Gilvary; age 11, of Montoursville, Pennsylvania, for her question:
Where does the wind start?
Air is much easier to move than water and several outside forces work together to start the winds a blowing. Another team of outside forces guides the winds to blow in this or that direction. The key factor that keeps the global atmosphere in such a breezy turmoil is the sun and what its heat does to the gaseous molecules of the atmosphere. The poles and tropics swap blowing breezes, so do the continents and oceans. The shape and rotation of the globe play a part in guiding the paths of winds.
Heat and cold help to start the winds because of the nature of gases. The molecules of the airy atmosphere use heat energy to move faster. With extra speed they spread farther apart and create pockets of thin,' lightweight air. When they lose heat, they slow down and tend to move closer together. This creates pockets of dense, heavier air. All gas molecules rush around, mingling together to create an even mixture. In the atmosphere, warm and cool air masses build up enormous contrasts. When the mingling process begins, the moving winds are started on their way.
What actually triggers the motion is atmospheric pressure, which is the weight of the immense shell of air pressing down on the earth's surface. Low pressure air is light and usually warm, high pressure air is heavier and usually cool. The natural motion of gases tends to push denser high pressure air toward thinner, low pressure air. Winds blow out from heavy piles of high pressure air, somewhat like water flowing downhill.
The global system of prevailing winds is set in motion because cool, heavy air above the poles starts toward the warm, light air above the tropics. But this simple basic plan is modified by the shape of the earth, its rotation and a system of upstairs winds that ascend and descend along certain latitudes. This team of planetary forces keeps six belts of prevailing winds in circulation around the globe.
The prevailing winds prevail all the time. But limited weather conditions build up and create breezy pockets of local winds. For example, the air sitting on a polar ice field becomes chilled and piles up in a dense mass. The winds from this high pressure cell blow out from the center and the spinning earth spirals them around.
Air above a sunny tropical sea becomes warm and rises aloft to create a pocket of low pressure. When winds rush in toward the center, the spinning earth spirals them around. If this weather event has enough force, it becomes a raging hurricane.
Other local winds are started by warm and cool contrasts between land and sea. During a summer night, the land loses its heat faster than the sea. Come morning, a cool land breeze blows from the shore, out over the ocean. During the day, the land gets hotter than the sea. Come evening, a cool sea breeze from the ocean blows over the land. Mountains, deserts and valleys also start up local winds and breezes ¬and always the denser high pressure air tends to blow downhill to lighter low pressure air.