That's the Way Winds Blow

Air Pressure and Wind
 

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Air Pressure helps forecasters
Not only do differences in air pressure help determine wind speed and direction, they help forecast precipitation and clear weather. High pressure indicates sinking air where clouds cannot form. Low pressure indicates rising air, which allows clouds to form, bringing rain or snow.

 

Air Flows From High Pressure to Low Pressure

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The wind blows because of differences in air pressure from one location to another. Wind blows from areas of high pressure toward areas of low pressure. If the high pressure area is very close to the low pressure area, or if the pressure difference is very great, the wind can blow very fast.

What is Air Pressure?

Imagine a group of acrobats at the circus. One climbs up and stands on another's shoulders. The weight of the acrobat on top puts more pressure on the one below. Then another acrobat climbs up and stands on the second acrobat's shoulders. Now there's even more pressure on the acrobat on the bottom because he is under the weight of the two acrobats above him. It's the same with air. Yes, air has weight, and probably more than you think. In fact, the weight of the air on your desk at school weighs about 11,000 pounds. That's about the same weight as a school bus! Since air pressure pushes in all directions, the air pressure pushing up from under your desk balances out the air pushing down on it, so the desk doesn't collapse under the weight. Just like an acrobat with two people stacked on his shoulders would want to move to where there wasn't so much pressure on him, air moves from areas where the pressure is higher to where it is lower.

What causes Air Pressure?

Air pressure depends on the density of the air, or how close together its molecules are. You know that a hard rubber ball is more dense than a Styrofoam ball and that ice cream is more dense than whipped cream. Air lower in the atmosphere is more dense than air above, so air pressure down low is greater than air pressure higher up. (Remember those acrobats; there's a lot more pressure on the one on bottom than on the one on top.) Temperature also makes changes in air pressure. In cold air, the molecules are more closely packed together than in warm air, so cold air is more dense than warm air.

Rising and Sinking Air

Since warm air is less dense and creates less air pressure, it will rise; cold air is denser and creates greater air pressure, and so it will sink. When warm air rises, cooler air will often move in to replace it, so wind often moves from areas where it's colder to areas where it's warmer. The greater the difference between the high and low pressure or the shorter the distance between the high and low pressure areas, the faster the wind will blow. Wind also blows faster if there's nothing in its way, so winds are usually stronger over oceans or flat ground. Meteorologists can forecast the speed and direction of wind by measuring air pressure with a barometer.

 Wind Direction

Although wind blows from areas of high pressure to areas of low pressure, it doesn't blow in a straight line.  That's because the earth is rotating.  In the northern hemisphere, the spin of the earth causes winds to curve to the right (to the left in the southern hemisphere). This is called the coriolis effect. So in the northern hemisphere, winds blow clockwise around an area of high pressure and counter-clockwise around low pressure.


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ęCopyright 2005 Nick Walker/Small Gate Media