What is atmospheric humidity and how is it measured?
Humidity is the amount of water vapor in the atmosphere. The main
sources of water vapor in the lower atmosphere are evaporation from
the Earth's surface and transpiration by plants. In the stratosphere,
the breakdown of methane by sunlight is another source. The main sink
is precipitation.
Atmospheric water vapor accounts for only about 1/10,000th of the
total amount of water in the global hydrological cycle. The total
volume of water in the atmosphere is about 1.3 x 1013 m3, the
overwhelming majority of which is in the vapor phase. For comparison,
the oceans contain about 1.35 x 1018 m3 of water. (The ratio of
global atmospheric and oceanic water volumes is approximately the same
as the ratio of the volume of water that can be held in a thimble to
that in a bath tub.) Nevertheless, atmospheric water vapor is one of
the most important factors in determining Earth's weather and climate,
because of its role as a greenhouse gas and because of the large
amounts of energy involved as water changes between the gaseous
(vapor) phase and liquid and solid phases.
Definitions of atmospheric humidity
Meteorologists have defined several different measures of humidity.
These can be divided into two categories: those that describe the
actual amount, or concentration, of water vapor in the air and those
that relate the actual amount to the potential amount that the air
could hold if it were saturated with respect to water vapor. Air is
saturated when it holds the maximum possible amount of water vapor.
At that point, the rate at which water molecules enter the air by
evaporation exactly balances the rate at which they leave by
condensation.
Definitions of water vapor concentration
Specific humidity is the concentration, by mass, of water vapor in
a sample of moist air. It is, therefore, the ratio of the mass of
water vapor in a sample to the total mass of the sample moist air,
including both the dry air and the water vapor. The mixing ratio is
the ratio of the mass of water vapor to the mass of dry air in the
sample. As ratios of masses, both specific humidity and mixing ratio
are dimensionless numbers. However, because atmospheric
concentrations of water vapor tend to be less than a few percent at
the surface, and much lower at higher elevations, they are both often
expressed in units of grams of water vapor per kilogram of (moist or
dry) air. Absolute humidity is the water vapor density, defined as
the ratio of the mass of water vapor to the volume of associated moist
air and generally expressed in grams per cubic meter.
Definitions of humidity referenced to saturation
The partial pressure of water vapor in saturated air is called the
saturation vapor pressure. The higher the air temperature, the
greater the saturation vapor pressure, as expressed by the
Clausius-Clapeyron relation, which gives the saturation vapor pressure
over a plane surface of pure water. Saturation vapor pressure
increases rapidly with temperature; the value at 90oF (32oC) is about
double the value at 70oF (21oC). The saturation vapor pressure over a
curved surface, such as a droplet, is greater than that over a plane.
And the saturation vapor pressure over pure water is greater than that
over water with a dissolved solute.
We define relative humidity as the ratio of the actual vapor
pressure to the saturation vapor pressure at the air temperature, and
the result is expressed as a percentage. Because of the temperature
dependence of the saturation vapor pressure, for a given value of
relative humidity, warm air has more water vapor than cooler air.
When air is cooled, at constant pressure and water vapor content, to
the saturation (or condensation) point, the resulting air temperature
is the dew point temperature. The difference between the actual
temperature and the dew point is called the dew point depression.
A final humidity definition is the wet-bulb temperature. The
principle behind the wet-bulb temperature is illustrated by the
operation of a psychrometer, a simple, and widely used, instrument for
measuring humidity. This is a pair of thermometers, one of which has
a wetted piece of cotton on the bulb, and so is called the wet bulb.
The other is called the dry bulb. As water is evaporated from the
cotton wick, the air near the wet bulb is cooled by the transfer of
heat required to evaporate water from the wet bulb from the air. The
resulting wet-bulb temperature is the lowest temperature that can be
achieved by evaporation. The evaporation rate depends on ambient
humidity. A sling psychrometeris a hand-operated instrument, and
evaporation is effected by swinging the two thermometers in the air.
The wet-bulb and dry- bulb temperature readings together uniquely
determine the humidity. At saturation, the wet-bulb, dry-bulb, and
dew point temperatures are all equal. Otherwise the dew point
temperature is less than the wet-bulb, which is less than the dry
bulb.
Variations in humidity
In the absence of significant changes in weather, the variations in
humidity over the course of the day can be easily understood. The
specific humidity remains relatively constant from day to night, and
so, therefore, does the dew point temperature. However, as the
temperature cools at night, the saturation vapor pressure decreases
and so relative humidity increases. The highest values usually occur
at the time of lowest temperature, near dawn.
Seasonal humidity variations depend on two factors: seasonal
changes in temperature, and seasonal changes in atmospheric
circulation. In temperate climates, the warmer air in summer has a
higher saturation vapor pressure than the cooler winter air. Average
summer and winter relative humidity values tend not to differ much, so
the specific humidity tends to be much larger in summer. In monsoon
climates, relative and specific humidity tend to increase dramatically
during the monsoon season, when onshore winds carry moisture-laden air
from the ocean to land areas.
