Winds

What is Wind? Wind- The horizontal movement of air from an area of high pressure to an area of lower pressure.

What causes winds?: Winds are caused by differences in air pressure. The pressure differences are caused by unequal heating of the Earth’s atmosphere.

More Dense air sinks –(Air Has High Pressure) Less Dense air rises- –(Air Has Low Pressure) What causes winds?:

How is wind measured? Weather Vane- an instrument used to determine the direction of the wind. Anemometer- An instrument used to measure wind speed.

Anemometer Cups blow in the wind and measure the speed of the wind.

Weather Vane Tells the direction the wind is blowing from. If it is pointing to the south- wind is blowing from the south to the north.

Global Winds

Belts of High and Low Pressure Global convection cells produce global belts of high and low pressure. –The Doldrums: The Doldrums is a belt low pressure (rising air) that runs along the equator. As the air rises along the equator, it cools. This causes water vapor in the air to condense, forming clouds and increased precipitation. As a result, most areas near the equator are warm and wet. Most tropical rain forests are near the equator. –The Horse Latitudes: The horse latitudes are belts of high pressure (sinking air) located at 30˚ north and south latitudes. Along these latitudes, warm, dry air sinks toward the ground. This sinking air produces belts of high pressure. Remember, that high-pressure systems are associated with dry weather. Most of the world’s deserts are located along these latitudes.

Prevailing winds are large-scale winds that blow in the same direction most of the time. Prevailing winds can significantly affect an area’s climate. There are three main groups of prevailing winds: –Trade Winds: The Trade winds are prevailing winds that blow westward (from east to west) carrying warm, moist air from the equator toward 30º north and south latitude. –Prevailing Westerlies: The Prevailing Westerlies blow eastward (from west to east) between 30˚ to 60˚. These winds carry warm air to the middle latitudes creating temperate climates. In the Northern Hemisphere, most of the United States lies within these latitudes. –Polar Easterlies: The Polar Easterlies are prevailing winds that blow westward (from east to west) from the poles to 60˚ latitude. At 60˚ latitude, the Westerlies collide with the Polar Easterlies. The warm air is forced up over the cold air. Water vapor in the warm air condenses, producing precipitation. The poles are extremely dry and cold areas because the air sinks there. Global Winds

Local Winds Are winds that blow over short distances. Caused by unequal heating of Earth’s surface with in a small area. –Land heats faster than water. –Water cools slower than land.

Sea Breeze- The flow of air from a body of water to land. –Water is cooler – more dense air sinks. –Land is warmer- less dense air rises. Local Winds

Land Breeze- The flow of air from land to a body of water. –Water is warmer – less dense air rises. –Land is cooler- more dense air sinks. Local Winds

Valley Breeze Occurs during the day. Uneven heating of the mountain slopes and the valley. Areas of lower pressure near mountain tops. The Valley Breeze flows from the valley up the slopes. Local Winds Low Pressure High Pressure

Local Winds Mountain Breeze Occurs at nightfall. Areas of higher pressure along the mountain slopes. The air around the mountain slopes cools creating higher pressure. Wind moves from the mountain slopes down into the valley. High Pressure Low Pressure

Reading Wind Maps Cloud Cover The line points into the weather station from the direction the wind came from. This example shows that the wind is coming from the east.

Reading Wind Maps Cloud Cover To determine wind speed. Read the flag on the end of the wind direction line. Use the chart to determine the wind speeds in knots and mph. Example above: 8-12 knots (9-14mph)