Precipitation forms when cloud droplets or ice particles in clouds grow and combine to become so large that the updrafts in the clouds can no longer support them, and they fall to the ground.

A thunderstorm is formed when a combination of moisture and warm air rise in the atmosphere and condense. While over land, thunderstorms are most likely to occur at the warmest, most humid part of the day, which is usually the afternoon or evening. Over the ocean they are most likely to occur in the early hours of the morning before dawn.

Thunderstorms form when an air mass becomes unstable (when air in the lowest layers is very warm and humid, or air in the upper layers is unusually cold, or if both occur). Rising near-surface air in an unstable air mass expands and cools, making it warmer than its environment, which causes it to rise even farther. If enough water vapor is present, some of this vapor condenses into a cloud, releasing heat, which makes the air parcel even warmer, forcing it to rise yet again.  Water vapor fuels the storm.

A tropical depression forms when a low pressure area is accompanied by thunderstorms that produce a circular wind flow with maximum sustained winds below 39 mph. An upgrade to a tropical storm occurs when cyclonic circulation becomes more organized and maximum sustained winds gust between 39 mph and 73 mph.

As rising water vapor condenses and latent heat is released, surrounding air is warmed and made less dense, causing the air to rise.  The thunderstorms that make up the hurricane’s core are strengthened by this process. As air rises within the storms, pressure at the surface decreases and moister, tropical air is drawn to the center of the circulation, providing even more water vapor to fuel the hurricane.  A hurricane has sustained wind gusts of at least 74 mph.

They are different names for the same type of storm, collectively known as tropical cyclones.

What they’re called is determined by where they form.  In the Atlantic Basin and east of the International Date Line in the Pacific Ocean, they’re called hurricanes.  Typhoons form in the North Pacific Ocean, west of the date line.  The storms are called cyclones in the Indian Ocean and in the Coral Sea off northeastern Australia.

Availability of water vapor and intensity of updrafts within a cloud determine the size of a raindrop. Larger drops tend to result from the vigorous updrafts within a thunderstorm and fall faster than smaller drops. Mist or drizzle produce smaller drops that fall at lower speeds.

Most hurricanes begin in the Atlantic as a result of tropical waves that move westward off the African coast.

Hail forms when thunderstorm updrafts are strong enough to carry water droplets well above the freezing level. This freezing process forms a hailstone, which can grow as additional water freezes onto it. Eventually, the hailstone becomes too heavy for the updrafts to support it and it falls to the ground.

The tropics receive a great amount of direct solar energy, which produces more evaporation than higher latitudes. The warm, moist air rises, condenses into clouds and thunderstorms, and falls back to earth as precipitation. More evaporation results in more precipitation.

The forecast of a hurricane's path is dependent upon the accuracy of the predicted winds from computer forecast models. The speed and direction of steering winds generally vary with altitude. Weak tropical cyclones tend to be steered more by lower-level winds, while upper-level winds usually influence the paths of stronger hurricanes.

Temperatures in the US are colder than they are near the equator, so air pressure is lower than it is in the tropics. Because wind flows counterclockwise around low pressure, winds usually blow from west to east, pushing weather systems to the east.

This important question is still under investigation. Much of the rain is produced by clouds whose tops do not extend to temperatures colder than 0° C. The mechanism responsible for rain formation in these "warm" clouds is merging or "coalescence" among cloud droplets, which are first formed by vapor condensation. Coalescence is probably the dominant rain-forming mechanism in the tropics. It is also effective in some mid-latitude clouds whose tops may extend to subfreezing temperatures. However, once a cloud extends to altitudes where the temperature is colder than 0° C, ice crystals can form and "ice-phase" processes become important. In favorable conditions, ice-involving processes can initiate precipitation in half the amount of time water-only processes would need. Hence, at mid-latitudes, cumulus cloud rain is probably initiated by ice-processes and melting of ice. Observations have shown, however, that precipitation can first appear at levels warmer that 0° C, where vapor condensation and coalescence are the main rain producers. Thus, precipitation may be initiated by either process.

Depending on their type, clouds can consist of dry air mixed with liquid water drops, ice particles, or both. Low, shallow clouds are mostly made of water droplets of various sizes. Thin, upper level clouds (cirrus) are made of tiny ice particles. Deep thunderstorm clouds which can reach up to 20 km in height contain both liquid and ice in the form of cloud and raindrops, cloud ice, snow, graupel and hail.

It is important to understand that even a cloud that looks impenetrably dark is almost entirely made of dry air. Water vapor and precipitation each make up a maximum of just a couple of percent of the mass of a cloud, except in a few very intense storms.

