sea level pressure of 1,013.2 mb, this means vapor pressure is, at best, a small frac-
tion of atmospheric pressure.
Dewpoint temperature is another measure of humidity. The dewpoint is the tem-
perature to which air must cool to bring it to saturation (see below). Dewpoint tem-
perature is an indirect measure of the amount of water vapor and latent heat
content. If dewpoint temperature and air temperature are the same, the air is satu-
rated. If not, the dewpoint will be lower than the air temperature and the air has
some additional capacity with which to store water vapor. The greater the spread
between air temperature and dewpoint temperature, the drier is the air; the farther
the air is from saturation, the faster evaporation will take place. Dewpoints up to
about 15°C are perceived as comfortable by most people, while dewpoints in
excess of 20°C cause discomfort because of slowness of evaporation of perspiration.
The U.S. National Oceanic and Atmospheric Administration calculates apparent
temperature, which quantifies human discomfort based on the combination
between air temperature and the relative humidity (see below). The mixing ratio
is measured in grams of water vapor per kilogram of air not including the water
vapor. This measure is frequently used in scientific work because it does not vary
in rising or sinking air. The mixing ratio varies from almost 0 in polar winters to
almost 20 over tropical oceans.
Relative humidity is the humidity measure most people hear about. It is the
percentage relationship between the actual amount of water vapor in the air and
the amount of water vapor possible in the airat that temperature.Itrangesupto
100 percent (saturation) and can represent varying amounts of moisture because
vapor capacity changes with temperatures. Two factors can change relative humidity:
amount of moisture in the atmosphere and temperature change. Of these two factors,
the atmosphere uses temperature decreases as the most ready way to bring itself to
saturation. Relative humidity is affected by the diurnal heating cycle. Under good
weather conditions, relative humidity is highest near dawn and lowest in the mid
afternoon; this is because relative humidity is inversely related to temperature.
Unlike the lower atmosphere’s gaseous mainstays of nitrogen and oxygen,
water vapor presents hugely complicated patterns over the planet over time and
place. Over 99 percent of humidity is in the atmosphere’s first layer, the tropo-
sphere. The other layers are very dry and offer little in the way of cloudiness or
precipitation. On the average, the water vapor content of the sky is greatest at
sea level. Thus, the tops of very tall mountains such as Everest (8,848 m) and
Denali (6,194 m) have vapor pressures substantially lower than near the surface.
The bottom of theatmospherehas the richest supply of water vapor because
vapor of the troposphere is from the evaporation, sublimation, and transpiration
from land and ocean surfaces. Geographically, vapor is most plentiful over tropical
oceans and least plentiful over polar land and ice surfaces. There is a huge daily176 Humidity