Chapter 4
Atmospheric Moisture and
Precipitation
Overall, this chapter considers the types and sources of moisture in the atmosphere.
While examining the mechanism of condensation of atmospheric moisture into
precipitation, the process of adiabatic cooling and heating during changes in air mass
elevation in the atmosphere is thoroughly examined. Additionally, this chapter explores
the effects of human activities on atmospheric air quality.
 Water exists in the atmosphere as water vapor, clouds, fog, and precipitation.
 The physical movement of water between the land, the oceans, and the atmosphere is
called the hydrologic cycle.
 Humidity refers to the amount of water vapor in the air.
 The amount of water the air can hold is dependant upon air temperature. Warm air
can hold more moisture than cold air.
 Specific humidity is the actual mass of water vapor per mass of air, usually stated in
grams of water vapor per kilogram of air. It is a measure of the amount of water vapor
that can be extracted from the atmosphere as precipitation.
 Relative humidity is a measure of the amount of water vapor in the air expressed as a
percentage of the amount of water vapor the air can hold given its present temperature.
 The dew point temperature is the temperature at which relative humidity would be
100%. Condensation will occur if the temperature is lowered resulting in dew or frost.
 Precipitation results when a large mass of air is lifted and cooled to a temperature
below its dew point.
 The adiabatic process causes heating or cooling solely by pressure change: air that
rises, expands, and cools as pressure decreases with altitude or air that descends,
encounters higher pressures, is compressed, and subsequently warmed.
 A parcel of air cooling without condensation occurring cools at the dry adiabatic
lapse rate of 10° C per 1000 meters (5.5° F per 1000 feet).
 Once air has cooled to its dew point temperature, condensation releases latent heat,
slowing the rate of cooling to the wet adiabatic lapse rate which, on average, varies
between 4° and 9° C per 1000 meters (2.2° and 4.9° F per 1000 feet) depending on the
temperature and pressure of the air and its moisture content.
 A cloud is made up of water droplets or ice crystals formed around tiny particles of
matter called condensation nuclei.
 Clouds are classified on the basis of height and form.
 Clouds at ground level are called fog. Radiation fog forms when the temperature of the
air near the ground falls below the dew point. Advection fog occurs when warm moist
air is cooled below its dew point temperature as it moves over a cold surface.
 Precipitation forms when either cloud droplets or ice crystals increase in size by
colliding with each other until they are heavy enough to descend through the
atmosphere.
 Precipitation that occurs as a result of air being forced over a topographic barrier is
called orographic precipitation. Air that rises because it is warmer than the air around
it produces convectional precipitation, and air that is forced to rise over another air
mass produces cyclonic precipitation.
 Thunderstorms are intense convectional storms associated with massive
cumulonimbus clouds. They may produce heavy rains, hail, thunder, lightening, and
intense downdrafts (microbursts) which may create hazards for humans.
 Air pollutants are undesirable gases, aerosols, and particulates injected into the
atmosphere by human and natural causes.
 The most important human source of pollutants is the combustion of fossil fuels for
the production of energy for transportation, heating, and industrial processes.
 Urban air pollution produces smog and haze which reduce visibility and
illumination. Urban areas also experience more fog, cloudiness, and precipitation than
adjacent rural areas.
 Acid deposition refers to acid rain and acidic dust particles produced by emissions of
sulfur dioxide and nitric oxide. Acid deposition is very damaging to natural
ecosystems.