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 and 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. Identify a region in the U.S. and outside the U.S. where each of the four types of lifting would be likely to occur. Be specific, e.g. south Florida for convection. You can not use a location more than once. United States World Convection Convergence Orographic Frontal H-2
E. Using both the Dry Adiabatic and Saturated Adiabatic 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? Part III. Precipitation World Precipitation Patterns A. The world annual precipitation map in Goode's World Atlas 20 th edition displays global precipitation patterns. Two different latitudinal regions stand out as receiving large amounts of precipitation annually. Identify these two latitudinal regions and explain why they receive so much rain? B. On the same map, several areas stand out as being very dry, receiving less than 25 cm of precipitation annually. Identify two of these regions and explain why they receive so little rain? H-3
United States Precipitation Patterns A. Understanding the climate controls on U.S. precipitation distribution. See page 72 of Goode's World Atlas 20 th edition 1. Of the contiguous 48 states, which three states have regions receiving greater than 100 inches of annual precipitation?,, & 2. Name and discuss three climate controls that operate in this region to produce such large precipitation totals. 3. Notice that this high rainfall region has two parallel bands with a region of lower precipitation between them. (Hint: compare page 72 to page 114). What causes the two high precipitation areas? What is the cause of the low precipitation strip between the higher areas? _ What is the name of the phenomenon responsible for this dry region? B. Using the 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-4
Arizona Precipitation Patterns The following 30-year average rainfall data (in inches) are for Phoenix, AZ and Flagstaff, AZ. Jan Feb Mar Apr May June July Aug Sept Oct Nov Dec PHX 0.83 0.77 1.07 0.25 0.16 0.09 0.99 0.94 0.75 0.79 0.73 0.92 FLG 2.18 2.56 2.62 1.29 0.80 0.43 2.40 2.89 2.12 1.93 1.86 1.83 A. Plot the data on the graphs below. The type of graph for precipitation is a bar graph. B. Explain the higher Flagstaff precipitation totals compared to Phoenix? C. What causes the summertime maximum rainfall for both cities? D. What causes wintertime precipitation for both cities? E. What causes the minimum rainfall totals during April, May, and June for both cities? F. Would you expect Tucson to have more or less rainfall totals than Phoenix? Why? 3.5 3.5 3 3 2.5 2.5 2 2 1.5 1.5 1 1 0.5 0.5 0 J F M A M J J A S O N D 0 J F M A M J J A S O N D Phoenix Flagstaff H-5