Introduction LAB H - ATMOSPHERE AND CLIMATE LAB II STABILITY AND PRECIPITATION PATTERNS 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. A. Using the diagram below, fill in the temperatures for the rising parcel of unsaturated air. Environmental lapse rate data measured on August 18, 2000. H-1
B. Now answer the following questions: 1. What is the Environmental Lapse Rate for August 18, 2000? 2. Is the Adiabatic Lapse Rate greater or less than the ELR on this date? 3. Is this a condition of stability or instability? C. There are four major causes of air mass lifting: convection, convergence, orographic, and frontal. Diagram each type of air mass lifting from a side view. Show the general direction of air movement with arrows. CONVECTION OROGRAPHIC CONVERGENCE FRONTAL D. For this question, you will use the North American and World maps at the end of this lab on pages H-9 and H-10. Match the marked locations to one of the four lifting mechanisms as discussed in lecture materials. Be sure that the selected lifting process is the most significant one in terms of producing precipitation at this location. North America Location # World Location # Convection Convergence Orographic Frontal H-2
E. Using the provided lapse rates, fill in the temperatures associated with an air mass that is forced over a topographic barrier. F. On the diagram (above), draw in the bottom of a cloud at the altitude at which condensation occurs and clouds would begin to form. Now answer the following 1. At what elevation did the air cool to the dew point? 2. At what elevation is the Relative Humidity = 100%? 3. At what elevation is the air saturated? 4. How do the windward and leeward sea level temperatures compare to each other? G. Using the provided lapse rates, fill in the temperatures associated with an air mass that is forced over a topographic barrier H. On the diagram (above), draw in the bottom of a cloud at the altitude at which condensation occurs and clouds would begin to form. Now answer the following 1. At what elevation did the air cool to the dew point? H-3
2. What did the increase in the dew point do to the height of the Lifting Condensation Level (LCL)? 3. How do the windward and leeward sea level temperatures compare to each other? 4. Is the leeward sea level temperature warmer or cooler than the answer you found for question F4 (mountain problem #1 in question E)? I. Using the provided lapse rates, fill in the temperatures associated with an air mass that is forced over a topographic barrier. J. On the diagram (above), draw in the bottom of a cloud at the altitude at which condensation occurs and clouds would begin to form. Then answer the following 1. At what elevation did the air cool to the dew point? 2. How do the windward and leeward sea level temperatures compare to each other in this problem? H-4
Part II. Precipitation World Precipitation Patterns A. Using the World Precipitation Map in Goode s World Atlas, locate the following geographic coordinates. You will find that each of these locations has a fairly rainy or dry climate. Using the climate controls listed in the box below, select the factors that are responsible for the precipitation characteristics observed at each location. Note: A given location may have one or more explanatory climate controls. Select all that apply. Climate Control Answer Choices Surface air temperature Access to moisture Prevailing wind direction Topographic barriers Orographic lifting Frontal lifting Convergence Convection Continentality Subsidence Ocean current temperatures Intertropical Convergence Zone (ITCZ) Subtropical High Pressure (STH) Subpolar Low Polar High Thermal High Thermal Low Cyclonic Storms 1. 80 N, 40 W 2. 40 N, 90 E H-5
3. 25 N, 0 4. 50 S, 70 W 5. 20 S, 70 W 6. 0, 70 W H-6
7. 50 N, 125 W 8. 60 N, 5 E 9. 20 N, 95 E H-7
United States Precipitation Patterns A. Understanding the climate controls on U.S. precipitation distribution. From Goode s World Atlas, use the U.S. and Canada Average Annual Precipitation map and the Northwestern U.S. regional physical map to answer the following questions. 1. Focus on the region of the Pacific Northwest and the parallel bands of high precipitation with a region of lower precipitation between them. What causes the two high precipitation areas? 2. What is the cause of the low precipitation strip between the higher precipitation areas? _ 3. What is the name of the phenomenon responsible for this dry region? B. Using a precipitation map from your text or the atlas, describe and explain the pattern of precipitation across the eastern U.S. from the Rocky Mountains to the Atlantic Ocean. Be as descriptive as possible. C. Utah and Wyoming contain large areas that receive less than 10 of precipitation per year. Since these areas are too far from the influence of STHP (Subtropical High Pressure), explain why they are so dry? H-8
PHYSICAL GEOGRAPHY GPH111 North America Map for Question D on page H-2 H-9
PHYSICAL GEOGRAPHY GPH111 World Map for Question D on page H-2 H-10