Air Pressure and Winds

1. Air Pressure and Winds Earth Science Ch. 19

2. 19.1 Understanding Air Pressure • Variations in air pressure generates winds. • Winds can bring changes in temperature and humidity • Air pressure in measured in atmospheres (ATM)

3. Measuring Air Pressure • When meteorologists measure atmospheric pressure they use a unit called millibars (mb). • Standard sea level pressure is 1013.2 mb. • A barometer is a device used for measuring air pressure. • A barometer measures air pressure in inches of mercury. • Standard air pressure at sea level when measured in inches of mercury is 29.92.

5. An aneroid barometer can also be used to measure air pressure. • An aneroid barometer uses a metal chamber with some air removed. This chamber is sensitive to changes in air pressure.

7. Factors Affecting Wind • Wind is the result of horizontal differences in air pressure. • Air flows from areas of high pressure to low pressure. • The unequal heating of Earth’s surface generates pressure differences. • Solar radiation is the ultimate energy source for most wind.

8. There are 3 factors that combine to control wind: • Pressure differences • The greater the differences in air pressure the greater the wind speed • Differences in air pressure are shown on a weather map using isobars. • Isobars are lines on a map that connect points of equal pressure. • Winds always blow from high pressure to low pressure.

10. Coriolis Effect • The change in directions of wind caused by the Earth’s rotation. • All winds are deflected to the right in the Northern hemisphere and to the left in the Southern hemisphere.

12. Friction • Friction slows air movement which changes wind direction • The changing terrain of the of the earth’s surface causes friction

13. The Jet Stream • The most prominent airflows high above the friction layer are the jet streams. • Jet streams are fast moving rivers of air that travel in a west to east direction and impact the weather of the US

15. 19.2 Pressure Centers and Winds • Lows • Cyclones • Pressure decreases at the surface. • Counterclockwise • Air flows inward and converges. • Air rises • Clouds and precipitation. • Highs • Anticyclones • Pressure increases at the surface • Clockwise • Air sinks and flows outward. • Clear skies and good weather.

18. Global Winds • The cause of wind is the unequal heating of Earth’s surface • Tropical regions receive more solar radiation than the poles. • The atmosphere balances these differences by moving warm air toward the poles and cool air toward the equator.

19. Each hemisphere has 3 convection cells • These 3 cells are called Hadley cells, Ferrel cells, and Polar cells • The winds in each cell move from high to low pressure.

20. Equitorial Low • Air rises at the equator • A lot of precipitation due to cooling and condensing of rising air • Caused by convergence of the trade winds • Intertropical convergence zone (ITCZ) • Occurs near the equator (within 5° N and S) • Also known as the doldrums due to lack of air movement • Location where trade winds come together

21. Subtropical Highs • Air sinks at around 30° north and south latitude • Dry conditions, common locations of deserts • The sinking air moves outward from the high • Winds created from the Subtropical Highs • Trade winds – move toward the equator • Prevailing westerlies – move toward the poles, directly effect our weather in the U.S. • Sometimes referred to as the horse latitudes • Sailors would throw horses overboard in order to conserve water.

22. Subpolar low • Air that rises at around 60° north and south • Caused by convergence of the prevailing westerlies and polar easterlies • Polar high • Air that sinks at the poles • The polar easterlies are created by the sinking air • The polar easterlies move from the polar high to the subpolar low

23. 90° 60° 30° 0° 30° 60° 90°

24. In polar regions cold polar air sinks and spreads toward the equator. • These cold winds interact with the warm winds coming from the equator • The interactions of these air masses produces a stormy belt in the middle latitudes called the polar front.

26. 19.3 Regional Wind Systems • Local winds are caused by either changes in topography or by differences between land and water. • There are 2 types of local winds: • Land and Sea Breezes • Valley and Mountain Breezes

27. Land and Sea Breezes • During the day the land is heated more than the water. • The air above the land rises creating an area of low pressure • The air over the water is cooler and more dense creating an area of high pressure • A sea breeze develops as the high pressure air over the water replaces the low pressure air over the land • The breeze comes from the sea which gives the name

28. At night the reverse takes place • The air over the water is warmer than the air over the land • The low pressure air over the water rises and is replaced by the high pressure air from the land • The breeze comes from the land which gives the name

30. Valley and Mountain Breezes • During the day the air on the slopes of mountains is warmer than the air over the valley • The breeze moves up the slope because the air is less dense • A valley breeze forms because the air is rising from the valley up the slope

31. At night the pattern reverses • The air along the mountain slopes cools • The cool air is more dense so it sinks from the slopes into the valley • This is called a mountain breeze

33. How wind is measured • Winds are always labeled with the direction that are coming from • The instrument most commonly used to determine wind direction is a wind vane • Wind vanes always point into the wind • Wind directions can either be given by points on a compass (N, S, E, W) or on a degree scale • 0° – North, 90° – East, 180° – South, 270° - West

35. Wind Direction • When the wind consistently blows from one direction it is called a prevailing wind • In the US the westerlies consistently move weather from west to east across the continent • Wind Speed • An anemometer is an instrument used to measure wind speed • When an anemometer is used to measure wind speed the speed is measured from a dial much like a speedometer. • A wind sock can be used to determine both wind direction and wind speed.