What causes the weather?

For example, look at these pictures of the average January global temperature, wind speeds and surface pressure from the model called BUGS. Near the equator, the temperatures are fairly constant - it is almost all orange (between 27 and 31 deg C). The surface pressure doesn't change much either, except for the green blob of lower pressure east of Australia where the ocean is very warm and storms are common.

But in the northern hemisphere, where it's winter, the temperature changes pretty quickly as you go north, especially above the oceans.

Look, for example, at the north Pacific Ocean. Sure enough, there is a large difference in pressure between Hawaii and Alaska, with a big low pressure center above the Alaskan Aleutian islands.

Since air flows from high to low pressure, you would expect the air to flow toward the north. But, because of the earth's rotation, the air is deflected as it flows northward, and instead flows to the east. That's why the air flows around high and low pressures, instead of directly between them. In the northern hemisphere, the air flows counterclockwise around a low and clockwise around a high. You can also see that the winds are the strongest where the differences in pressure and temperature are the strongest.

Next page   -> under pressure, continued

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