OVERVIEW

This chapter examines the types and sources of moisture in the atmosphere and how that moisture becomes precipitation.

• 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.
• 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 falls producing dew or frost.
• 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.
• Precipitation results when a large mass of air is lifted and cooled to a temperature below its dew point.
• Adiabatic processes cause heating or cooling solely by pressure change. Air that rises expands and cools as pressure decreases with altitude. Air that descends encounters higher pressures and is compressed and warms.
• A parcel of air cooling without condensation cools at the dry adiabatic lapse rate of 10° C per 1000 m (5.5° F per 1000 ft.).
• 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 m.(2.2° and 4.9° F per 1000 ft) 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.

KEY TERMS

sublimation        radiation fog      thunderstorm       
deposition         advection fog      microburst         
hydrologic cycle   orographic         smog               
humidity           precipitation      temperature        
relative humidity  convectional       inversion          
specific humidity  precipitation      pollution dome     
dew point          cyclonic           pollution plume    
temperature        precipitation      acid deposition    
adiabatic          rainshadow                            
processes          unstable air                          
condensation                                             
nuclei                                                   

STUDY QUESTIONS

1. Draw a diagram of the hydrologic cycle. Label where water is stored on the earth. Label the pathways water moves through from the ocean to land and from land to the ocean.
2. Explain the global distribution of specific humidity. How do you think relative humidity would vary globally?
3. What is the adiabatic process? Why does the dry adiabatic lapse rate differ from the wet adiabatic lapse rate?
4. List the most common high, medium and low cloud types.
5. What are the two processes through which precipitation forms?
6. Describe the mechanisms that force air to rise, cool and produce precipitation.
7. How is an unstable situation in the atmosphere produced?
8. Explain the formation and effects of thunderstorms.
9. What are the main air pollutants? How are these pollutants produced?
10. What effect does a temperature inversion have on pollution in an area?
11. What are the effects of acid deposition?