Air Pressure and Wind Atmospheric pressure Force exerted

Air Pressure and Wind

Atmospheric pressure Ø Force exerted by the weight of the air above Ø Decreases with increasing altitude Ø Units of measurement l l Millibar (mb) – standard sea level pressure is 1013. 2 mb Inches of mercury – standard sea level pressure is 29. 92 inches of mercury

Wind Ø Horizontal movement of air l l Out of areas of high pressure Into areas of low pressure Ø Controls of wind l Pressure gradient force • Isobars – lines of equal air pressure • Pressure gradient – pressure change over distance

Wind Ø Controls of wind l l Coriolis effect Friction

Wind Ø Upper air winds l l Generally blow parallel to isobars – called geostrophic winds Jet stream • • • "River" of air High altitude High velocity (120 -240) kilometers per hour

Rotating Air Bodies Ø Low Pressure Zone Formation l l l Warm air rises Creates a low pressure zone At the Earth’s surface, air “feeds” the low pressure zone, moves counterclockwise Ø High Pressure Zone Formation l l l Cool air sinks Creates a high pressure zone At the Earth’s surface, winds blow clockwise

Rotating Air Bodies Ø Bends in the polar jet create troughs and ridges Ø Forms cyclones and anticyclones

Rotating Air Bodies Ø Cyclones l l Low pressure zone in polar jet trough Winds at surface flow counterclockwise towards the core Air is updrafted and cooled Forms clouds, rain and upper level outflow of air

Rotating Air Bodies Ø Anticyclones l l l High pressure zone at ridge of polar jet Air converges in upper atmosphere Descends towards the ground Flows outward at surface Dry, windy conditions

Cyclones and Anticyclones

Cyclonic and anticyclonic winds in the Northern Hemisphere

General atmospheric circulation Ø Idealized global circulation l Equatorial low pressure zone • Rising air • Abundant precipitation

General atmospheric circulation Ø Idealized global circulation l Subtropical high pressure zone • Subsiding, stable, dry air • Location of great deserts • Air traveling equatorward from the subtropical high produces the trade winds • Air traveling poleward from the subtropical high produces the westerly winds

General atmospheric circulation Ø Idealized global circulation l Subpolar low pressure zone • Warm and cool winds interact • Polar front – an area of storms

General atmospheric circulation Ø Idealized global circulation l Polar high pressure zone • Cold, subsiding air • Air spreads equatorward and produces polar easterly winds • Polar easterlies collide with the westerlies along the polar front

General atmospheric circulation Ø Influence of continents l Seasonal temperature differences disrupt the • Global pressure patterns • Global wind patterns l Influence is most obvious in the Northern Hemisphere

General atmospheric circulation Ø Influence of continents l Monsoon • Seasonal change in wind direction • Occur over continents l l During warm months • Air flows onto land • Warm, moist air from the ocean Winter months • Air flows off the land • Dry, continental air

Local winds Ø Produced from temperature differences Ø Small scale winds Ø Types l l l Land sea breezes Mountain and valley breezes Chinook and Santa Ana winds

Illustration of a sea breeze and a land breeze

The Santa Ana Winds

Wind measurement Ø Two basic measurements l Direction • Winds are labeled from where they originate • Direction indicated by either l l Compass points (N, NE, etc. ) Scale of 0º to 360º • Prevailing wind comes more often from one direction l Speed • often measured with a cup anemometer

Wind measurement Ø Changes in wind direction l Associated with locations of • Cyclones • Anticyclones l Often bring changes in • Temperature • Moisture conditions

El Niño

Normal conditions in the tropical Pacific Ocean Ø Surface winds move from east to west Ø From high pressure in S. America to low pressure in Australia Ø Drags water westward Ø Warm water pools in the western Pacific

Normal conditions

Every 3 – 8 years, system reverses Ø Called the Southern Oscillation Ø Trade winds weaken or reverse Ø Warm water migrates from Australia to S. America Ø Arrives in time for Christmas – Corriente del Niño

El Niño

What is El Niño? Ø Basically, it's a giant puddle (or pod) of heated water that sloshes across the Pacific Ocean Ø Similar to an iceberg l l l Bulge on the surface Most of “pod” beneath the surface Due to difference in density Ø National Geographic’s Model

ENSO - El Niño-Southern Oscillation Ø Typically lasts 1 year Ø May last up to 3 Ø In multi-year events, first year not as affected Ø Affects both hemispheres

Recognizing an El Niño Ø Sea Surface Temperatures (SST) Ø Normal: 6 -8° C warmer in the western tropical Pacific than in the eastern tropical Pacific Ø Check SST to see if in “normal” range

La Niña Return to “normal” conditions from an El Niño strong Ø Produces: Ø l l l Strong currents Powerful upwelling Chilly and stormy conditions along S. American coast Eastern Pacific cools rapidly, Western Pacific warms rapidly Ø Renewed Trade Wind activity spreads the cooler eastern Pacific waters westward Ø

Global distribution of precipitation Ø Relatively complex pattern Ø Related to global wind and pressure patterns l High pressure regions • • Subsiding air Divergent winds Dry conditions e. g. , Sahara and Kalahari deserts

Global distribution of precipitation Ø Related to global wind and pressure patterns l Low pressure regions • • Ascending air Converging winds Ample precipitation e. g. , Amazon and Congo basins

Average annual precipitation in millimeters

Global distribution of precipitation Ø Related to distribution of land water l l Large landmasses in the middle latitudes often have less precipitation toward their centers Mountain barriers also alter precipitation patterns • Windward slopes receive abundant rainfall from orographic lifting • Leeward slopes are usually deficient in moisture

~ End ~