Winds The Movement of Air Wind Air in

Winds The Movement of Air

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.

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.

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”.

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.

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.

Local & Seasonal Winds on a Relatively Small Scale

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 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.

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 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.

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”.

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.

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.

Global Winds Air Circulation Around the World

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.

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!

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!

Major Surface Wind Patterns § § § Trade Winds Prevailing Westerlies Polar Easterlies Doldrums Horse Latitudes

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.

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.

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.

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.

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. . .

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/