of the instrument can be hair (hair hygrometer), a plate coated with carbon (electrical hygrometer), or an infrared sensor (infrared hygrometer).

Hypothermia⁽⁶⁾
	The deterioration in one's mental and physical condition brought on by a rapid lowering of human body temperature.

Ice Age⁽²⁾
	Period during which polar ice extends to much lower latitudes than normal.

Ice Cap Climate⁽⁶⁾
	A climate that has no monthly means above freezing and supports no vegetative cover except in a few scattered high mountain areas. This climate, with its perpetual ice and snow, is confined largely to the ice sheets of Greenland and Antarctica.

Ice core⁽¹⁾
	A cylindrical section of ice removed from a glacier or an ice sheet in order to study climate patterns of the past. By performing chemical analyses on the air trapped in the ice, scientists can estimate the percentage of carbon dioxide and other trace gases in the atmosphere at that time.

Ice fog⁽⁶⁾
	A type of fog composed of tiny suspended ice particles that forms at very low temperatures.

Ice nuclei⁽⁶⁾
	Particles that act as nuclei for the formation of ice crystals in the atmosphere.

Ice pellets⁽⁶⁾
	See Sleet

Ice sheet (continental glacier)⁽¹⁾

A glacier of considerable thickness and more than 50,000 sq km in area. It forms a continuous cover of ice and snow over a land surface. An ice sheet is not confined by the underlying topography but spreads outward in all directions. During the Pleistocene Epoch, ice sheets covered large parts of North America and northern Europe but they are now confined to polar regions (e.g., Greenland and Antarctica).

Ice shelf⁽¹⁾
	A thick mass of ice extending from a polar shore. The seaward edge is afloat and sometimes extends hundreds of miles into the sea.

In situ⁽¹⁾
	Latin for 'in original place.' Refers to measurements made at the actual location of the object or material measured. Compare remote sensing.

Indian summer⁽⁶⁾
	An unseasonably warm spell with clear skies near the middle of autumn. Usually follows a substantial period of cool weather.

Infrared radiation⁽⁴⁾

The heat energy that is emitted from all solids, liquids, and gases. In the context of the greenhouse issue, the term refers to the heat energy emitted by the Earth's surface and its atmosphere. Greenhouse gases strongly absorb this radiation in the Earth's atmosphere, and radiate some back towards the surface, creating the greenhouse effect. See radiation, greenhouse effect, enhanced greenhouse effect, global warming.

Insolation⁽⁶⁾
	The incoming solar radiation that reaches the earth and the atmosphere.

Insolation⁽⁶⁾
	Solar radiation incident upon a unit horizontal surface on or above the Earth's surface. Kelvin Waves cause variations in the depth of the oceanic thermocline, the boundary between warm waters in the upper ocean and cold waters in the deep ocean.

Intergovernmental Panel on Climate Change (IPCC)⁽⁵⁾
	The IPCC was established in 1988 by the World Meteorological Organization and the UN Environment Programme. The IPCC is responsible for providing the scientific and technical foundation for the United Nations Framwork Convention on Climate Change (UNFCCC), primarily through the publication of periodic assessment reports (see "Second Assessment Report" and "Third Assessment Report").

International System of Units (SI)⁽¹⁾
	The International System of Units prescribes the symbols and prefixes shown in the table to form decimal multiples and submultiples of SI units. The following examples illustrate the use of these prefixes: 0.000,001 meters = 1 micrometer = 1¾m; 1000 meters = 1 kilometer = 1 km; 1,000,000 cycles per second = 1,000,000 hertz = 1 megahertz =1 MHz..

Intertropical convergence zone ((ITCZ))⁽⁶⁾
	The boundary zone separating the northeast trade winds of the Northern Hemisphere from the southeast trade winds of the Southern Hemisphere.

Intraseasonal Oscillations⁽³⁾
	Variability on a timescale less than a season. One example is the Madden-Julian Oscillation.

Inversion⁽⁶⁾
	An increase in air temperature with height.

Ion⁽⁶⁾
	An electrically charged atom, molecule, or particle.

Ionosphere⁽⁶⁾
	An electrified region of the upper atmosphere where fairly large concentrations of ions and free electrons exist.

Isobar⁽⁶⁾
	A line connecting points of equal pressure.

Isotach⁽⁶⁾
	A line connecting points of equal wind speed.

Isotherm⁽⁶⁾
	A line connecting points of equal wind temperature.

January thaw⁽⁶⁾
	A period of relatively mild weather around January 20 to 23 that occurs primarily in New England; an example of a singularity in the climatic record.

Jet Stream⁽³⁾
	Strong winds concentrated within a narrow zone in the atmosphere in the upper troposphere, about 30,000 feet aloft that generally move in an easterly direction that drive weather systems around the globe. In North America jet streams are more pronounced in winter.

Katabatic wind⁽⁶⁾
	Any wind blowing downslope. Usually cold.

Kelling Curve⁽⁶⁾
	The name given to a graph that measured the rise of carbon dioxide gas in our atmosphere. Carbon Dioxide has been measured continuously since 1958 and follows an oscillating line. The line was named after Dr. Charles David Keeling, professor at Scripps Institution of Oceanography.

Kelvin Waves⁽³⁾
	Fluctuations in wind speed at the ocean surface at the Equator result in eastward propagating waves, known as Kelvin Waves. Kelvin Waves cause variations in the depth of the oceanic thermocline, the boundary between warm waters in the upper ocean and cold waters in the deep ocean. They play an important role in monitoring and predicting El Niño episodes.

Kyoto Protocol⁽⁵⁾
	An international agreement adopted in December 1997 in Kyoto, Japan. The Protocol sets binding emission targets for developed countries that would reduce their emissions on average 5.2% below 1990 levels.

La Niña⁽³⁾

La Niña, a phase of ENSO, is a periodic cooling of surface ocean waters in the eastern tropical Pacific along with a shift in convection in the western Pacific further west than the climatological average. These conditions affect weather patterns around the world. The preliminary CPC definition of La Niña is a phenomenon in the equatorial Pacific Ocean characterized by a negative sea surface temperature departure from normal (for the 1971-2000 base period), averaged over three months, greater than or equal in magnitude to 0.5°C in a region defined by 150°W-160°E and 5°N-5°S (commonly referred to as Niño 4).

