H_stability_precip_online_updt_3_10-2.pdf - PHYSICAL...

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PHYSICAL GEOGRAPHY GPH111 H-1 LAB H - ATMOSPHERE AND CLIMATE LAB II STABILITY AND PRECIPITATION PATTERNS Introduction This lab will provide students with the opportunity to become familiar with the concepts of atmospheric stability and precipitation patterns at different locations during different seasons. Part I. Lapse Rates and Stability From an earlier lesson, students learned that still air cools at a rate termed the Average Environmental Lapse Rate ( ELR ) - 3.6°F per 1,000 ft. This is analogous to a balloonist ascending through the atmosphere and measuring the temperature every 1,000 ft. The measurements would reflect a decrease of 3.6°F for every 1,000 ft. In reality, the Environmental Lapse Rate is quite variable, changing from day to day and from place to place. We know that all air is not still. We can look at what happens to a “parcel” of air as it moves in the atmosphere. For example, air that is warmed by insolation will rise and air that flows over topographic barriers (mountain ranges) will be forced aloft over the barrier. Rising bodies of air will cool at one of two different rates: at the Dry Adiabatic Lapse Rate, DALR ( 5.5°F per 1,000 ft. ) for unsaturated air, or at the Saturated Adiabatic Lapse Rate, SALR ( 3.3°F per 1,000 ft. ) for saturated air. The rising air parcel contrasts with the surrounding air by being either warmer or cooler. If it becomes cooler than the surrounding air, the rising air parcel will begin to sink. If the rising air parcel is warmer than the surrounding air, it will continue to rise. Whether a rising air body becomes warmer or cooler than the surrounding air depends on the Environmental Lapse Rate of the surrounding air, which is measured with a weather balloon (radiosonde). If the Environmental Lapse Rate were high, perhaps 10°F per 1,000 ft., then air cooling at only 5.5°per 1,000 ft. would be warmer than the surrounding air and would continue to rise. This condition is termed instability . If the Environmental Lapse Rate were low, perhaps only 2°F per 1,000 ft., then the rising air would soon become colder than the surrounding air and would cease rising and eventually sink back to earth. This condition is termed stability . Cloud formation and precipitation will result when rising air cools to the dew point (air becomes saturated) and condensation can then occur.
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