How do these precipitation particles form? First, tiny cloud droplets are born when the water vapor in the air is cooled and starts to condense around tiny "condensation nuclei" (particles so small they are invisible to the naked eye). The presence of these aerosols is crucial: without them, in absolutely clean air, condensation would not start until the relative humidity has reached several hundred percent (this suggests that the "saturation" level of 100% humidity is poorly defined; in fact, the atmosphere always contains more than enough nuclei of all sorts for condensation to start as soon as the dew point temperature is reached). The more particles there are in the atmosphere, the easier cloud droplets will be formed and the smaller they will be (since more particles will be competing for the same amount of water, so each one of them will attract less). This is why clouds over land have more droplets of smaller sizes than clouds over oceans where the air is generally much cleaner.

The process of ice formation similarly requires the presence of nuclei. However, there are much fewer particles which make suitable ice nuclei. This is why freezing often does not start until the temperature of the air reaches -15° C (if there are no ice nuclei at all, freezing will not occur before the temperature drops to -40° C). Hence, clouds with temperatures below 0° C can still consist of water droplets called "supercooled" water. These drops freeze immediately upon contact with any surface. When they fall to the ground as freezing rain, they can form a thin layer of sleet on roadways, an almost invisible and very dangerous hazard for drivers.

Precipitation is measured from space using a combination of active and passive remote-sensing techniques, improving the spatial and temporal coverage of global precipitation observations.

Reliable ground-based precipitation measurements are difficult to obtain because most of the world is covered by water and many countries don’t have precise rain measuring equipment (i.e., rain gauges and radar).  Precipitation is also difficult to measure because precipitation systems can be somewhat random and evolve very rapidly.  During a storm, precipitation amounts can vary greatly over a very small area and over a short time span.

Visible and infrared space-borne sensors can provide precipitation information inferred from cloud-top radiation, and microwave sensors provide direct precipitation measurement based on radiative signatures of precipitating particles.  This type of information is not available through ground-based measuring systems. GPM will advance space-based measurement even further by combining active and passive sensing capabilities.

The Tropical Rainfall Measuring Mission (TRMM) is a joint mission between NASA and the National Space Development Agency of Japan. TRMM primarily measures tropical and subtropical rainfall and is the only current satellite that carries weather radar.

Rising temperatures will intensify the Earth’s water cycle, increasing evaporation.  Increased evaporation will result in more storms, but also contribute to drying over some land areas. As a result, storm-affected areas are likely to experience increases in precipitation and increased risk of flooding, while areas located far away from storm tracks are likely to experience less precipitation and increased risk of drought.

A flash flood is a rapid rise of water along a stream or low-lying urban area. Flash flooding occurs within six hours of a significant rain event and is usually caused by intense storms that produce heavy rainfall in a short amount of time.

Densely populated areas are at a high risk for flash floods. Buildings, highways, driveways, and parking lots increase runoff by reducing the amount of rain absorbed by the ground. This runoff increases potential for a flash flood.

A landslide is the movement of rock, debris, or earth down a slope.

A drought is a period of unusually persistent dry weather that continues long enough to cause serious problems such as crop damage and/or water supply shortages.

Droughts are caused by low precipitation over an extended period of time.  Atmospheric conditions such as climate change, ocean temperatures, changes in the jet stream, and changes in the local landscape are all factors that contribute to drought.

The water cycle (or hydrologic cycle) is the path that water follows as it evaporates into the air, condenses into clouds, and returns to Earth as rain, snow, sleet, or hail.

Water molecules are heated by the sun and turn into water vapor that rises into the air through a process called evaporation.  Next, the water vapor cools and forms clouds, through condensation.  Over time, the clouds become heavy because those cooled water particles have turned into water droplets. When the clouds become extremely heavy with water droplets, the water falls back to earth through precipitation (rain, snow, sleet, hail, etc).  The process continues in a cyclical manner.

The water cycle is extremely important process because it ensures the availability of water for all living organisms and regulates weather patterns on our planet.   If water didn’t naturally recycle itself, we would run out of clean water, which is essential to life.

Tornadoes and hurricanes appear to be similar in their general structure. Both are characterized by extremely strong horizontal winds swirling around the center, strong upward motion dominating the circulation with some downward motion in the center. The tangential winds far exceed the radial inflow or the vertical motion, and can cause much damage. Hurricanes always rotate counterclockwise in the northern hemisphere (clockwise in the southern), the direction of their rotation being determined by the Earth's rotation. This is almost always true of tornadoes too, although on rare occasions "anticyclonic" tornadoes spinning in the opposite direction do occur (tornadic circulation is determined by the local winds). This is where the similarities end.