Lake breeze⁽⁶⁾
	A wind blowing onshore from the surface of a lake.

Lake-effect snows⁽⁶⁾
	Localized snowstorms that form on the downwind side of a lake. Such storms are common in late fall and early winter near the Great Lakes as cold, dry air picks up moisture and warmth from the unfrozen bodies of water.

Land breeze⁽⁶⁾
	A coastal breeze that blows from land to sea, usually at night.

Lapse rate⁽⁶⁾
	The rate at which an atmospheric variable (usually temperature) decreases with height. (See Environmental lapse rate.)

Latent heat⁽¹⁾
	The heat that is either released or absorbed by a unit mass of a substance when it undergoes a change of state, such as during evaporation, condensation, or sublimation.

Latitude⁽¹⁾
	The angle between a perpendicular at a location, and the equatorial plane of the Earth.

Lenticular cloud⁽⁶⁾
	A cloud in the shape of a lens.

Lightning⁽⁶⁾
	A visible electrical discharge produced by thunderstorms.

Little Ice Age⁽¹⁾
	A cold period that lasted from about A.D. 1550 to about A.D. 1850 in Europe, North America, and Asia. This period was marked by rapid expansion of mountain glaciers, especially in the Alps, Norway, Ireland, and Alaska. There were three maxima, beginning about 1650, about 1770, and 1850, each separated by slight warming intervals.

Long Wave⁽⁴⁾
	In meteorology, a long wave in atmospheric circulation in the major belt of westerlies has different characteristics than rapidly moving storms nearer the EarthÕs surface. (or Planetary Wave)","l

Longitude⁽¹⁾
	The angular distance from the Greenwich meridian (0°), along the equator. This can be measured either east or west to the 180th meridian (180°) or 0° to 360°W.

Longwave radiation⁽¹⁾
	The radiation emitted in the spectral wavelength greater than 4 micrometers corresponding to the radiation emitted from the Earth and atmosphere. It is sometimes referred to as terrestrial radiation or infrared radiation, although somewhat imprecisely. See infrared radiation.

Magnetosphere⁽⁶⁾
	The region around the earth in which the earth's magnetic field plays a dominant part in controlling the physical processes that take place.

Mammatus clouds⁽⁶⁾
	Clouds that look like pouches hanging from the underside of a cloud.

Marine climate⁽⁶⁾
	A climate dominated by the ocean, because of the moderating effect of water, sites having this climate are considered relatively mild.

Maritime air mass⁽⁶⁾
	An air mass that originates over the ocean. These air masses are relatively humid.

Maritime polar air⁽⁶⁾
	Cool, humid air mass that forms over the cold ocean waters of the North Pacific and North Atlantic.

Maritime tropical air⁽⁶⁾
	Warm, humid air mass that forms over tropical and subtropical oceans.

Mauna Loa Record⁽⁵⁾
	The record of measurement of atmospheric CO₂ concentrations taken at Mauna Loa Observatory, Mauna Loa, Hawaii, since March 1958. This record shows the continuing increase in average annual atmospheric CO₂ concentrations.

Mean⁽³⁾
	The arithmetic average, or the middle point between two extremes.

Mean annual temperature⁽⁶⁾
	The average temperature at any given location for the entire year.

Mean sea level⁽¹⁾

The average height of the sea surface, based upon hourly observation of the tide height on the open coast or in adjacent waters that have free access to the sea. In the United States, it is defined as the average height of the sea surface for all stages of the tide over a nineteen-year period. Mean sea level, commonly abbreviated as MSL and referred to simply as 'sea level,' serves as the reference surface for all altitudes in upper atmospheric studies.

Mesoscale⁽⁶⁾
	The scale of meteorological phenomena that ranges in size from a few km to about 100 km. It includes local winds, thunderstorms, and tornadoes.

Mesosphere⁽¹⁾
	The atmospheric layer between the stratosphere and the thermosphere. Located at an average elevation between 50 and 80 km above the earth's surface.

Meteorology⁽³⁾
	The scientific study of the physics, chemistry, and dynamics of the EarthÕs atmosphere, especially weather and climate.

Methane (CH₄⁽⁵⁾

CH₄ is among the six greenhouse gases to be curbed under the Kyoto Protocol. Natural processes produce atmospheric CH₄, but there are also substantial emissions from human activities such as landfills, livestock and livestock wastes, natural gas and petroleum systems, coalmines, rice fields, and wastewater treatment. CH₄ has a relatively short atmospheric lifetime of approximately 10 years, but its 100-year GWP is currently estimated to be approximately 23 times that of CO₂.

Microburst⁽⁶⁾
	A strong localized downdraft less than 4 km wide that occurs beneath severe thunderstorms. A strong downdraft greater than 4 km across is called a downburst.

Microclimate⁽⁶⁾
	The climate structure of the air space near the surface of the earth.

Microscale⁽⁶⁾
	The smallest scale of atmospheric motions.

Millibar⁽³⁾
	A pressure unit of 1000 dynes cm^-2, often used for reporting atmospheric pressure where one millibar equals 1 hectopascal (hPA).

Mirage⁽⁶⁾
	A refraction phenomenon that makes an object appear to be displaced from its true position. When an object appears higher than it actually is, it is called a superior image. When an object appears lower than it actually is, it is an inferior mirage.

Mist⁽⁶⁾
	Very thin fog in which visibility is greater than 1.0 km (0.62 mi).

Mistral⁽⁶⁾
	A katabatic wind that flows from the Alps down the Rhone River Valley of France to the Mediterranean coast.

Mixing ratio⁽⁶⁾
	The ratio of the mass of water vapor in a given volume of air to the mass of dry air.

Model (noun)⁽⁶⁾
	A mathematical representation of a process, system, or object developed to understand its behavior or to make predictions. The representation always involves certain simplifications and assumptions.

Moist adiabatic rate⁽⁶⁾
	The rate of change of temperature in a rising or descending saturated air parcel. The rate of cooling or warming varies but a common value of 6C per 1000 m (3.3F/1000 ft) is used. "Molecular viscosity

Monsoons⁽³⁾
	Seasonal winds. They are caused primarily by the greater annual variation in air temperature over large land surfaces compared to ocean surfaces though other factors like land-relief are important.

