Earth image Wind and Global Wind Systems

Dr. Pamela Gore
Georgia Perimeter College

Objectives

  1. Explain the causes of winds on the Earth.
  2. Explain the differences between land breezes and sea breezes, and their causes.
  3. Describe the Coriolis Effect.
  4. Explain how the Coriolis Effect and the unequal heating of the Earth affect the global wind systems.
  5. Discuss the trade winds.
This section addresses, in whole or in part, the following Georgia GPS standard(s):
  • S6E6a. Explain the role of the sun as the major source of energy and its relationship to wind and water energy.
  • S6E4b. Relate unequal heating of land and water surfaces to form large global wind systems and weather events such as tornados and thunderstorms.

This section addresses, in whole or in part, the following Benchmarks for Scientific Literacy:
  • Air is a substance that surrounds us, takes up space, and whose movement we feel as wind.
  • Weather (in the short run) and climate (in the long run) involve the transfer of energy in and out of the atmosphere. Solar radiation heats the land masses, oceans, and air. Transfer of heat energy at the boundaries between the atmosphere, the land masses, and the oceans results in layers of different temperatures and densities in both the ocean and atmosphere. The action of gravitational force on regions of different densities causes them to rise or fall-and such circulation, influenced by the rotation of the earth, produces winds and ocean currents.
  • Waves, wind, water, and ice shape and reshape the earth's land surface by eroding rock and soil in some areas and depositing them in other areas, sometimes in seasonal layers.
  • Some changes in the earth's surface are abrupt (such as earthquakes and volcanic eruptions) while other changes happen very slowly (such as uplift and wearing down of mountains). The earth's surface is shaped in part by the motion of water and wind over very long times, which act to level mountain ranges.

This section addresses, in whole or in part, the following National Science Education Standards:
  • The sun is the major source of energy for phenomena on the earth's surface, such as growth of plants, winds, ocean currents, and the water cycle. Seasons result from variations in the amount of the sun's energy hitting the surface, due to the tilt of the earth's rotation on its axis and the length of the day.
  • Global patterns of atmospheric movement influence local weather. Oceans have a major effect on climate, because water in the oceans holds a large amount of heat.

Causes of Wind

The tropics, near the equator, receive more direct rays from the sun throughout the year (see course notes on Seasons). As a result, near the equator, the ocean "heats up" more than at higher latitudes. This creates a temperature imbalance (warmer water near the equator and cooler water near the poles). The temperature imbalance creates a flow of heat towards the poles by way of winds and the oceanic currents. Unequal heating of the atmosphere on land and the oceans creates wind and global wind systems.

Wind is the result of horizontal differences in air pressure. Air flows form areas of higher pressure to areas of lower pressure. Differences in air pressure are caused by uneven heating of the Earth's surface. Therefore, we can say that the sun (solar energy) is the ultimate cause of wind.

Something to remember: Wind direction is given as the direction from which the wind comes. For example, a "north wind" blows from north to south.


Land and Sea Breezes

The land heats and cools more rapidly than does the sea. Water retains heat longer than land, and also takes longer to heat and cool.

This causes temperature differences between the land and the sea, which leads to a thermal circulation (or wind based on temperature differences).

During the day, the land gets hotter faster, and the hot air rises, creating an area of lower pressure. Wind blows from the sea to the land. This is a sea breeze.

At night, the land cools off faster than the sea. Cooler air descends creating an area of higher pressure. Wind blows from the land to the sea. This is a land breeze.


Global circulation and wind systems

Much of the solar energy that the earth receives causes intense heating in the equatorial regions.  This intense heat produces powerful convection in these areas.

As the warm, moist air rises, it creates a zone of low pressure, clouds, and precipitation along the equator.  As that warm air rises, it eventually reaches the troposphere and can rise no higher.  It spreads outwards towards the poles.

As it spreads, it cools and sinks back down to the surface at about 30 degrees north and south of the equator.  This sinking air produces areas of higher pressure with drier conditions.

Many of the world's deserts are located in these high pressure areas, around 30o north and south of the equator.
Examples?
Sahara Desert
Great Victoria Desert, Australia
Kalahari Desert
Sonoran Desert

Some of this air, as it sinks, moves back towards the equator. This air flowing back towards the equator produces what is know as the trade winds.

The trade winds obtained their name from the sailing ships used in foreign trade, which were called traders. These winds propelled the ships from Europe to the New World.

The area near the equator where these winds die out is referred to as the doldrums.

These areas where air rises at the equator, sink at 30 degrees north and south latitude, and then flow back to the equator are known as Hadley cells. Although most of the air that sinks at 30 degrees north and south latitude returns to the equator, some of it continues to move poleward.

At approximately 60 degrees north and south, this air meets cold polar air. The areas where these air masses meet form polar fronts. The air moving in from the lower latitudes is generally warmer and will rise. It then moves back towards the equator, sinking at about 30 degrees north and south. This sinking air contributes to the high pressure systems located there.

The circulation cells that form between 30 degrees and 60 degrees north and south are called Ferrel cells.

Some of the air that rises at the polar fronts continues to move poleward, sinking at the poles and then moving back towards 60 degrees north and south.
These Hadley cells are weaker than the tropical ones.

As the air that sinks at these locations flows back along the surface of the earth, it does not flow in a straight north-south path. This flow of air is affected by the Coriolis Effect.


The Coriolis Effect

Because of the earth's rotation, any freely moving object or fluid will appear to: (For example, note the curved wind arrows in the diagram below.)

This causes winds to travel clockwise around high pressure systems in the Northern Hemisphere, and counter-clockwise in the Southern Hemisphere.

Low pressure winds travel in the opposite direction (counter-clockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere).

Because of the Coriolis Effect, winds traveling along the surface of the earth from 30 - 60 degrees north and south of the equator flow from the west to the east.  These winds are referred to as the westerlies.


Idealized global circulation

The air currents moving along the surface of the Earth from the poles (90 degrees) to 60 degrees north and south of the equator, flow from east to west and are referred to as the polar easterlies.


Jet Stream

The jet streams are the result of the poleward-flowing air at latitudes between 30 - 60 degrees north and south of the equator (westerlies). The speed of this wind may exceed 160 km/hr (or 100 mi/hr).


Return to Earth & Space Science page

Return to Georgia Geoscience Online


Page created by Pamela J.W. Gore
Georgia Perimeter College,
Clarkston, GA

Page created March 31 - April 1, 2005
Web links updated to facstaff, email updated August 23, 2008