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Name _____________________________________________ Period: _________ Date: _________________ Factors Affecting Wind Wind is created through differences in horizontal pressure. Air flows from areas of higher pressure to lower pressure. Pressure can be visualized using an isobar map. The isobar map shows lines of similar pressure. The pressure for each isobar is written in millibars (mb). Without external forces, wind flows at right angles to isobars from high pressure to low pressures. This rule allows us to determine the direction of wind from looking at isobar maps. Closely spaced isobars represent a higher difference in pressure, which means higher wind speeds. Isobars that are farther apart indicate low wind speeds. The spacing of these isobars over a certain distance shows the pressure gradient. Another factor affecting winds is the Coriolis effect. Objects in motion appear to become deflected from their expected path as the earth rotates under them. Objects in the Northern Hemisphere are deflected to the right of their intended path, and objects in the Southern Hemisphere are deflected to the left of their path. Deflection from the intended path becomes greater the closer you get to the poles. The map below shows atmospheric pressure at sea level for the eastern United States. Pressures are expressed in millibars (e.g., 1020 mb). Using the map, answer questions 1–5: 1. Directly on the map, draw a capital L in the center of a low pressure system, and draw a capital H in the center of a high pressure system. 2. Four locations on the map are indicated by the capital letters A–D. Based on the information on the map, which of these locations will experience the highest wind speed? (write capital letter in blank) Highest wind speed: __________ 3. Explain how you know which location will have the highest wind speed: ____________________________________________________________________________________ 4. Find the location marked with an X on the map. At the location marked X, the wind will be blowing (circle letter of correct answer): To the north To the west To the northwest To the southeast Explain how you know which direction the wind is blowing (don’t forget changes due to the Coriolis effect!): ____________________________________________________________________________________ 5. What are the highest/lowest air pressures represented on this map? Highest air pressure: __________ Lowest air pressure: __________ Using the map below, answer questions 6–10: D A X B C 6. Directly on the map, draw a capital L in the center of a low pressure system, and draw a capital H in the center of a high pressure system. 7. Four locations on the map are indicated by the capital letters A–D. Based on the information on the map, which of these locations will experience the highest wind speed? (write capital letter in blank) Highest wind speed: __________ 8. Explain how you know which location will have the highest wind speed: ____________________________________________________________________________________ 9. Find the location marked with an X on the map. At the location marked X, the wind will be blowing (circle letter of correct answer): To the north To the west To the northwest To the southeast Explain how you know which direction the wind is blowing (don’t forget changes due to the Coriolis effect!): ____________________________________________________________________________________ 10. What are the highest/lowest air pressures represented on this map? Highest air pressure: __________ Lowest air pressure: __________ Drawing Conclusions! Surface Air Pressure Map This map shows the sea level pressures for various locations over the contiguous United States. The values are in whole millibars. In this part of the exercise, you will be drawing lines connecting identical values of sea level pressure. Remember, these lines, called isobars, do not cross each other. Isobars are usually drawn for every 4mb, using 1000mb as the starting point (e.g., 1000, 1004, 1008, etc., or 996, 992, 988, etc.). You will then fill in details, as described by the procedures and analysis below. You will need colored pencils in black, red, blue, green, yellow, and two additional colors of your choice. Procedure 1. Use a black colored pencil to begin drawing from the 1024mb station pressure over Salt Lake City, Utah (in grey box). Draw a line to the next 1024 value located to the northeast (upper right). Without lifting your pencil, draw a line to the next 1024 value located to the south, and then to the one located southwest, finally returning to the Salt Lake City value. Remember, isobars are smooth lines with few, if any, kinks. The result is an elongated circle, centered approximately over eastern Utah. The line that was drawn represents the 1024mb line. You can expect the pressure to be 1024mb everywhere along that line. Repeat the procedure with the next isobar value. Remember, the value between isobars is 4mb. As there are no 1028mb values on the map, your next line will follow the 1020mb reports. Then continue with the remaining values until you have all the reports connected with an isobar. 2. Label each isobar with the appropriate value. Traditionally, only the last two digits are used for labels. For example, the label on the 1024 mb isobar would be 24. A 992 mb isobar will be labeled 92. These labels can be placed anywhere along the isobar but are typically placed around edges of the map at the end of each line. For closed isobars (lines that connect) a gap is placed in the isobar with the value inserted in the gap. (See Ms. Nevel’s example on the board.) Analysis Isobars can be used to identify “Highs” and “Lows.” The pressure in a High is greater than the surrounding air. The pressure in a Low is lower than the surrounding air. 3. Label the center of the high pressure area with a large blue “H.” Label the center of the low pressure area with a large red “L.” High pressure regions are usually associated with dry weather because as the air sinks it warms and the moisture evaporates. Low pressure regions usually bring precipitation because the air rises cools, and the water vapor condenses. 4. Shade, in green, the general area(s) where you would expect to see rain or snow. Shade, in yellow, the general area(s) where would you expect to see clear skies. In the northern hemisphere the wind blows clockwise around centers of high pressure. The wind blows counterclockwise around lows. 5. Using a new color, draw arrows around the “H” on your map to indicate the wind direction. Using a different new color, draw arrows around the “L” on your map to indicate the wind direction.