Montreal Protocol⁽¹⁾

An international agreement to drastically reduce CFC production, the Protocol was adopted in Montreal in 1987. It was significantly strengthened at a subsequent meeting in London in 1990 that called for a complete elimination of CFCs by the year 2000. The agreement was again amended by a Meeting of the Parties in Copenhagen in November 1992. Consumption of controlled substances--such as CFCs and halons--was greatly reduced or eliminated, and many accountability dates were moved forward, often from 1 January 2000 to 1 January 1996.

Mountain and valley breezes⁽¹⁾
	A local wind system of a mountain valley that blows downhill (mountain breeze) at night and uphill (valley breeze) during the day.

Nacreous clouds⁽⁶⁾
	Clouds of unknown composition that have a soft, pearly luster and that form at altitudes about 25 to 30 km above the earth's surface. They are also called mother-of-pearl clouds.

NAO Index⁽³⁾

This index measures the anomalies in sea level pressure between the Icelandic low-pressure system and the Azores high-pressure system. The nine centers are: Aviation Weather Center, Climate Prediction Center, Environmental Modeling Center, Hydrometeorological Prediction Center, Marine Prediction Center, NCEP Central Operations, Space Environment Center, Storm Prediction Center and Tropical Prediction Center (formerly the National Hurricane Center).

National Centers for Environmental Prediction (NCEP)⁽³⁾

The National Centers for Environmental Prediction (NCEP), an arm of the NOAA's National Weather Service (NWS) provides national and international weather and climate guidance products to National Weather Service field offices, government agencies, emergency managers, private sector meteorologists, and meteorological organizations and societies throughout the world. The nine centers are: Aviation Weather Center, Climate Prediction Center, Environmental Modeling Center, Hydrometeorological Prediction Center, Marine Prediction Center, NCEP Central Operations, Space Environment Center, Storm Prediction Center and Tropical Prediction Center (formerly the National Hurricane Center).

National Climatic Data Center (NCDC)⁽³⁾
	NCDC maintains the world's largest active archive of weather data. NCDC produces numerous climate publications and responds to data requests from all over the world.

National Oceanic and Atmospheric Administration (NOAA)⁽³⁾
	NOAA's historical role has been to predict environmental changes, protect life and property, provide decision makers with reliable scientific information, and foster global environmental stewardship. Today NOAAÕs mission remains unchanged as it describes and predicts changes in the Earth's environment, and conserves and wisely manages the Nation's coastal and marine resources.

National Weather Service (NWS)⁽³⁾
	National Weather Service.

Negative Feedback⁽⁵⁾
	A process that results in a reduction in the response of a system to an external influence. For example, increased plant productivity in response to global warming would be a negative feedback on warming, because the additional growth would act as a sink for CO₂, reducing the atmospheric CO₂ concentration.

Nimbostratus⁽⁶⁾
	A dark, gray cloud characterized by more or less continuously falling precipitation. It is not accompanied by lightning, thunder, or hail.

Noctilucent clouds⁽⁶⁾
	Wavy, thin, bluish-white clouds that are best seen at twilight in polar latitudes. They form at altitudes about 80 to 90 km above the surface.

Nocturnal inversion⁽⁶⁾
	See Radiation inversion.

Normal⁽³⁾

To understand whether precipitation and temperature is above or below normal for seasons and longer timescales, normal is defined as the average weather over 30 years. These averages are recalculated every ten years. The National Weather Service has just recalculated the baseline period for normal from 1961 to 1990 to 1971 to 2000. Since the cool decade of the 1960's has been replaced with the mild 1990's, normal temperatures in many areas have increased.

North Atlantic Oscillation (NAO)⁽³⁾

The NAO is a large-scale fluctuation in atmospheric pressure between the subtropical high-pressure systems located near the Azores in the Atlantic Ocean and the sub-polar low-pressure system near Iceland and is quantified in the NAO Index. The surface pressure drives surface winds and wintertime storms from west to east across the North Atlantic affecting climate from New England to Western Europe as far eastward as central Siberia and eastern Mediterranean and southward to West Africa

Numerical Forecasting⁽³⁾
	A computer forecast or prediction based on equations governing the motions and the forces affecting motion of fluids. The equations are based, or initialized, on specified weather or climate conditions at a certain place and time. (Also called mathematical forecasting, dynamical forecasting, physical forecasting, and numerical weather prediction).

Obliquity⁽²⁾
	The angle between the planes of the celestial equator and the ecliptic, currently the earth has a 23.4 degree obliquity cycle.

Offshore breeze⁽⁶⁾
	A breeze that blows from the land out over the water. Opposite of an onshore breeze.

Onshore breeze⁽⁶⁾
	A breeze that blows from the water onto the land. Opposite of an offshore breeze.

Orbital forcing⁽²⁾
	Theory that proposes large scale climate changes are due in part to the variations in precession, eccentricity and obliquity that affects the amount of solar radiation received by the earth.

Orographic precipitation⁽⁶⁾
	Rainfall or snowfall from clouds, induced by topographic uplift.

Orographic uplift⁽⁶⁾
	The lifting of air over a topographic barrier. Clouds that form in this lifting process are called orographic clouds.

Oscillations⁽³⁾
	A shift in position of various high and low pressure systems that in climate terms is usually defined as an index (i.e., a single numerically-derived number, that represents the distribution of temperature and pressure over a wide ocean area, such as the El Niño-Southern Oscillation, North Atlantic Oscillation, and Pacific Decadal Oscillation).

Ozone⁽³⁾

A molecule consisting of three oxygen atoms that is formed by a reaction of oxygen and ultraviolet radiation. In the stratosphere, ozone has beneficial properties where it forms an ozone shield that prevents dangerous radiation from reaching the Earth's surface. Closer to the planetÕs surface, ozone is considered an air pollutant that adversely affects humans, plants and animals as well as a greenhouse gas.

Ozone Hole⁽³⁾
	A severe depletion of stratospheric ozone over Antarctica that occurs each spring. The possibility exists that a hole could form over the Arctic as well. The depletion is caused by a chemical reaction involving ozone and chlorine, primarily from human produced sources, cloud particles, and low temperatures.

