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 A high, otherwise known as a high pressure area, is characterized by generally fair weather. One way to think of a high is as a sort of giant mound of air, one to two thousand miles across and about eight miles high. This is that big H you see on weather maps.
Atlantic hurricanes usually start as weak tropical disturbances off the coast of West Africa and intensify into rotating storms with weak winds, called tropical depressions. We can follow these tropical depressions as they move towards the United States by looking at a movie of Global Satellite IR images from the 2004 Hurricane Season. This data set was created using infrared satellites, which measure emitted heat. Where there are clouds, the satellites measure the heat emitted by the cloud rather than the ground below it. Because clouds are so much colder than the ground, they are easy to detect on IR satellite images. Areas that show up in the bright colors are extremely cold and the gray shades are areas that are warmer. The higher the cloud, the colder the cloud top will be and so it shows up vividly on the IR satellite image. Hurricanes and tropical storms are well developed weather systems and therefore have high clouds that are easy to detect in the IR satellite images. Watch the NOAA IR satellite movie from 8/27 - 9/6 2004.
Using this Hurricane Applet, plot the center of one of these highs over the island of Bermuda which is about 800 miles east of Charleston, South Carolina during the hurricane season in the United States. What are some possible impacts on tropical depressions and potential Hurricanes developing off the coast of Africa?
You can also help determine the strength of a hurricane by seeing how cold its cloud tops are. Infrared satellite imagery lets us do this. To learn some more about how satellite images can help, see the page entitled "What does the brightness of a cloud mean on the TV weather shows?".
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