Understanding Winds: From Local Breezes to Global Circulation

1. Winds The Movement of Air

2. Wind • “Air in motion.” • The main factor that causes winds is uneven heating of the earth’s surface! • Convection currents drive all wind patterns. • Masses of air travel from one place to another because of temperature and pressure gradients. • Differences in air pressure are caused by differences in temperature.

3. Properties of Air Masses • As air touching the ground heats up, it expands and becomes less dense. • Less dense air rises. • Warm air can hold more moisture. • Air aloft cools down, and condenses or shrinks, becoming more dense. • More dense air sinks. • Cool air can hold less moisture.

4. Convection Currents • Driven by differences in density. • As air moves from one place to another, more air rushes in to replace it from the side. • This sets up a continuous cycle of moving air called a convection cell. • Air moving from the side is called “wind”.

5. Pressure Gradients • Differences in pressure that push or pull air masses from places with more air (high pressure) to places with less air (low pressure). • Wind always moves from areas of high pressure to areas of low pressure. • The greater the difference in pressure, the stronger the wind.

6. Naming Winds • Winds are named according to the place the come from. • A north wind (northerly) comes from the north, a westerly from the west, and so on. • There are very few exceptions to this rule.

7. Local & Seasonal Winds Winds on a Relatively Small Scale

8. Sea Breeze • Wind that moves from the sea to the land. • Carries cool, moist air and frequently causes rain. • Driven by convection cells. • Land heats faster than water, so the land has low pressure while the sea has high pressure. • Air rises over the land and sinks over the sea. • Air moves in sideways to fill in the low pressure area over the land. • Happens at beaches and lakes during the day.

9. Land Breeze • Wind that moves from the land to the sea. • Carries warm, dry air and frequently causes clear weather. • Driven by convection cells. • Land cools faster than water, so the land has high pressure while the sea has low pressure. • Air sinks over the land and rises over the sea. • Air moves in sideways to fill in the low pressure area over the sea. • Happens at beaches and lakes during the night.

10. Monsoon • Seasonal winds that affect the tropical regions of western Asia. • A special case of a land/sea breeze pattern that lasts for an entire season. • The warm western Pacific Ocean causes a land breeze all winter long, creating a “dry season”. • The tropical land heats up more during the summer, causing a “rainy season”.

11. Mountain Breeze • Wind that moves down from a mountain to the valley below. • Also called a “katabatic wind”, or a downslope wind. • Carries cold, moist air and frequently causes fogs and severe frosts. • Driven by convection cells. • The tops of mountains cool faster than the valleys below, so the mountain has high pressure while the valley has low pressure. • Air sinks over the mountain, falling into the valley. • Happens at mountains during the night.

12. Valley Breeze • Wind that moves up from a valley to a mountain. • Also called an “anabatic wind”, or an upslope wind. • Driven by convection cells. • The tops of mountains heat up faster than the valleys below in the morning, so the mountain has low pressure while the valley below has high pressure. • Air rises over the mountain, then circulates and falls back into the valley. • Happens at mountains during the day.

13. Global Winds Air Circulation Around the World

15. Uneven Heating • The poles receive little direct sunlight, and are always cool. • The cold air over the poles is constantly sinking. • A permanent high pressure system sits over both poles, bringing clear, dry weather. • The equator receives lots of direct sunlight, and is always warm. • The hot air over the equator is constantly rising. • A permanent low pressure system sits over the equator, bringing rainy weather.

16. Permanent Pressure Systems • Permanent low pressure at 0° (equator). • Permanent high pressure at 30° north and south (horse latitudes). • Permanent low pressure at 60° north and south. • Permanent high pressure at 90° north and south (poles). • The differences in pressure between these zones drives global winds!

17. Coriolis Effect • The deflection of free moving objects to one side or the other due to the earth’s rotation. • Winds and currents are curved to the right north of the equator. • Winds and currents are curved to the left south of the equator. • The Coriolis Effect breaks the global circulation pattern into three distinct bands from the equator to the poles, and generates the pressure bands at 30° and 60°. • Without the earth’s rotation, global winds would only travel in one direction!

18. Major Surface Wind Patterns • Trade Winds • Prevailing Westerlies • Polar Easterlies • Doldrums • Horse Latitudes

20. Trade Winds • Found between 0° and 30° north and south. • Blow from the northeast in the northern hemisphere, and from the southeast in the southern hemisphere (towards the equator and from the east). • Named because they aided trading ships in the great sailing days.

21. Prevailing Westerlies • Found between 30° and 60° north and south. • Blow from the southwest in the northern hemisphere, and from the northwest in the southern hemisphere (towards the poles and from the west). • Provided reliable route from Americas to Europe.

22. Polar Easterlies • Found between 60° and 90° north and south. • Blow from the northeast in the northern hemisphere, and from the southeast in the southern hemisphere (from the poles and east). • Exact same direction as the Trade Winds, opposite of the Prevailing Westerlies.

23. Doldrums • Windless area around the equator. • Permanent low pressure area means air rises instead of moving to the side. • Air rises swiftly from the surface: air masses go up before they get there, leaving a belt with no surface winds. • Rising, moist air causes a band of rain along the doldrums much of the time. • Sailors would get stuck there for days because of the lack of wind.

24. Horse Latitudes • Windless area around 30° north and south. • Permanent high pressure area means air sinks instead of moving to the side. • Air sinks swiftly: air masses go down immediately surrounding the area, leaving a belt with no surface winds. • Sinking, dry air causes a band of deserts to exist along the Horse Latitudes around the globe. • Named because sailors would get stuck there and would often have to dump their cargo to lighten the load...

25. Video Bibliography • Investigating Weather. United Learning (1995). Retrieved January 27, 2006, from unitedstreaming: http://www.unitedstreaming.com/ • Weather Smart: Heat, Wind, and Pressure. United Learning (2001). Retrieved January 27, 2006, from unitedstreaming: http://www.unitedstreaming.com/ • Science Investigations Earth Science: Investigating Weather and Climate. Discovery Channel School (2004). Retrieved January 27, 2006, from unitedstreaming: http://www.unitedstreaming.com/ • Physical Science: Heat. Discovery Channel School (2002). Retrieved January 27, 2006, from unitedstreaming: http://www.unitedstreaming.com/ • Exploring Weather: The Atmosphere in Motion. United Learning (1993). Retrieved January 27, 2006, from unitedstreaming: http://www.unitedstreaming.com/