Ozone Layer⁽¹⁾

The layer of ozone that begins approximately 15 km above Earth and thins to an almost negligible amount at about 50 km, shields the Earth from harmful ultraviolet radiation from the sun. The highest natural concentration of ozone (approximately 10 parts per million by volume) occurs in the stratosphere at approximately 25 km above Earth. The stratospheric ozone concentration changes throughout the year as stratospheric circulation changes with the seasons. Natural events such as volcanoes and solar flares can produce changes in ozone concentration, but man-made changes are of the greatest concern.

Pacific Decadal Oscillation⁽³⁾

A recently described pattern of climate variation similar to ENSO though on a timescale of decades and not seasons. It is characterized by SST anomalies of one sign in the north-central Pacific and SST anomalies of another sign to the north and east near the Aleutians and the Gulf of Alaska. It primarily affects weather patterns and sea surface temperatures in the Pacific Northwest, Alaska, and northern Pacific Islands. Two main characteristics distinguish PDO from El Niño/Southern Oscillation (ENSO): first, 20th century PDO "events" persisted for 20-to-30 years, while typical ENSO events persisted for 6 to 18 months; second, the climatic fingerprints of the PDO are most visible in the North Pacific/North American sector, while secondary signatures exist in the tropics- the opposite is true for ENSO. Several independent studies found evidence of just two full PDO cycles in the past century: cool" PDO regimes prevailed from 1890-1924 and again from 1947-1976, while "warm" PDO regimes dominated from 1925-1946 and from 1977 through (at least) the mid-1990's. Causes for the PDO are not currently known. Likewise, the potential predictability for this climate oscillation is not known.

Paleoclimate⁽²⁾
	Past or ancient climates.

Paleoclimatologist⁽²⁾
	One who studies ancient (paleo-) climate. For example, increased atmospheric water vapor in response to global warming would be a positive feedback on warming, because water vapor is a GHG.

Paleoenvironmental proxy⁽²⁾
	An environmental remnant of the past (pollen grains, tree rings, lake sediments, pack rat middens, ice cores, coral skeletons) that assist researchers in deciphering past climate conditions through the use of scientifically proven dating techniques.

Palmer Drought Severity Index (PDSI)⁽⁸⁾
	An indicator, based on temperature, precipitation, and soil type, of long-term deficits or surpluses of soil moisture.

Particulate matter (PM)⁽⁴⁾
	Solid particles or liquid droplets suspended or carried in the air (e.g., soot, dust, fumes, mist). See aerosol, sulfate aerosols.

PDSI⁽⁶⁾
	See Palmer Drought Severity Index.

Permafrost⁽¹⁾
	Perennially frozen ground that occurs wherever the temperature remains below 0°C for several years.

Phenology⁽⁸⁾
	A branch of science dealing with the relations between climate and periodic biological phenomena, such as bird migration or plant flowering.

Photodissociation⁽⁶⁾
	The splitting of a molecule by a photon.

Photon⁽⁶⁾
	A discrete quantity of energy that can be thought of as a packet of electromagnetic radiation traveling at the speed of light.

Pileus cloud⁽⁶⁾
	A smooth cloud in the form of a cap. Occurs above, or is attached to, the top of a cumuliform cloud.

Planetary albedo⁽¹⁾
	The fraction of incident solar radiation that is reflected by a planet and returned to space. The planetary albedo of the Earth-atmosphere system is approximately 30 percent, most of which is due to backscatter from clouds in the atmosphere.

Polar air mass⁽⁶⁾
	A cold air mass that forms in a high-latitude source region.

Polar climates⁽⁶⁾
	Climates in which the mean temperature of the warmest month is below 10C; climates that are too cold to support the growth of trees.

Positive Feedback⁽⁵⁾
	A process that results in an amplification of the response of a system to an external influence. For example, increased atmospheric water vapor in response to global warming would be a positive feedback on warming, because water vapor is a GHG.

Potential energy⁽⁶⁾
	The energy that a body possesses by virtue of its position with respect to other bodies in the field of gravity.

Potential evapotranspiration ((PE))⁽⁶⁾
	The amount of moisture that, if it were available, would be removed from a given land area by evaporation and transpiration.

Potential temperature⁽⁶⁾
	The temperature that a parcel of dry air would have if it were brought dry adiabatically from its original position to a pressure of 1000 mb.

PRCP⁽⁶⁾
	Abbreviation for precipitation.

Precession⁽⁴⁾
	The tendency of the Earth's axis to wobble in space over a period of 23,000 years. The Earth's precession is one of the factors that results in the planet receiving different amounts of solar energy over extended periods of time.

Precipitable water vapor⁽⁶⁾
	The depth of water that would result if all the vapor in the atmosphere above a location were condensed into liquid water.

Precipitation⁽⁶⁾
	Any form of water particles-liquid or solid-that falls from the atmosphere and reaches the ground.

Prevailing wind⁽⁶⁾
	The wind direction most frequently observed during a given period.

Probability⁽⁶⁾
	A chance, or likelihood, that a certain event might happen.

Probability forecast⁽⁶⁾
	A forecast of the probability of occurrence of one or more of a mutually exclusive set of weather conditions.

Psychrometer⁽⁶⁾
	An instrument used to measure the water vapor content of the air. It consists of two thermometers (dry bulb and wet bulb). After whirling the instrument, the dew point and relative humidity can be obtained with the aid of tables.

Pyranometer⁽⁶⁾
	An instrument that measures the amount of radiation.

Radar⁽⁶⁾
	An instrument useful for remote sensing of meteorological phenomena. It operates by sending radio waves and monitoring those returned by such reflecting objects as raindrops within clouds.

Radiant energy (radiation)⁽⁶⁾
	Energy propagated in the form of electromagnetic waves. These waves do not need molecules to propagate them, and in a vacuum they travel at nearly 300,000 km per sec.

Radiation⁽¹⁾
	Energy transfer in the form of electromagnetic waves or particles that release energy when absorbed by an object. See ultraviolet radiation, infrared radiation, solar radiation, longwave radiation, terrestrial radiation.

