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Geography

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001

Name____________________________

Continuing Eduation

Exercise 4

Atmospheric Pressure and Wind

Isobars and Wind. (refer to text Ch. 5)

Lines of equal barometric pressure are called isobars and are typically drawn at 4 mb intervals. These isobars can be used to interpret wind speed and direction, as air tends to be driven away from air of high pressure toward areas of low pressure. Air flow is affected by the pressure gradient force, acting perpendicular to isobars, as well as by the Coriolis effect and surface friction (see text fig. 5.9)

1. The four diagrams below depict the wind pattern around surface high and low pressure areas. Put an H in the middle of each of the high pressure cells, and an L in the middle of each of the low pressure cells.

2. Based on direction of wind flow, identify whether each is in the Northern or Southern Hemisphere by placing an NH or SH beside each letter.

A B

C D

3. The figure below shows maps of pressure distributions in several situations. Keeping in mind the influence of pressure gradient force, Coriolis effect, and surface friction, use arrows to indicate the appropriate resultant wind directions corresponding to the isobaric patterns for geostrophic wind (left column) and surface wind (right column) on maps below

High

1000 mb

1012 mb

1012 mb
1000 mb

510 mb

512 mb

Low

512 mb
510 mb

High

Low

Drawing Isobars. Practice in drawing isobars (lines of equal pressure) on a weather map and figuring out how surface winds move diagonally across them will increase your ability to read and interpret not only daily weather maps, but also global maps of pressure and winds. The map on the following page shows barometric pressures observed simultaneously at many National Weather Service stations. Pressures at each location are given in millibars (mb), but only the last two digits are given. Thus “10” designates 1010 mb; “96” designates 996, etc. Each station is located at the dot alongside the number.

4. On the map on the following page: (1) draw isobars for the entire map using a 4 mb interval, starting at 992 mb. In other words, draw isobar lines for values 992, 996, 1000, 1004, 1008, 1012, etc. (2) Label each isobar. (3) Label areas of highest and lowest pressure with an H and an L, respectively. Note: In drawing the isobars, use a light pencil to start in order to allow for corrections. Then draw the final isobars as smooth, flowing curves. Alternatively, use a digital drawing tool to complete the map.

5. Interpret the following conditions for Boulder (indicated by asterisk on map):

Approximate pressure ___________

Approximate wind direction ___________

(i.e., from which direction?)

Based on interpretation of the map, what general area of the U.S. is probably experiencing the least wind? Why?

Global Air Pressure/Wind. Refer to the map of global barometric pressures for January and July in your textbook (Figure 5-12) to answer the following questions.

6. Notice the red dashed line marking the ITCZ.

a. What do the letters ‘ITCZ’ mean?

b. Why does the ITCZ change position north and south between July and January?

c. Why is it positioned furthest north over India and South Asia during July?

7. High pressure tends to dominate in the subtropics, especially over subtropical oceans; these are known as the subtropical high pressure cells. In the Northern Hemisphere, during which season (January or July) are the subtropical high pressure cells strongest?

8. Still looking at the two maps, find the location of the single highest pressure (hint: it is not in the subtropics).
(a) When and where does the strongest high pressure form (give the approx. latitude and longitude)?

(b) What causes this severe high pressure?

9. Again looking at figure 5.12, during the Southern Hemisphere summer, describe the pressure gradient over the Southern Ocean (South Pacific, South Atlantic, Indian Oceans) by answering the following two questions:
(a) What is the maximum range in pressure between the subtropical highs and the subpolar lows across this area?

(b) What is the likely effect of this strong pressure gradient?

10. On the figure below, label the following major global pressure zones: Equatorial Low Pressure (ITCZ), Subtropical High Pressure, Subpolar Low Pressure, Polar High Pressure

0°

30°N

60°N

30°S

60°S

11. Draw and label arrows to show the following surface wind systems on the Earth diagram: Trade Winds, Westerlies, Polar Easterlies.

12. Indicate how each of these major pressure areas is caused (thermally-induced or dynamically-induced).

Equatorial Low:

Subtropical High:

Subpolar Low:

Polar High:

13. In the figure below sketch in the thermal circulation associated with a land/sea breeze system (see fig. 5.18). Draw arrows depicting vertical and horizontal air movement in a coastal environment during the summer. Label the time of day as either day or night. Also, indicate the relative temperatures and surface pressures between the water and land surfaces.

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