WIND
Including Lab: Make a ping
pong ball anemometer
Wind
• Wind occurs when the flow of air over the Earth's surface
is unevenly heated.
• Warm air expands, becoming lighter. When this happens,
the air rises.
• As the warm air rises, cooler air rushes in to fill the space
left by the rising warm air.
• This movement is called convection.
• Wind speed can be measured with an anemometer.
• At higher elevations, the air gets thinner and there is less
air pressure.
• This reduction in air pressure reduces frictional drag and
wind speed increases.
• The rotation of the Earth also affects wind.
• There are six major wind belts around the Earth
(easterlies, westerlies, and trade winds in both the
northern and southern hemispheres) that form by
convection.
How wind forms
Wind moves from an area of high pressure to an area of low
pressure
1. Sunlight strongly heats an area of the ground. The ground
heats the air. The warm air rises, and an area of low
pressure forms.
2. Sunlight heats an area of ground less strongly. The cooler,
dense air sinks slowly and an area of low pressure forms.
3. Air moves as wind across the surface, it moves from higher
to lower pressure.
Animation:
cold and
warm fronts
Another
animation
Land & Sea Breezes
• Unequal heating of air over land and water
results in breezes near shorelines.
• While the land is warm during the day, air
above it rises, and a cool breeze blows in
from the sea.
• As the land cools off at night, air pressure
over it increases, and a cool land breeze
blows out to the sea.
• Examine the changing temperature of the
land throughout the 24 hours represented in
the animation.
• Animation
Global Wind Belts
• The distance wind travels varies.
• Global winds travel thousands of kilometers in steady
patterns.
• Formed by two main factors:
– unequal heating of the earth by sunlight
– the earth's spin.
• The unequal heating makes the tropical regions warmer
than the polar regions. As a result, there is generally
higher pressure at the poles and lower at the equator.
Wind flows from high to low pressure.
• So the atmosphere tries to send the cold air toward the
equator at the surface and send warm air northward
toward the pole at higher levels.
• The spin of the earth prevents this from being a direct
route, and the flow in the atmosphere breaks into three
zones between the equator and each pole.
Global Wind Belts
• A series of
wind belts
circles Earth.
Between the
wind belts
are calm
areas where
air is rising
or falling.
Bands of calm air separate
global wind belts
• The earth’s rotation and the uneven heating of its
surface cause a pattern of wind belts separated by
calm regions.
• Calm Regions
– Air usually stays calm; winds are light
– Doldrums
– Horse Latitudes
• Wind Belts
– Curve to the east or west because of Coriolis Effect.
– Coriolis Effect: if the earth did not rotate, the wind would
flow directly from the poles to the equator. The earth’s
rotation changes the direction of wind and they curve to the
east or west.
– Name after the direction from which they blow
– Trade winds
– Westerlies
– Easterlies
Coriolis Effect
• As Earth
rotates, the
Coriolis effect
turns winds in
the Northern
Hemisphere
toward the
right.
Animation
Another animation
Wind Belts
• 6 total: 3 in the Northern Hemisphere (NH) & 3 in the
Southern (SH).
• The Trade winds: Blow from the east moving from the
horse latitudes toward the equator. The strong,
steady winds die out as they come near the equator.
• The Prevailing Westerlies: blow from the west,
moving from the horse latitudes toward the equator.
They bring storms across much of the US. Most of
North America fits into this belt and that is why our
weather usually comes from west.
• The Polar Easterlies: which blow from the east,
moving from the polar regions toward the middle
latitudes. Stormy weather often occurs when the cold
air of the easterlies meets the warmer air of the
westerlies.
Calm Regions
• Doldrums: Low-pressure zone near the equator.
–
–
–
–
–
Warm air rises to the top of the troposphere
Since much of the air movement is vertical, winds are light.
The air and then spreads out toward the poles.
The rising, moist air produces clouds and heavy rain.
During the hottest months, heavy evaporation from warm ocean
water in the region fuels tropical storms.
– Sailors noticed the stillness of the rising (and not blowing) air near
the equator and gave the region the depressing name "doldrums."
• Horse Latitudes: High-pressure zones located about 30 ° north
and 30 ° of the equator.
– Warm air traveling away from the equator cools and sinks in these
regions.
– Again, since much of the air movement is vertical, winds are light.
– The weather tends to be clear and dry. und 30 degrees from the
equator, cool air sinks.
– Tradition states that sailors gave the region of the subtropical high
the name "horse latitudes" because ships relying on wind power
stalled; fearful of running out of food and water, sailors threw their
horses and cattle overboard to save on provisions.
A circulation cell is a giant
loop of moving air that
includes a wind belt and the
calm regions that border it.
The trade winds
blow from the horse
latitudes toward the
equator
Air rises in the
doldrums
Air sinks in the horselatitudes; a high
pressure zone
The westerlies blow
from the horse
latitudes toward the
poles
The easterlies blow
away from the polar
regions
Ping Pong Ball Anemometer
• Materials
–
–
–
–
–
Masking Tape
Pencil
Protractor
Sewing Thread
Ping Pong Ball
Procedure
1.
2.
3.
4.
5.
6.
Tape the pencil to the straight edge of the protractor.
Tape one end of the thread to the ping-pong ball.
Tape the other end of the thread to the center of the
straight edge of the protractor. The thread should hang
so that it crosses the 90-degree mark on the protractor.
Use the pencil as a handle and hold the instrument so that
the ball is about 12 inches from your helper’s face.
Ask your helper to gently blow a stream of air toward the
ball on the instrument. Observe where the string crossed
the protractor. Record angle on the chart below and
determine the wind speed using the conversion table on
the back of this sheet.
Ask your helper to blow harder as you observe the
position of the string on the protractor, again. Record
results below. Take turns and complete five separate
trials at varying wind speeds.
Create Data Collection Table
Trial
Angle
(Degree)
Wind Speed
(km/h)
Trial
1
1
2
2
3
3
4
4
5
5
Angle
(Degree)
Wind Speed
(km/h)
Conversion Table
Angle (Degrees)
Wind Speed (Approximate) (km/h)
90
0
85
6
80
8
75
10
70
12
65
13
60
15
55
16
50
18
45
20
40
21
35
23
30
26
25
29
20
33