Evaporation and condensation
Humidity in the atmosphere is constantly changing. Water vapor
enters and leaves the atmosphere at the surface of the Earth, but it
can also change phase to or from the liquid state (e.g., dew, rain,
fog, cloud) or solid states (e.g., snow, ice, sleet, graupel) while in
the atmosphere. The fluxes of water to the atmosphere from the
Earth's surface and to the surface from the atmosphere (by
precipitation) are controlled, in part, by atmospheric humidity.
Evaporation from open water surfaces, like the ocean or lakes, can be
simply estimated as proportional to the difference between the
saturation vapor pressure at the water surface and the actual vapor
pressure above the surface, although the higher the wind speed and the
greater the turbulent mixing of the atmosphere, the greater the
evaporation. On land, the availability of water may be limited, and
the type of land surface and vegetation will influence evaporation and
evapotranspiration rates. When a cold surface underlies a warm, moist
atmosphere, the atmospheric vapor pressure may exceed the surface
saturation vapor pressure and condensation to the surface can occur in
the form of dew or rime ice.
When relative humidity is near or exceeds 100%, atmospheric water
vapor is likely to condense on nuclei to form cloud droplets or ice
crystals, depending on the ambient temperature. The relative humidity
required to sustain droplet growth depends on the characteristics of
the droplet. If the humidity is not high enough, droplets will begin
to evaporate.
If cloud droplets grow to drop size, and if the drops become
sufficiently large, they fall as rain. Once outside the cloud, in air
of lower relative humidity, drops are likely to evaporate. As they
are evaporating, falling drops are cooled and their temperature is
close to the wet-bulb temperature. The rate of evaporation from
falling drops depends on the drop size, fall speed, updraft speeds and
ambient humidity. Thus there is a cycling of water among the vapor,
liquid and solid phases in the atmosphere.
Humidity and human comfort
As anyone who has spent a summer in the southeastern United States
can attest, high humidity can aggravate the effects of high
temperature on human comfort. People may complain that humid air
feels "heavy", but in fact, the more moisture in the air, the lower
the air density. That is because the molecular weight of water vapor
is lower the average molecular weight of the constituents dry air.
The discomfort associated with high humidity is somewhat analogous
to the wind chill effect, where high winds make people feel colder.
Various indices, comparable to the wind chill index, have been
developed to quantify the humidity effect; these include the apparent
temperature, heat stress index, "humiture", and "humidex". Some of
these can be adjusted to take into account the effects on solar
radiation, wind speed, and barometric pressure on human comfort.
The U.S. National Weather Service currently employs the "heat
index." As relative humidity increases, so does human discomfort.
For example, at an air temperature of 90oF (32oC) and 50% relative
humidity, the air "feels" as if it were 96oF (36oC). The reason is
that the moister the air, the larger the resistance to moisture loss
(and therefore to heat loss via evaporation) from the human body to
the air, because the air is closer to saturation. The humidity effect
on comfort operates at low temperatures as well: people are more
comfortable in cold air when humidity is high than low.
At very high temperatures, air is rarely if ever close to
saturation because the saturation vapor pressure is very large. Thus
there are blank entries in the Table for high temperature and high
relative humidity combinations. In the United States, the highest dew
point temperatures to persist for at least 12 hours are in the upper
70'soF, which, combined with a temperature of 90oF, corresponds with
relative humidities about 70%. Only where both air and nearby water
surfaces become very warm (such as in the Persian Gulf or the Red Sea,
or immediately following a rain shower that saturates the soil on a
hot summer day) are higher temperatures and relative humidities seen.
Humidity and plants
Plants also respond to changes in humidity. Transpiration of water
vapor through leaf stomata depends on the gradient of moisture between
the leaf interior (which is saturated) and the overlying air, as well
as the availability of moisture in the soil. The lower the
atmospheric humidity, the greater the transpiration rate. The
transpiration rate is determined by a balance between the amount of
energy available to convert water from the liquid to vapor phase and
the moisture gradient.
Transpiration rates also depend on the resistance to water movement
between the leaf and the air. By analogy with the flow of electricity
through a circuit with elements of different electrical resistance,
the flow of water is modeled by considering the resistance of the leaf
stomata, the leaf cuticle, and the air in the boundary layer adjacent
to the leaf. Extension of this leaf model to crop fields or to
natural plant ecosystems can be problematic, and more sophisticated
micrometeorological methods are used.
Bibliography
Byers, H.R., Elements of Cloud Physics, Univ. of Chicago Press,
Chicago, 1965.
Quayle, R., and F. Doering, "Heat Stress: A Comparison of Indices."
Weatherwise (June 1981): 120-124.
Rosenberg, N.J., Microclimate: The Biological Environment, John Wiley
and Sons, New York, 1974.
United Kingdom Meteorological Office, Handbook of Meteorological
Instruments: Vol. 3, Measurement of Humidity, Met.O. 919c, Her
Majesty's Stationery Office, London, 1981.
Dian Seidel