Radiation budget⁽¹⁾
	A measure of all the inputs and outputs of radiative energy relative to a system, such as Earth. See Earth Radiation Budget Experiment.

Radiation fog⁽⁶⁾
	Fog produced over land when radiational cooling reduces the air temperature to or below its dew point. It is also known as ground fog and valley fog.

Radiation inversion⁽⁶⁾
	An increase in temperature with height due to radiational cooling of the earth's surface. Also called a nocturnal inversion.

Radiative cooling⁽¹⁾
	Cooling process of the Earth's surface and adjacent air, which occurs when infrared (heat) energy radiates from the surface of the Earth upward through the atmosphere into space. Air near the surface transfers its thermal energy to the nearby ground through conduction, so that radiative cooling lowers the temperature of both the surface and the lowest part of the atmosphere.

Radiative Forcing⁽¹⁾

A change in the balance between incoming solar radiation and outgoing infrared radiation. Without any radiative forcing, solar radiation coming to the Earth would continue to be approximately equal to the infrared radiation emitted from the Earth. The addition of greenhouse gases traps an increased fraction of the infrared radiation, radiating it back toward the surface and creating a warming influence (i.e., positive radiative forcing because incoming solar radiation will exceed outgoing infrared radiation).

Radiosonde⁽⁶⁾
	A balloon-borne instrument that measures and transmits pressure, temperature, and humidity to a ground-based receiving station.

Rain⁽⁶⁾
	Precipitation in the form of liquid water drops that have diameters greater than that of drizzle.

Rain gage⁽⁶⁾
	A device-usually a cylindrical container-for measuring rain-fall.

Rain Shadow⁽⁶⁾
	The region on the leeside of a mountain where the precipitation is noticeable less than on the windward side.

Rawinsonde⁽⁶⁾
	An instrument carried by weather balloons to measure the temperature, humidity, pressure, and winds of the atmosphere.

Reflection⁽⁶⁾
	The process whereby a surface turns back a portion of the radiation that strikes it.

Refraction⁽⁶⁾
	The bending of light as it passes from one medium to another.

Refractive index⁽⁶⁾
	The ratio of the speed of light in a vacuum to its speed in a transparent medium.

Relative humidity⁽⁴⁾

An estimate of the amount of moisture in the air relative to the amount of moisture that the air can hold at a specific temperature. For example, if it's 70°F near dawn on a foggy summer morning, the relative humidity is near 100%. During the afternoon the temperature soars to 95°F and the fog disappears. The moisture in the atmosphere has not changed appreciably, but the relative humidity drops to 44% because the air has the capacity to hold much more moisture at a temperature of 95°F than it does at 70°F. But even when the relative humidity is "low" at 44%, it's a very humid day when the temperature is 95°F. For this reason, a better measure of comfort is dew point.

Retrogression or retrograde motion⁽³⁾

Motion that is backwards from the usual way things move in the Northern Hemisphere extratropics which is from west to east. In meteorology, the term is used in relation to atmospheric waves or pressure systems. When meteorologists say that a pattern will retrograde, they mean that the troughs and ridges will end up further west than they were previously. Normal motion (over the United States) is progressive, or prograde, which means (weather systems move) from west to east.

Rime ice⁽⁶⁾
	A white, granular deposit of ice formed by the freezing of water drops when they come in contact with an object.

Salinity⁽⁶⁾
	The degree of salt in water. The rise in sea level due to global warming would result in increased salinity of rivers, bays and aquifers.

Santa Ana⁽⁶⁾
	The local name given a foehn wind in southern California.

Saturation vapor pressure⁽⁶⁾
	The maximum amount of water vapor necessary to keep moist air in equilibrium with a surface of pure water or ice. It represents the maximum amount of water vapor that the air can hold at any given temperature and pressure. (See Equilibrium vapor pressure.)

Scattering⁽⁶⁾
	The process by which small particles in the atmosphere deflect radiation from its path into different directions.

Scintillation⁽⁶⁾
	The apparent twinkling of a star due to its light passing through regions of differing air densities in the atmosphere.

Sea breeze⁽⁶⁾
	A coastal local wind that blows from the ocean onto the land. The leading edge of the breeze is termed a sea breeze front.

Sea level⁽¹⁾
	The datum against which land elevation and sea depth are measured.

Sea level pressure⁽⁶⁾
	The atmospheric pressure at mean sea level.

Sea surface temperature (SST)⁽⁶⁾
	The temperature of the layer of seawater (approximately 0.5 m deep) nearest the atmosphere.

Sea surface temperature anomaly⁽⁶⁾
	Temperature of emitted energy from the sea surface. SST anomaly = (SST-SST mean), where SST = sea surface temperature. Snow water content and snow water equivalent are different terms for the same parameter.

Semiarid⁽⁶⁾
	See Steppe.

Sensible heat transfer⁽⁶⁾
	Movement of heat from one place to another as a consequence of conduction or convection or both.

Sensible temperature⁽⁶⁾
	The sensation of temperature that the human body feels in contrast to the actual temperature of the environment as measured with a thermometer.

Sequestration⁽⁵⁾
	Opportunities to remove atmospheric CO₂, either through biological processes (e.g. plants and trees), or geological processes through storage of CO₂ in underground reservoirs.

Shear⁽⁶⁾
	See wind shear.

Sheet lightning⁽⁶⁾
	A fairly bright lightning flash from distant thunderstorms that illuminates a portion of the cloud.

Shortwave radiation⁽⁶⁾
	A term most often used to describe the radiant energy emitted from the sun, in the visible and near ultraviolet wavelengths.

Shower⁽⁶⁾
	Intermittent precipitation from a cumuliform cloud, usually of short duration but often heavy.

Sinks⁽⁴⁾
	A reservoir that uptakes a chemical element or compound from another part of its cycle. For example, soil and trees tend to act as natural sinks for carbon.

Sleet⁽⁶⁾
	A type of precipitation consisting of transparent pellets of ice 5 mm or less in diameter. Same as ice pellets.

Smog⁽⁶⁾
	Originally smog meant a mixture of smoke and fog. Today, smog means air that has restricted visibility due to pollution, or pollution formed in the presence of sunlight-photochemical smog.

SNOpack TELemetry (SNOTEL)⁽⁸⁾
	A near real-time hydrometeorological data collection network in the West that collects SWE, precipitation, and temperature data from nearly 600 remote high-elevation stations.

Snow⁽⁶⁾
	Solid precipitation in the form of minute ice flakes that occur below 0C.

Snow flurries⁽⁶⁾
	Light showers of snow that fall intermittently.

Snow grains⁽⁶⁾
	Precipitation in the form of very small, opaque grains of ice. The solid equivalent of drizzle.

Snow pellets⁽⁶⁾
	White, opaque, approximately round ice particles between 2 and 5 mm in diameter that form in a cloud either from the sticking together of ice crystals or from the process of accretion.

Snow rollers⁽⁶⁾
	A cylindrical spiral of snow shaped somewhat like a child's muff and produced by the wind.

Snow squall (shower)⁽⁶⁾
	An intermittent heavy shower of snow that greatly reduces visibility.

Snow water content (SWC)⁽⁸⁾
	How much liquid water is contained in a volume of solid snow (in other words, how much water would be measured if a known amount of snow was melted). Snow water content and snow water equivalent are different terms for the same parameter.

Snow water equivalent (SWE)⁽⁸⁾
	How much liquid water is contained in a volume of solid snow (in other words, the amount of water measured from melting a known amount of snow). Snow water content and snow water equivalent are different terms for the same parameter.

Snowflake⁽⁶⁾
	An aggregate of ice crystals that falls from a cloud.

Snowpack⁽⁸⁾
	A horizontally layered accumulation of snow from snowfall events, which may be modified by meteorological conditions over time.

Solar constant⁽¹⁾
	Aka total solar irradiance. The constant expressing the amount of solar radiation reaching the Earth from the sun, approximately 1370 watts per square meter. It is not, in fact, truly constant and variations are detectable.

Solar cycle⁽¹⁾
	Eleven-year cycle of sunspots and solar flares that affects other solar indexes such as the solar output of ultraviolet radiation and the solar wind. The Earth's magnetic field, temperature, and ozone levels are affected by this cycle.

Solar radiation⁽¹⁾
	Energy received from the sun is solar radiation. The energy comes in many forms, such as visible light (that which we can see with our eyes). Other forms of radiation include radio waves, heat (infrared), ultraviolet waves, and x-rays. These forms are categorized within the electromagnetic spectrum.

Solar variability⁽²⁾
	changes in the sun's radiation due to the sun's internal dynamics.

Solstice⁽⁶⁾
	Either of the two times of the year when the sun is the greatest distance from the celestial equator, occurring about June 22 and December 22. See winter solstice and summer solstice.

Source⁽⁵⁾
	Any process or activity that results in the net release of greenhouse gases, aerosols, or precursors of greenhouse gases into the atmosphere.

Southern Oscillation (ENSO)⁽⁶⁾
	Shifting of pressure zones in the Pacific during an El Niño event.

Southern Oscillation Index (SOI)⁽³⁾
	SOI is based on the (atmospheric) pressure difference between Tahiti and Darwin, Australia.

Spatial⁽¹⁾
	A characteristic that refers to a location (which may be a specific location on the Earth's surface, or relative to an arbitrary point).

Specific heat⁽⁶⁾
	The ratio of the heat absorbed (or released) by the unit mass of the system to the corresponding temperature rise (or fall).

Specific humidity⁽⁶⁾
	The ratio of the mass of water vapor in a given parcel to the total mass of air in the parcel.

Spontaneous nucleation (freezing)⁽⁶⁾
	The freezing of pure water without the benefit of any nuclei.

Spring freeze date⁽⁶⁾
	The date of occurrence in the spring of the last minimum at or below a temperature threshold.

Squall line⁽⁶⁾
	Any nonfrontal line or band of active thunderstorms.

Station pressure⁽⁶⁾
	The actual air pressure computed at the observing station.

Steam fog⁽⁶⁾
	See Evaporation fog.

Steppe⁽⁶⁾
	One of the two types of dry climate. A marginal and more humid variant of the desert that separates it from bordering humid climates. Steppe also refers to the short-grass vegetation associated with this semiarid climate.

Storm surge⁽⁶⁾
	An abnormal rise of the sea along a shore. Primarily due to the winds of a storm, especially a hurricane.

Stratocumulus⁽⁶⁾
	A low cloud, predominantly stratiform with low, lumpy, rounded masses, often with blue sky between them.

Stratopause⁽⁶⁾
	The boundary between the stratosphere and the mesosphere.

Stratosphere⁽⁶⁾
	The region of the atmosphere extending from the top of the troposphere to the base of the mesosphere, an important area for monitoring stratospheric ozone.

Stratospheric ozone⁽³⁾
	In the stratosphere, ozone has beneficial properties where it forms an ozone shield that prevents dangerous radiation from reaching the Earth's surface. Recently, it was discovered that in certain parts of the world, especially over the poles, stratospheric ozone was disappearing creating an ozone hole.

Stratus⁽⁶⁾
	A low, gray cloud layer with a rather uniform base whose precipitation is most commonly drizzle.

Subarctic climate⁽⁶⁾
	A climate found north of the humid continental climate and south of the polar climate and characterized by bitterly cold winters and short cool summers. Places within this climatic realm experience the highest annual temperature ranges on earth.

Sublimation⁽⁶⁾
	The process whereby ice changes directly into water vapor without melting. In meteorology, sublimation can also mean the transformation of water vapor into ice. (See Deposition.)

Subsidence⁽⁶⁾
	The slow sinking of air, usually associated wit high-pressure areas.

Subsidence inversion⁽⁶⁾
	A temperature inversion produced by the adiabatic warming of a layer of sinking air.

Subtropical⁽³⁾
	A climate zone adjacent to the tropics with warm temperatures and little rainfall. Recent theory suggests that sulfate aerosols may lower the earth's temperature by reflecting away solar radiation (negative radiative forcing). General Circulation Models which incorporate the effects of sulfate aerosols more accurately predict global temperature variations.

