Air Pressure and Wind

Atmospheric pressure Force exerted by the weight of the air above Decreases with increasing altitude Units of measurement Millibar (mb) – standard sea level pressure is 1013.2 mb Inches of mercury – standard sea level pressure is 29.92 inches of mercury

Atmospheric pressure Instruments for measuring Barometer Mercury barometer Aneroid barometer Barograph (continuously records the air pressure)

A recording aneroid barometer A mercury barometer A recording aneroid barometer

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

A weather map showing isobars and wind speed/direction

Wind Controls of wind Coriolis effect Friction

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

The geostrophic wind

Comparison between upper-level winds and surface winds

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

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

Rotating Air Bodies Cyclones 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 Anticyclones 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 To view this animation, click “View” and then “Slide Show” on the top navigation bar.

Cyclonic and anticyclonic winds in the Northern Hemisphere

General atmospheric circulation Underlying cause is unequal surface heating On the rotating Earth there are three pairs of atmospheric cells that redistribute the heat

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

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

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

Average surface pressure and associated winds for January

Average surface pressure and associated winds for July

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

Circulation in the mid-latitudes The zone of the westerlies Complex Air flow is interrupted by cyclones Cells move west to east in the Northern Hemisphere Create anticyclonic and cyclonic flow Paths of the cyclones and anticyclones are associated with the upper-level airflow

Local winds Produced from temperature differences Small scale winds Types Land and 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 Two basic measurements Direction Speed Winds are labeled from where they originate Direction indicated by either Compass points (N, NE, etc.) Scale of 0º to 360º Prevailing wind comes more often from one direction Speed often measured with a cup anemometer

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

El Niño

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

Normal (Non-El Niño) conditions Trade winds blow towards the west across the tropical Pacific. These winds pile up warm surface water in the west Pacific, so that the sea surface is about 1/2 meter higher at Indonesia than at Ecuador. The sea surface temperature is about 8 degrees C higher in the west, with cool temperatures off South America, due to an upwelling of cold water from deeper levels. This cold water is nutrient-rich, supporting high levels of primary productivity, diverse marine ecosystems, and major fisheries. Rainfall is found in rising air over the warmest water, and the east Pacific is relatively dry. The observations at 110 W (left diagram of 110 W conditions) show that the cool water (below about 17 degrees C, the black band in these plots) is within 50m of the surface.

Normal conditions

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

During El Niño, the trade winds relax in the central and western Pacific leading to a depression of the thermocline in the eastern Pacific, and an elevation of the thermocline in the west. The observations at 110W show, for example, that during 1982-1983, the 17-degree isotherm dropped to about 150m depth. This reduced the efficiency of upwelling to cool the surface and cut off the supply of nutrient rich thermocline water to the euphotic zone. The result was a rise in sea surface temperature and a drastic decline in primary productivity, the latter of which adversely affected higher trophic levels of the food chain, including commercial fisheries in this region. The weakening of easterly tradewinds during El Niño is evident in this figure as well. Rainfall follows the warm water eastward, with associated flooding in Peru and drought in Indonesia and Australia. The eastward displacement of the atmospheric heat source overlaying the warmest water results in large changes in the global atmospheric circulation, which in turn force changes in weather in regions far removed from the tropical Pacific.

El Niño

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

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

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

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

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

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

Average annual precipitation in millimeters

Global distribution of precipitation Related to distribution of land and water 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

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