Download 16.3 Winds

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Section 3: Winds
1. What is Wind?
a. Wind: the horizontal movement of air from an area of high pressure to an area
of lower pressure
i. Caused by differences in air pressure
1. Which are mostly caused by unequal heating of the atmosphere
ii. As air becomes less dense, its air pressure decreases
iii.Cool, dense air with a higher pressure flows underneath the warm, less
dense air—forcing warm air to rise
b. Measuring wind
i. Described by their direction and speed
1. Direction is determined with a wind vane
a. The name of a wind tells you where the wind is coming from
i. Ex: a south wind blows from the south toward the north
2. Anemometer: an instrument used to measure wind speed
a. Has three or four cups mounted at the ends of spokes that spin on an axle
b. A meter on the axle shows the wind speed
c. Wind-chill factor: the increased cooling a wind can cause
i. The wind blowing over your skin removes body heat
1. The stronger the wind, the colder you feel
2. Local winds: winds that blow over short distances
a. Caused by the unequal heating of Earth’s surface within a small area
i. Form only when large-scale winds are weak
b. Sea breeze: a local wind that blows from an ocean or lake
i. Unequal heating often occurs along the shore of a large body of water
ii. It takes more energy to warm up a body of water than it does to warm up
land
iii.The air over land becomes warmer than the air over the water
1. The warm air expands creating a low-pressure area
2. Cool air blows from over the water and moves underneath the warm
air causing a sea breeze
c. Land breeze: the flow of air from land to a body of water
i. The process of a sea breeze is reversed at night
1. Land cools more slowly than water, so the air over the land becomes
cooler than the air over the water.
2. As the warmer air over the water expands and rises, cooler air from
the land moves beneath it
3. Global winds: wind that blow steadily from specific directions over long distances
a. Created by the unequal heating of Earth’s surface, and occur over a large area
b. Global Convection Currents
i. Temperature differences between the equator and the poles produce giant
convection currents in the atmosphere
ii. Warm air rises at the equator, and cool air sinks at the poles
1. Air pressure tends to be lower near the equator
2. Difference in pressure causes winds at Earth’s surface to blow from the
equator
3. Higher in the atmosphere, air flows away from the equator toward the
poles producing global winds
c. Coriolis effect: the way Earth’s rotation makes winds curve
i. If Earth didn’t rotate, global winds would blow in straight lines from poles
to the equator
ii. Global winds in the Northern Hemisphere gradually turn toward the right
1. In the Southern Hemisphere, winds gradually curve toward the left
4. Global Wind Belts
a. The Coriolis effect and other factors combine to produce a pattern of calm
areas and wind belts
i. The major global winds are the trade winds, the polar easterlies, and the
prevailing westerlies.
b. Doldrums
i. A calm area above the equator where warm air rises
c. Horse Latitudes
i. Warm air that rises at the equator divides and flows both north and south
1. At about 30° north and south latitudes, the air sinks
ii. Latitude: distance from the equator, measured in degrees
iii.Hundreds of years ago, sailors becalmed in these waters and ran out of food
and water for their horses, hence the name horse latitudes
d. Trade Winds
i. The area of high pressure created when the cold air over the horse latitudes
sinks
1. Causes winds to blow toward the equator and away from it
a. Toward the equator are turned west by the Coriolis effect
i. Winds in the Northern Hemisphere generally blow from the
northeast
ii. Southern Hemisphere—blow from the southeast
2. Sailors have relied on these winds to move ships of cargo from Europe to
the West Indies and South America
e. Prevailing Westerlies
i. Between 30° and 60° north and south
ii. Blow toward the poles and are turned toward the east by the Coriolis effect
iii.Blow generally from the southwest in north latitudes and from the
northwest in south latitudes
iv.Play an important role in the weather of the US
f. Polar Easterlies
i. Cold air near the poles sinks and flows toward the equator
ii. The Coriolis effect shifts these polar winds to the west, producing polar
easterlies
iii.Meet polar westerlies at about 60° north and south latitudes along the
prevailing westerlies
1. Mixing of cold or warm air has a major effect on weather in the US
g. Jet streams: bands of high-speed winds
i. Hundreds of kilometers wide but only a few kilometers deep
ii. Generally blow from west to east at speeds of 200 to 400 kilometers per
hour
* What is wind?
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* Compare the conditions that cause a sea
breeze with those that cause a land breeze.
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* Briefly describe the three major global wind
belts and where they are located.
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