Sulfate aerosols⁽¹⁾

Particulate matter that consists of compounds of sulfur formed by the interaction of sulfur dioxide and sulfur trioxide with other compounds in the atmosphere. Sulfate aerosols are injected into the atmosphere from the combustion of fossil fuels and the eruption of volcanoes like Mt. Pinatubo. Recent theory suggests that sulfate aerosols may lower the earth's temperature by reflecting away solar radiation (negative radiative forcing). General Circulation Models which incorporate the effects of sulfate aerosols more accurately predict global temperature variations. See particulate matter, aerosol, General Circulation Models.

Summer solstice⁽⁶⁾
	Approximately June 22 in the Northern Hemisphere when the sun is highest in the sky and directly overhead at latitude 23.5°N, the Tropic of Cancer.

Sun pillar⁽⁶⁾
	A vertical streak of light extending above (or below) the sun. It is produced by the reflection of sunlight of ice crystals.

Sundog⁽⁶⁾
	A colored luminous spot produced by refraction of light through ice crystals that appears on either side of the sun. Also called parhelion.

Supersaturated air⁽⁶⁾
	A condition that occurs in the atmosphere when the relative humidity is greater that 100 percent.

Surface air temperature⁽²⁾
	The temperature of the air near the surface of the Earth, usually determined by a thermometer in an instrument shelter about 2 m above the ground. The true daily mean, obtained from a thermograph, is approximated by the mean of 24 hourly readings and may differ by 1.0 degrees C from the average based on minimum and maximum readings. The global average surface air temperature is 15 degrees C.

Surface inversion⁽⁶⁾
	See Radiation inversion

Synoptic scale⁽⁶⁾
	Used to classify large-scale weather systems more than 200 miles across.

Taiga⁽⁶⁾
	The northern coniferous forest; also a name applied to the subarctic climate.

Teleconnection⁽³⁾
	A strong statistical relationship between weather in different parts of the globe. For example, there appears to be a teleconnection between the tropics and North America during El Niño.

Temperature (TEMP)⁽⁶⁾
	A measure of the energy in a substance. The more heat energy in the substance, the higher the temperature. The Earth receives only one two-billionth of the energy the sun produces. Much of the energy that hits the Earth is reflected back into space. Most of the energy that isn't reflected is absorbed by the Earth's surface. As the surface warms, it also warms the air above it.

Temperature inversion⁽⁶⁾
	An extremely stable air layer in which temperature increases with altitude, the inverse of the usual temperature profile in the troposphere.

Terminal velocity⁽⁶⁾
	The constant speed obtained by a falling object when the upward drag on the object balances the downward force of gravity.

Terrestrial radiation⁽¹⁾
	The total infrared radiation emitted by the Earth and its atmosphere in the temperature range of approximately 200 to 300 Kelvin. Terrestrial radiation provides a major part of the potential energy changes necessary to drive the atmospheric wind system and is responsible for maintaining the surface air temperature within limits of livability.

Theory⁽¹⁾
	An explanation for some phenomenon that is based on observation, experimentation, and reasoning.

Thermal⁽⁶⁾
	Of, making use of, producing, or caused by heat.

Thermal expansion⁽⁵⁾
	Expansion of a substance as a result of the addition of heat. In the context of climate change, thermal expansion of the world's oceans in response to global warming is considered the predominant driver of current and future sea-level rise.

Thermocline⁽³⁾

As one descends from the surface of the ocean, the temperature remains nearly the same as it was at the surface, but at a certain depth temperature starts decreasing rapidly with depth. This boundary is called the thermocline. In studying the tropical Pacific Ocean, the depth of 20°C water ("the 20°C isotherm") is often used as a proxy for the depth of the thermocline. Along the equator, the 20°C isotherm is typically located at about 50m depth in the eastern Pacific, sloping downwards to about 150m in the western Pacific.

Thermograph⁽⁶⁾
	A recording instrument that gives a continuous trace of temperature with time.

Thermohaline Circulation ((THC))⁽²⁾

Density-driven circulation system for the world's oceans. Warm Atlantic water moves northward along the axis of the Gulf Stream, evaporation makes the water more and more dense while releasing heat to the colder atmosphere in the North Atlantic. Once dense enough, the water sinks into the deep ocean, forming a downward limb of a giant conveyor-like circulation that extends around the world's oceans.

Thermometer⁽⁶⁾
	An instrument used to measure temperature.

Thermosphere⁽⁶⁾
	The atmospheric layer above the mesosphere. It extends from 90 km to outer space.

Thunder⁽⁶⁾
	The sound due to rapidly expanding gases along the channel of a lightning discharge.

Tipping bucket rain gage⁽⁶⁾
	A device that accumulates rainfall in increments of 0.01 in. by containers that alternately fill and empty (tip).

Tornado⁽⁶⁾
	An intense, rotating column of air that protrudes from a cumulonimbus cloud in the shape of a funnel or a rope and touches the ground. (See Funnel cloud.)

Tradewinds⁽⁶⁾
	A system of low-level winds occurring in the tropics; the tradewinds blow from the northeast to the equator in the Northern Hemisphere and from the southeast to the equator in the Southern Hemisphere.

Transpiration⁽⁶⁾
	The release of water vapor to the atmosphere by plants.

Tropical air mass⁽⁶⁾
	A warm-to-hot air mass that forms in the subtropics.

Tropical depression⁽⁶⁾
	A mass of thunderstorms and clouds generally with a cyclonic wind circulation of between 20 and 34 knots

Tropical disturbance⁽⁶⁾
	An organized mass of thunderstorms with a slight cyclonic wind circulation of less than 20 knots.

Tropical storm⁽⁶⁾
	Organized thunderstorms with a cyclonic wind circulation between 35 and 64 knots.

Tropopause⁽⁶⁾
	The boundary between the troposphere and the stratosphere.

Troposphere⁽³⁾
	The lowest portion of the atmosphere which lies next to the earth's surface where most weather occurs. The absorption of UV by stratospheric ozone and atmospheric oxygen prevents very little ultraviolet radiation to reach earth's surfaces where it cans detrimental effects on human health and property.

Tundra Climate⁽⁶⁾
	Found almost exclusively in the northern hemisphere or at high altitudes in many mountainous regions. A treeless climatic realm of sedges, grasses, mosses, and lichens that is dominated by a lng, bitterly cold winter.

