So What Causes Winds? • Air is a fluid so it move’s easily from place to place. • Wind is the horizontal movement of air from an area of high pressure to an area of lower pressure. • All winds are caused by differences in air pressure.
What Causes Winds? • Most differences in air pressure are caused by unequal heating of the atmosphere.
As air becomes less dense, its air pressure decreases. • If a nearby area is not heated as much, the air above the less-heated area will be cooler and denser. • Cool dense air has a higher air pressure so it flows underneath the warm, less dense air. • This process forces the warm air to rise.
Measuring Wind • Winds are described by their direction and speed. • Wind direction is determined with a wind vane. • Wind speed is measured with an anemometer • The increased cooling that a wind can cause is called the wind-chill factor.
Local Winds • Local winds are winds that blow over short distances. • Local winds are caused by unequal heating of Earth's surface within a small area.
Local Winds • Local winds form only when no winds are blowing from farther away. • Unequal heating often occurs on land that is next to a large body of water.
Local Winds • A wind that blows from an ocean or lake onto land is known as a sea breeze or a lake breeze. • The flow of air from land to a body of water is called a land breeze.
Monsoons • Sea and land breezes over a large region that change direction with the seasons are called monsoons. • Monsoons produce heavy rains that supply the water needed by rice and other crops.
Global Winds • Winds that blow steadily from specific directions over long distances are called global winds. • Global winds are created by unequal heating of Earth's surface.
Global Convection Currents • Temperature differences between the equator and the poles produce giant convection currents in the atmosphere. The movement of air between the equator and the poles produces global winds.
The Coriolis Effect • Because Earth is rotating, global winds do not follow a straight path. • The way Earth's rotation makes winds curve is called the Coriolis effect.
Global Wind Belts • The Coriolis effect and other factors combine to produce a pattern of calm areas and wind belts around Earth. • The calm areas include the doldrums and the horse latitudes. • The major global wind belts are the trade winds, the prevailing westerlies, and the polar easterlies.
Jet Streams • 10 kilometers above Earth's surface are bands of highspeed winds called jet streams. • Jet streams blow from west to east at speeds of 200 to 400 kilometers per hour. • Airplanes are aided by a jet stream when traveling east. • Pilots can save fuel and time by flying east in a jet stream. • However, airplanes flying at jet stream altitudes are slowed down when traveling west against the jet stream winds.
Jet Streams Jet streams can significantly affect weather and climate!