In the final days of October 1991, a large low-pressure area formed in the western Atlantic Ocean off the northeast coast of the U.S. and Canada. At the same time, hurricane Grace was moving northwestward through the Atlantic toward the North Carolina coast. In response to the winds generated on the south side the low-pressure system, Hurricane Grace made a hairpin turn to head eastward along the southern side of the low-pressure system.

The resulting weather pattern, caused by the combined effects of the two systems, was labeled by the National Weather Service as "the perfect storm." Later it became the subject of a best selling novel, and then a movie, by Sebastian Junger.

Exploring the Effects of High-pressure Areas

In this activity, you will explore high-pressure areas and how they affect the weather.

  1. In the Gizmotm, select Show fronts and high/low-pressure areas. Unselect all other options. Click New until a high-pressure area (an "H") appears near the center of the screen. The is the center of a region of high air pressure. Select Show Mobile Station B data. Drag Station B to the center of the high-pressure area, lying on the H-symbol.
    1. Data from Station B are shown in the left-hand panel. What is the current location of the station, in degrees latitude and degrees longitude?
    2. What is the temperature at the center of the high-pressure system? What is the air pressure?
    3. What is the wind speed? (You can read the wind speed on the left, or by reading the flags on the "tail" of the Station B symbol. Each short flag stands for 5 mph of wind, each longer flag stands for 10 mph, and each triangular flag stands for 50 mph. Add all the flags up to get the actual wind speed.)
    4. Cloud cover is represented by how much the circle representing Station B is filled in. Is there any cloud cover at this location?
  2. Drag Station B just slightly toward the north (toward the top of the screen), about 0.5 degrees north of the center of the high-pressure system.
    1. What is the temperature in this new location? What is the pressure?
    2. What are the wind speed and direction? (The "tail" of the Station B symbol points in the direction that the wind is coming FROM.)
    3. How do the values for pressure and wind speed compare to the values at the center of the system?
  3. Move Station B 0.5 degrees further toward the top of the screen. Record the temperature, the pressure, and the wind speed and direction in this new location. Then keep moving the station northward in increments of 0.5 degrees. At each location, collect data on pressure, wind, and temperature.
    1. How does the pressure change as you move north from the center of the high-pressure system?
    2. What is the wind direction at locations north of the system?
    3. Describe how the wind speed changes as you move north from the high-pressure system. Where is the wind speed highest?
  4. Now move the station to a position 0.5 degrees to the right (east) of the center of the high-pressure area.
    1. What are the temperature, the pressure, and the wind speed and direction at this location?
    2. Continue moving the station to the right in 0.5-degree increments. Record the temperature, the pressure and the wind speed and direction at each location. How do the pressure changes as you move east compare to the pressure changes as you move north?
    3. How do the wind speeds change as you move east? How does this compare to moving north from the center of the system?
    4. How does the wind direction on the east side of the high-pressure system compare to the wind direction on the north side?
  5. Collect the same data, first moving south and then moving west from the high.
    1. In general, how would you describe the direction of the wind in the region surrounding a high? (The motion of the wind is circular around a high, so your answer should be "clockwise" or "counterclockwise.")
    2. Describe the wind speeds in the region surrounding a high.
    3. Where is the temperature lowest around the high? Where is it highest? Explain your findings.
    4. Turn on Show additional display options. Select Show radar (to see precipitation), and then Show satellite image (to see clouds). Is there any precipitation or cloud cover near a high-pressure area?

Exploring the Effects of Low-pressure Areas

In this activity, you will explore low-pressure areas and how they affect the weather.

  1. Select Show fronts and high/low-pressure areas and turn off all other options. Click New until a low-pressure area (an "L") appears. Low-pressure areas are accompanied by a front. Lines with semicircles are warm fronts. Lines with triangles are cold fronts. The semicircles or triangles point in the direction that the front is moving.
    1. Click New repeatedly until you have seen several examples of lows with warm fronts as well as those with cold fronts. Where is a warm front always located, relative to the center of the low-pressure area? In what direction does a warm front move?
    2. Where is a cold front always located, relative to the center of the low-pressure area? In what direction does a cold front move?
  2. Click New until a low with a cold front appears near the center of the screen. Select Show Mobile Station B data. Drag Station B to the center of the low-pressure area, so it is on the L-symbol. Record the temperature, pressure, wind speed, and wind direction.
    1. How does the pressure compare to the pressure in the center of the high-pressure area that you examined in the previous exercise?
    2. Move Station B to different locations around the low. How does the wind move around a low, clockwise or counterclockwise? How does this compare to the wind direction surrounding a high?
    3. Where surrounding the low is the wind speed highest?
  3. Drag Station B just behind the cold front (on the side that does NOT have the triangles).
    1. What is the temperature on this side of the front?
    2. Move the station just ahead of the front. What is the temperature? How does it compare to the temperature behind the front?
  4. Drag Station B to different locations around the low. Explore the cloud cover (shown by how much of the Station B circle is filled in) as you do so.
    1. Is it cloudier ahead of the front or behind it?
    2. Turn on Show additional display options. Select Show satellite image. Where do the clouds lie?
    3. Select Show radar. What type of precipitation occurs with this front?
  5. Turn off Show additional display options. Click New repeatedly until a low-pressure system with a warm front appears near the center of the screen. Use Mobile Station B to explore each side of the warm front.
    1. How do the temperatures ahead of the warm front compare with the temperatures behind it?
    2. Is there cloud cover accompanying this front? On which side of the front? Turn on Show additional display options and select Show satellite image to confirm this.
    3. Based on the temperatures in this region, what type of precipitation would you expect to be falling? Select Show radar to confirm.