Turbulence⁽⁶⁾
	Any irregular or disturbed flow in the atmosphere that produces gusts and eddies.

Twilight⁽⁶⁾
	The time immediately before sunrise and after sunset when the sky remain illuminated.

Typhoon⁽⁶⁾
	A hurricane that forms in the western Pacific Ocean.

Ultraviolet (UV)⁽³⁾

Ultraviolet radiation from the sun plays a role in the formation of the ozone layer by acting as a catalyst for a chemical reaction that breaks apart oxygen molecules which then recombine to form ozone. The absorption of UV by stratospheric ozone and atmospheric oxygen prevents very little ultraviolet radiation to reach earth's surfaces where it cans detrimental effects on human health and property.

Uncertainty⁽⁵⁾
	Uncertainty is a prominent feature of the benefits and costs of climate change. Decision makers need to compare risk of premature or unnecessary actions with risk of failing to take actions that subsequently prove to be warranted. This is complicated by potential irreversibilities in climate impacts and long term investments.

Upper-level⁽³⁾
	In weather observing, the term applies to the portion of the atmosphere that is above the lower troposphere, generally 850 hPa and above.

Upslope fog⁽⁶⁾
	Fog formed as moist, stable air flows upward over a topographic barrier.

Upslope precipitation⁽⁶⁾
	Precipitation that forms due to moist, stable air gradually rising along an elevated plain. Upslope precipitation is common over the western Great Plains, especially east of the Rock Mountains.

Upwelling⁽³⁾
	In ocean dynamics, the upward motion of sub-surface water toward the surface of the ocean. This is often a source of cold, nutrient-rich water. Strong upwelling occurs along the equator where easterly winds are present. Upwelling also can occur along coastlines, and is important to fisheries and birds in California and Peru.

Urban heat island⁽⁶⁾
	The increased air temperatures in urban areas as contrasted to the cooler surrounding rural areas.

UTC⁽³⁾
	Universal Time Coordinated is the same as Greenwich Mean Time.

Validation⁽⁶⁾
	Comparing a climate model's predictions with observations of the real climate, in order to test the reliability and accuracy of the model. The most obvious way to test a climate model is to use it to analyze past events, and then see whether its simulated prediction 'came true,' or how close it was to being correct.

Vapor pressure⁽⁶⁾
	The pressure exerted by the water vapor molecules in a given volume of air.

Vernal equinox⁽⁶⁾
	The equinox at which the sun approaches the Northern Hemisphere and passes directly over the equator. Occurs around March 20.

Virga⁽⁶⁾
	Precipitation that falls from a cloud but evaporates before reaching the ground. (See Fall streaks.)

Virtual temperature⁽⁶⁾
	An adjustment applied to the real air temperature to account for a reduction in air density due to the presence of water vapor.

Viscosity⁽⁶⁾
	The resistance of fluid flow.

Visibility⁽⁶⁾
	The greatest distance an observer can see and identify prominent objects.

Visible light⁽⁶⁾
	That portion of the electromagnetic spectrum from 0.4 to 0.7 m wavelengths that is visible.

Vorticity⁽⁶⁾
	A measure of the spin of a fluid, usually small air parcels. Absolute vorticity is the combined vorticity due to the earth's rotation and the vorticity due to the air's circulation relative to the earth. Relative vorticity is due to the curving of the air flow and wind shear.

Warm front⁽⁶⁾
	The leading edge of a warm air mass.

Water balance⁽⁶⁾
	The comparison of actual and potential evapotranspiration with the amount of precipitation, usually on a monthly basis.

Water budget⁽⁶⁾
	Balance sheet for the inputs and outputs of water to and from the various global water reservoirs.

Water Cycle⁽¹⁾
	The process by which water is transpired and evaporated from the land and water, condensed in the clouds, and precipitated out onto the earth once again to replenish the water in the bodies of water on the earth.

Water equivalent⁽⁶⁾
	The depth of water that would result from the melting of a snow sample. Typically about 10 inches of snow will melt to 1 inch of water, producing a water equivalent of 10 to 1.

Water Vapor (H₂⁽⁵⁾
	Water vapor is the primary gas responsible for the greenhouse effect. It is believed that increases in temperature caused by anthropogenic emissions of greenhouse gases will increase the amount of water vapor in the atmosphere, resulting in additional warming (see "positive feedback").

Weather⁽⁶⁾
	Describes the short-term (i.e., hourly and daily) state of the atmosphere. Weather is not the same as climate. ","w

Weighing bucket rain gage⁽⁶⁾
	A device that is calibrated so that the weight of rainfall is recorded directly in terms of rainfall in millimeters or in inches.

Wet-bulb depression⁽⁶⁾
	The difference in degrees between the air temperature (dry-bulb temperature) and the wet-bulb temperature.

Wet-bulb temperature⁽⁶⁾
	The lowest temperature that can be obtained by evaporating water into the air.

White frost⁽⁶⁾
	Ice crystals that form on surfaces instead of dew when the dew point is below freezing.

Wind Chill⁽³⁾
	The portion of the cooling of the human body caused by air motion. Wind chill becomes important for human health as air motion accelerates the rate of heat loss from a human body, especially when temperatures are below 45°F.

Wind chill equivalent temperature⁽⁶⁾
	A theoretical air temperature at which the heat loss from exposed skin under calm conditions is equivalent to the heat loss at the actual air temperature and under the actual wind speeds.

Wind Chill Index⁽³⁾
	A means of quantifying the threat of heat loss from the human body during windy and cold conditions.

Wind shear⁽⁶⁾
	A difference in wind speed or direction between two wind currents in the atmosphere.

Wind Vane⁽⁶⁾
	An instrument used to determine wind direction.

Wind-chill factor⁽⁶⁾
	The cooling effect of any combination of temperature and wind, expressed as the loss of body heat. Also called wind-chill index.

Windsock⁽⁶⁾
	A large, conical, open bag designed to indicate wind direction and relative speed; usually used at small airports.

Winter solstice⁽⁶⁾
	Approximately December 22 in the Northern Hemisphere when the sun is lowest in the sky and directly overhead at latitude 23.5°S, the Tropic of Capricorn.

Z=Zulu⁽³⁾
	See GMT or Greenwich Mean Time.