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WX61s Why Wind Pm.doc
Early sailors depended on the wind to carry them across the oceans. why the wind blows
They learned there are wind patterns that they could rely on to navigate across
the oceans. Columbus was aware of these patterns and used them for his great
voyages. Figure 2 Global Wind Patterns, shows where the winds blows. What causes the wind to blow? From light breezes to strong gusts,
winds are the result of uneven heating of the earth’s surfaces.
Sailors found that near the equator there was little or no wind.
The process begins as the sun warms the earth. As the layer of air
It also rained nearly every afternoon. They called this windless wet above the ground or water heats, it expands. As it expands, it becomes less
region the doldrums. Doldrums means low spirits, or a period of dense and rises. The heat energy moves with the fluid. Meteorologists use the
inactivity. That is how sailors felt when trapped in the doldrums with word “convection current” to describe this movement of heat through fluids
drinking water supplies running low. The trade winds, Westerlies and (gases and liquids). As the air heats, it becomes less dense and is pushed
Easterlies, are named of different kinds of global winds that form upwards as cooler dense air is pulled to the earth by gravity. As the warm air
because of earth’s rotation and the sun’s energy. Westerlies flow from rises, the air pressure goes down and the air expands. This expansion causes
the west. Easterlies flow from the east. Note, winds are named for the the air to get colder.
direction they flow from. As the warm air rises from the surface of the earth, cold air moves in
to replace the rising warm air. The earth warms this layer of incoming cool air
SPELLING WORDS & VOCABULARY WORDS and it too expands and rises. This air heats and rises and is replaced by more
absorb: To keep completely, without reflecting or moving. cold air. This cycle goes on and on. Convection currents can form in gases or
conduction: The transfer of energy through contact (touch). liquids if there is a temperature difference and gravity.
convection: The transfer of energy in fluids by the circulation of molecules
because of differences in density. Local Winds: Land Breezes and Sea Breezes
global wind: Giant convection currents that circulate in the Northern and Land breezes and sea breezes are caused by convection. These
Southern Hemispheres. breezes occur where large bodies of land and water meet. The different rates
heat: energy in the form of moving molecules at which land and water heat and cool cause these winds. This happens more
local wind: Convection current that circulate in local areas, usually caused by often in early summer than at other times of the year.
large water bodies near land. During the day, land heats up faster than the water in lakes or oceans.
meteorologist: Scientist that studies the atmosphere, with a focus on weather. This heats the air above the land, and causing it to rise. As the air rises, the air
radiation: The movement of energy by waves. pressure drops. The cool dense air over the water moves toward the land and
replaces the rising warmed air. This flow of air is called a sea breeze.
References: http://eosweb.larc.nasa.gov/EDDOCS
QUESTIONS
On a separate sheet of paper, write out each question and then answer it.
1. What is a meteorologist?
2. What is a convection current?
3. Why do some parts of earth, like the equator, get more heat than other
regions?
4. How does the Coriolis effect influence wind circulation?
5. As air temperature goes down, what happens to density and pressure?
6. What is a global wind?
7. What is a sea breeze? What is a land breeze?
WX61s Why Wind Pm.doc
At night, the land cools faster than the water. The air above the land The Coriolis Effect
now becomes cooler than the air above the water. As the warm air over the The uneven heating between the equator and the poles does not
water rises, cool air from the land moves towards the water to replace the air completely explain the global wind patterns. If you were on a moving
that is rising. The flow of air from land to water is called a land breeze. merry-go round, and threw a ball to a friend on the opposite side, what would
Other factors can cause local winds. One factor is color. You may happen? By the time the ball got to the opposite side, your friend would have
have observed that on sunny days, black clothing and black cars become moved. It would appear to you as if the ball had curved away from your
warmer faster than light colored clothing or cars. This is because the black is friend.
absorbing more energy from the sunlight and becoming warmer. This Like the merry-go round, the rotation of Earth causes moving air and
happens with the earth as well. Dark soils absorb more energy and heat up water to appear to turn to the right when looking north of the equator, and to
faster than light-green fields. The air above these surfaces is warmed or the left south of the equator. This is called the Coriolis Effect. The flow of air
cooled accordingly, and this creates local winds. caused by differences in the amount of solar radiation received and by and by
the Coriolis effect create distinct wind patterns on the surface of the Earth.
Global Winds These winds not only influence the weather, they also determine when and
There are other winds than just local winds. Winds are continually where ships and planes travel most efficiently.
being formed around the world. These winds move in a particular direction
and travel over long distances. These winds, which form between the equator
and poles, are called, global winds.
The equator and poles are not heated evenly. Near the equator, the
sun’s rays are more intense. This makes the areas nearby, called the tropics,
very warm. Near the poles, sunlight that reaches the earth’s surface has to pass
through more atmosphere than at the equator. Because the atmosphere both
absorbs and reflects the sun’s heat, the poles are not as warm.
Circling the Globe
Global winds are the result
of giant convection currents that
circulate within the Northern and
Southern Hemispheres of the earth.
As warm air is heated at the
equator, it rises and flows both
north and southward toward the
poles. If the earth didn’t rotate, the
hot air at the equator would rise to
the poles, cool sink, and flow back
to the equator again (See Figure 1).
But the earth does rotate, which
means that air and water currents Figure 2 Global Wind Patterns.
on the earth are deflected. The Sun’s uneven heating of Earth’s surface forms giant
loops, or cells, of moving air. The Coriolis effect deflects the
Fig. 1 Global winds if Earth did surface winds to the west or east. This makes belts of wind
not rotate. that distribute heat and moisture around the world.
