Chapter 4

Atmospheric Moisture and Precipitation

This chapter considers the types and sources of moisture in the atmosphere. It examines in detail the mechanisms by which atmospheric moisture becomes precipitation: in particular, the important process of adiabatic cooling that occurs when air moves upward in the atmosphere. This chapter also discusses the effect that human activities can have on air quality.

  • Water exists in the atmosphere as water vapor, clouds, fog, and precipitation.
  • The 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 depends on 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.
  • The dew point temperature is the temperature at which relative humidity would be 100%. Condensation will occur if the temperature falls producing dew or frost.
  • 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.
  • 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 warms.
  • A parcel of air cooling without condensation 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, condensation releases latent heat, slowing the rate of cooling to the wet adiabatic lapse rate which 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 formed on 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 dew point 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 fall.
  • 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.