The most obvious difference between tornadoes and hurricanes is that they have drastically different scales. They form under different circumstances and have different impacts on the environment. Tornadoes are "small-scale circulations", the largest observed horizontal dimensions in the most severe cases being on the order of 1 to 1.5 miles. They most often form in association with severe thunderstorms which develop in the high wind-shear environment of the Central Plains during spring and early summer, when the large-scale wind flow provides favorable conditions for the sometimes violent clash between the moist warm air from the Gulf of Mexico with the cold dry continental air coming from the northwest. However, tornadoes can form in many different circumstances and places around the globe. Hurricane landfalls are often accompanied by multiple tornadoes. While tornadoes can cause much havoc on the ground (tornadic wind speeds have been estimated at 100 to more than 300 mph), they have very short lifetimes (on the order of minutes), and travel short distances. They have very little impact on the evolution of the surrounding storm, and basically do not affect the large-scale environment at all. Hurricanes, on the other hand, are large-scale circulations with horizontal dimensions from 60 to well over 1000 miles in diameter. They form at low latitudes, generally between 5 and 20 degrees, but never right at the equator. They always form over the warm waters of the tropical oceans (sea-surface temperatures must be above 26.5° C, or about 76° F) where they draw their energy. They travel thousands of miles, persist over several days, and, during their lifetime, transport significant amounts of heat from the surface to the high altitudes of the tropical atmosphere. While their sporadic occurrence prevents them from drastically impacting the large-scale circulation, they still affect it in ways which must be accounted for and need to be better understood.

Global Precipitation Measurement (GPM) is an international satellite mission to unify and advance precipitation measurements from space for scientific discovery and societal applications.

GPM will be able to measure precipitation globally; over the land and ocean, in both the tropics, mid-latitudes, and cold locations near the poles. GPM will be able to measure both light and heavy precipitation including the microphysical properties of precipitation particles. This wide range of locations and precipitation types presents a host of challenges not encountered by TRMM, which only measures moderate to heavy rainfall in the tropics.

Currently, The Tropical Rainfall Measuring Mission (TRMM) measures heavy to moderate rain over tropical and subtropical oceans.  GPM will provide advanced measurements, including coverage over medium to high latitudes, improved estimates of light rain and snowfall, advanced estimates over land and ocean, and coordination of radar and microwave retrievals to unify and refine precipitation estimates from a constellation of research and operational satellites.  GPM will also provide more frequent observations, every 2 to 4 hours.

GPM data will primarily be used by operational forecasters, but the information will also benefit numerical weather prediction models, climate prediction patterns, crop monitoring, and other research applications. In addition, scientists will use GPM data to advance our understanding of precipitation and its role in the Earth's environment.

The increased sensitivity of the Dual-frequency Precipitation Radar (DPR) and the high-frequency channels on the GPM Microwave Imager (GMI) will enable GPM to improve forecasting by estimating light rain and falling snow outside the tropics, even in the winter seasons, over land, which current satellites are unable to measure.  These advanced measurements will extend current capabilities in monitoring and predicting hurricanes and other extreme weather events, as well as improved forecasting for floods, landslides, and droughts.

The GPM Core Observatory is scheduled for launch in 2014. It will be delivered to orbit by an H-IIA rocket provided by JAXA.

GPM will advance precipitation measurement capability from space using a combination of active and passive remote-sensing techniques.  These measurements will be used to calibrate, unify, and improve precipitation measurements by a constellation of research and operational microwave sensors.

The primary GPM instruments are the Dual-frequency Precipitation Radar (DPR) and GPM Microwave Imager (GMI).  The DPR will be able to make detailed measurements of rainfall, and rain rates, while the GMI will use a set of optimized frequencies to retrieve heavy, moderate, and light precipitation measurements.

Learn more about the GPM instruments.

GV activity is designed to support pre-launch algorithm development and post-launch product evaluation.   There will be a series of pre- and post-launch field campaigns to carry out validation activities.

GPM can help numerical models predict some aspects of chemical/biological/nuclear (C/B/N) agent dispersal and assess removal of these agents from the air by rainfall.

Accurate estimates of precipitation amount are important for crop growth assessments and timing of precipitation is critical for assessing crop productivity.

GPM will provide precipitation information that the health community will use to identify weather and climate patterns associated with disease outbreaks and provide advanced warning of outbreaks.

GPM can improve climate prediction through better understanding of surface water fluxes, soil moisture storage, cloud/precipitation microphysics, and latent heat release in the Earth’s atmosphere.

By providing four-dimensional measurements of space-time variability of global precipitation, GPM will allow for better understanding of precipitation systems, water cycle variability, and freshwater availability.

By providing more accurate and timely precipitation observations, GPM will lead to improved weather forecasting, allowing for more accurate monitoring of transportation hazards and timelier travel advisories.