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Douglas Rattray, age 11, of Montreal, Quebec, Canada for his question:

What causes chances in atmospheric pressure?

The atmosphere is changeable by nature. Its changing pressures create the turbulent winds and weather and play a part in the seasons and the global climate zones. But atmospheric pressure does not work alone to keep these changes in continuous motion. It is merely one factor in a cycling planetary operation. Naturally, the restless atmosphere needs energy to keep it moving.. Its power sources are the sun and the gravity of the found, rotating earth. These outside energies interact with the gaseous air to create changes in pressure, plus the complex global weather activity.

Atmospheric pressure is the weight: of the atmosphere pressing down upon the surface of the globe. The total weight of the entire airy shell is estimated to be 5,600 million million..toas.w For ~ convcai.euce, we use a small sample as a standard measuring unit. It is the average weight of a column of. atmosphere pressing down upon one square inch at sea level. This standard unit of atmospheric pressure is 14 1/2 pounds. At higher levels it decreases, and everywhere on earth it changes with every change in the weather.

The factors that cause these changes are unbelievably complex. They involve the luminous sun, 93 million miles away. Its radiant energy causes changes in the density of the air, changes that create masses of lighter or heavier air. Warm air is lighter than cool air    and the sun warms the earth fn uneven patches. Here the operation is complicated by the shape and the axis of our rotating planet    also by its lands and oceans, its plains and mountains. The atmosphere absorbs only about 15 per cent of the solar radiation that passes down through it. The lands and oceans convert its short waves to long, warm rays of infrared. This solar heat radiates upward and some is absorbed by the air.

The earth's rotation and spotty geography create a global patchwork of warm and cool air masses. Warm air expands and becomes lighter, and its atmospheric pressure higher. Temperature triggers other changes in the gaseous sir. Masses of heavier high pressure flow and blow toward regions of lighter low. pressure.

On a global scale, the atmosphere strives to keep its pressure evenly distributed. But the sun and the moving earth continuously create changing pockets of high and low pressure. What's more, as the atmosphere mixes and mingles, its air masses often reverse their atmospheric pressures. Warm, low pressure air above the equator rises and chills aloft. There it blows north and south and later descends, bringing dense layers of high pressure air down to ground level. All these and several other factors work to keep the pressure of the global atmosphere in a state of turbulent change.

Day and night, local geography and other factors help to create warm and cool patches of air. But the energy that drives the airy, global engine comes from the sun. And the major portion falls within the tropics. On a global scale, changing atmospheric pressure is governed by weathery interchanges between the equator and the poles.

 

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