Global Winds and Local Winds Chapter menu Resources

THINK ABOUT IT! • We have learned that the Earth is heated from the sun unevenly. • Warm air is less dense (lighter), and exerts less air pressure than cool air. Because of this, warm air rises and cool air sinks. How do you think this affects our weather locally and globally? Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved.

• Wind is moving air caused by differences in air pressure. Air moves from areas of high pressure to areas of low pressure. (This is why your ears “pop” when you ascend in an airplane – as the plane rises to where there is less air pressure, the air in your ears balance and you feel it as a “pop”) The greater the pressure difference, the faster the air moves, and the stronger the wind blows. Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved.

• Most differences in air pressure are caused by differences in air temperatures. Temperature differences happen because some parts of the Earth get more energy from the sun than others. Since the sun shines more directly on the equator than on the poles, the air is warmer near the equator. Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved.

• Since the air near the equator is less dense, it rises forming areas of low pressure. • The cold air near the poles sinks because it is more dense, forming areas of high pressure. • The air moves in large circular patterns called convection cells. Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved.

Chapter 15 Section 3 Global Winds and Local Winds Why Air Moves • Air Rises at the Equator and Sinks at the Poles As the cold air sinks, it creates areas of high pressure around the poles. This cold polar air then flows toward the equator. Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved.

Chapter 15 Section 3 Global Winds and Local Winds Chapter menu Resources Copyright ©

• Wind • Pressure Belts and Convection Cells Chapter menu Resources Copyright ©

Chapter 15 Section 3 Global Winds and Local Winds Why Air Moves, continued • The Coriolis Effect The apparent curving of the path of currents due to the Earth’s rotation is called the Coriolis effect; the deflection, or curving, of moving objects from a straight path. Wind moving from the poles to the equator is deflected to the west. Wind moving from the equator to the poles is deflected to the east. In the Northern Hemisphere the winds travel in a clockwise direction, and in the Southern Hemisphere the winds travel in a counter-clockwise direction. Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved.

• Coriolis Effect Chapter menu Resources Copyright © by Holt, Rinehart and Winston.

Chapter 15 Section 3 Global Winds and Local Winds Global Winds • Polar Easterlies are the wind belts that extend from the poles to 60° latitude in both hemispheres. They form as cold, sinking air moves away from the poles. They can carry cold arctic air over the U. S. which can produce snow and freezing weather. • Westerlies are the wind belts found between 30° and 60° latitude in both hemispheres. They blow toward the poles from west to east. These winds can carry moist air of the U. S. producing rain and snow. • Trade Winds are the winds that blow from 30° latitude almost to the equator in both hemispheres. They curve to the West as they blow toward the equator. Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved.

Chapter 15 Section 3 Global Winds and Local Winds Global Winds, continued The Doldrums The trade winds of the Northern and Southern Hemispheres meet in an area around the equator called the doldrums. there is very little wind because the warm, rising air creates an area of low pressure. The name doldrums means “dull” or “sluggish. ” • The Horse Latitudes At about 30° north and 30° south latitude, sinking air creates an area of high pressure called the horse latitudes. According to legend, this name was given to these areas when sailing ships carried horses from Europe to the Americas. When the ships were stuck in this windless area, horses were sometimes thrown overboard to save drinking water for the sailors. Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved.

• Jet Streams are narrow belts of high-speed winds that blow in the upper troposphere and lower stratosphere. Knowing the path of a jet stream is important not only to pilots but also to meteorologists. Because jet streams affect the movement of storms, meteorologists can track a storm if they know the location of a jet stream. Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved.

• Types of Wind Chapter menu Resources Copyright © by Holt, Rinehart and

Chapter 15 Section 3 Global Winds and Local Winds • Local winds generally move short distances and can blow from any direction. • Mountain and valley breezes are examples of local winds caused by an area’s geography. • Sea and land breezes are affected by temperature. Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved.