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High & Low Pressure Circulation

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“Why do low pressure and high pressure rotate in opposite directions?”
Dave Wilson
Central Point

Because the sun heats the earth’s surface unevenly it causes air to expand and contract at different places across the globe. As a result, air pressure is different at all points on the earth. It’s these pressure differences that cause the wind to blow. Wind is created as air moves from high to low pressure.

High Pressure is seen on a weather map as a Blue Capital “H”. Near high pressure, the wind is usually light and circulates clockwise (in the Northern Hemisphere) away from the high. Because the air is leaving the high pressure area, air from above comes down to replace the air that left. This sinking air keeps the skies mostly clear and free from clouds that could develop into showers or thunderstorms. High pressure is usually associated
with nice weather.

Low Pressure is seen on a weather map as a Red Capital “L”. Since the pressure is low, air needs to fill it in, so it moves toward the low, but around it counterclockwise (in the northern hemisphere) at the same time. Often the winds can be quite strong around low pressure. Because the air is piling up at the low, some of the air is forced up. This rising air often becomes clouds and even produces rain and snow if cold enough. If the atmosphere is “unstable” then the rising air can become thunderstorms. Low pressure is associated with stormy weather.

Friction is a force that acts to slow down the movement of an object. Ice has a very low amount of friction, so walking across it is very slick, but sandpaper has a high friction and you could get good traction walking across
sandpaper.

Since the Earth is constantly rotating, the wind “turns” as the world turns. This is known as the Coriolis force. But there is friction at the surface. Friction not only slows the speed of the wind, but it also changes
the effect of the spin of the Earth.

1 comment

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  1. Dave Wilson says:

    Thanks for your explanation, (and the cool back pack!) I find meteorology quiet fascinating and being a naturally curious person, have another: If friction slows wind speed, how is it the jet stream can outpace the rotation of the earth? Both travel west to east, right?

    Thanks again for taking my question and for your time to answer.

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