Download Winds - GEO

Chapter 5
Winds and Global Circulation
This chapter considers winds and ocean currents. It examines how unequal surface
heating and the rotation of the Earth generate global circulation systems in the
atmosphere and oceans.
 The weight of air and the force of gravity pulling air towards the Earth create air
pressure. Air pressure is greatest at the Earth's surface and decreases with altitude.
 Differences in pressure cause air to move horizontally. This air in motion is called
wind. Winds move from areas of high pressure to areas of low pressure.
 Pressure differences between two places create pressure gradients and the resulting
pressure gradient force causes air to move from high pressure areas to low pressure
areas.
 Land and sea breezes are examples of winds caused by pressure differences that
result from temperature differences over land and water surfaces.
 Wind direction is measured by a wind vane, and wind speed is measured by an
anemometer.
 The Coriolis effect is due to the Earth's rotation and causes objects in motion to
appear to be deflected off course. This apparent deflection is to the right in the
northern hemisphere and to the left in the southern hemisphere. The effect is absent at
the equator and increases as you move towards the poles.
 Another force affecting the direction of wind is that of friction.
 Air flow spirals into a low-pressure center and rises while the air descends and flows
out of a high pressure center.
 The inward spiral at a low-pressure center is counterclockwise in the northern
hemisphere and clockwise in the southern hemisphere.
 The outward spiral at a high-pressure center is clockwise in the northern
hemisphere and counterclockwise in the southern hemisphere.
 Cyclones (low pressure centers) are associated with cloudy or rainy weather.
Anticyclones (high pressure centers) are associated with clear, dry weather.
 At the equator, heating causes air to rise creating an area of low pressure called the
Intertropical Convergence zone (ITCZ).
 At 30° latitude, air descends creating areas of high pressure in the subtropical high
pressure belt. Air moves out of these high pressure areas toward the equator creating
the Trade Winds. Winds also move toward the midlatitudes creating the Westerlies.
 The monsoon is a seasonally reversing wind pattern that brings heavy rains onto the
Asian subcontinent in summer and hot, dry conditions in the winter.
 Winds at an altitude of five to seven kilometers above the Earth’s surface are
influenced by pressure gradient force and Coriolis force but not by the force of
friction. These winds are the geostrophic winds that flow parallel to isobars.
 Rossby waves are large undulations in the flow of the upper air Westerlies along the
zone of contact between cold and warm air. They allow warm air to penetrate
northward and cold air to penetrate southward.
 Jet streams are narrow bands of high velocity air that form primarily along the polar
front and above the Hadley cell in the subtropics.
 The uppermost layer of ocean water is the warmest. Below this warm layer,
temperatures decline rapidly to around 0° and remains cold in a layer extending to the
ocean floor.
 Ocean currents are persistent, mainly horizontal flows of ocean water set in motion
by the prevailing surface winds. Coreolis force causes the flows to be deflected about
45 ° from the direction of the wind.
 Gyres are circular movements of water that are driven by the subtropical high pressure
cells.
 El Niño occurs when warm water replaces the usual upwelling cold water that flows
along the South American coast. El Niño affects climate in other parts of the world.
Thermohaline circulation refers to slowly moving, deep ocean currents driven by the
sinking of cold, salty water in the northern Atlantic. This circulation is thought to play an
important role in the storage and release of CO2.