Air Pressure and WindChapter 19

What is Air Pressure? Air pressure weight of air

above

Exerted in all directions (up, down, and sideways)

Measuring Air Pressure

Unit: called the millibar

Barometer: device used for measuring air pressure

Types of barometers Mercury

When air pressure increases, the mercury in the tube rises

When air pressure decreases, the mercury in the tube goes down

Aneroid recording mechanism

providing a continuous record of pressure change over time

Wind The result of horizontal

differences in air pressure Air flows from areas of

high to low. The unequal heating of

Earth’s surface generates pressure differences.

Solar energy is the ultimate energy source for most wind.

Three Factors that Combine to Control Wind

Pressure Differences Coriolis Effect Friction

Pressure Differences Wind is created from differences in pressure.

The greater the difference the greater the wind speed.

Shown using isobars: Lines on a map

that connect places of equal air pressure

Closely spaced isobars indicate high winds.

Isobars Closely spaced isobars:

indicate a steep pressure gradient and high winds.

Widely spaced isobars: indicate a weak pressure gradient and light winds.

Coriolis Effect The Coriolis effect

describes how Earth’s rotation affects moving objects.

Northern Hemisphere Deflected to right

Southern Hemisphere Deflected to the left

Strongest at the poles, weakest at the equator

Coriolis video

Friction Important only

within a few km of Earth’s surface

Slows air movement, which changes wind direction

Jet Streams Jet streams: fast-

moving rivers of air

Travel between 120 and 240 kilometers per hour

High and Low Pressure System

Cyclones Anticyclones

Centers of Low Pressure High PressureWind Behavior

Winds blow inward and counterclockwise

Winds blow outward and clockwise

Weather Associated

Severe and stormy Fair and Sunny

Symbol “L” that is RED “H” that is BLUE

Underlying cause of wind is the unequal heating of Earth’s surface

Atmosphere balances these differences by acting as a giant heat-transferring system

Warm air towards the poles

Cool air toward equator

Global Winds

Global Winds Trade winds

Blow constantly from east Westerlies

Middle latitudes Blow from the west

Polar Easterlies Not constant Blow east to west

Polar Front: Interaction of war and cool air

masses produces a stormy belt

Four Pressure Zones

Highs: Subtropical and Polar Dry subsiding air Prevailing winds

Lows: Equatorial and Subpolar Inward and upward air Clouds and precipitation

Influence of Continents Only true continuous

pressure belt is the subpolar low.

Uninterrupted by landmasses

Can produce seasonal temperature differences

Monsoons Seasonal changes in

wind direction Summers: rainy Winters: dry

Caused by topographic effects or variations in surface composition

Types of Local Winds Land Breezes Sea Breezes Valley Breezes Mountain Breezes

Local Winds

Sea and Land Breezes

Sea Breezes Day Land heated more

intensely So air above the land:

heats, expands, and rises Cool air over water moves

toward land.

• Land Breezes Night Land cools more quickly. Cooler Air over land

moves to the sea

Valley and Mountain Breezes

ValleyDayLand is heated more intenselyAir moves up along the slop

MountainNightRapidly cooling air along the slopesMoves downward

Measuring Wind

Direction Always labeled by the

direction from which they blow

Prevailing Wind: wind consistently blows from one direction.

Wind Speed Anemometer

Measures how fast wind is blowing

El Niño Episodes of ocean warming that affect the eastern

tropical Pacific. Warm countercurrents replace normally cold

offshore waters.

El Nino