Chapter One WHAT IS WIND Wind is a familiar element of weather one that pretty much everyone has some experience with However many people don’t know how to really define wind or what causes it Simply put wind is the movement of air across Earth’s surface LOCAL WINDS All winds from gentle breezes to raging hurricanes are caused by differences in the temperature of the atmosphere by rotation of the Earth and by unequal heating of the continents and the oceans Simple examples of the effect of temperature on wind are land and sea breezes The sun heats land and water differently Over oceans and lakes most 8
WHAT IS WIND warm air of the heat energy is consumed in evaporation cool sea breeze evaporation or is absorbed by the water The air is not heated very much Land however day land warmer sea cooler absorbs only six tenths as much heat as water warm air does and evaporation is less Hence air over cool land breeze land tends to receive more heat than does air over water night land cooler sea warmer Heated air expands 2014 Encyclopædia Britannica Inc and becomes lighter This starts to occur over land soon after By day the land is warm and the sea sunrise Air over the cool The warm air rises and the cool air descends The breeze is off the sea does not heat sea At night the land is cool and the as rapidly and so it sea warm The breeze is off the land remains denser or heavier than the land air Its pressure near the Earth's surface is greater At length this extra pressure starts it flowing landward where it pushes the lighter land air up out of its way This movement is called a sea breeze
WIND POWER warm air warm air cool air During the night the opposite action occurs The land cools more rapidly than the sea and so does arming mountainside day the air above it At some time in the warm air cool air cool air night the breeze is reversed and blows out to sea as a land breeze Similar day Cooling mountainside and night changes night cause mountain 2014 Encyclopædia Britannica Inc and valley breezes By day the greatest Day and night changes in heating greatest heating occurs mountains cause breezes By day heated air rises along the warm slopes along mountainsides but descends into the valley At night mountainsides and at the the circulation is reversed heads of valleys Cooler heavier air on the lower levels then blows up the valleys At night this action is reversed
WHAT IS WIND PROOF OF WIND’S POWER While they can be a power source winds are also important for other reasons These masses of moving air change the face of the Earth Winds like water constantly wear down rocks and scoop out canyons The powerful blasts of air carry away sand and soil This is called wind erosion In some places winds bury fertile soil under wind-borne sand In others they lay down rich deposits ATMOSPHERIC PRESSURE AND HEAT In general all the winds near the Earth’s surface are caused by differences in atmospheric pressure brought about by uneven heating Air is pushed from regions where pressure is high toward regions where pressure is low Weather maps make this plain with the labels high and low The notion that heating of air by the sun makes it lighter and decreases its pressure may seem contrary contrary to experience When we compress air in a tire 11
