ES 106 Air Pressure and Wind I Pressure A 14 7 lab in2 exerted in all directions up down sideways B Measuring air pressure with barometer 1 millibars standard sea level pressure 1013 2 mb 2 inches of mercury a rises in evacuated tube from pressure on open dish b standard sea level pressure 29 92 inches 3 aneroid barometer uses partly evacuated metal chamber a high fair b low storm c overgeneralization 4 barograph records pressure continuously II Wind A Horizontal movement of air advection 1 flows due to pressure differences Pressure Gradient Force a from high to low b created by unequal heating of Earth s surface 2 affected by surface friction 3 affected by Coriolis Effect B pressure gradient force 1 maps drawn of pressure shown with isobars equal pressure lines 2 spacing of isobars shows the pressure gradient 3 wind blows more strongly with larger pressure gradients 4 initial direction from high pressure toward low pressure but C Coriolis Effect begins to affect direction 1 general mechanism a deflected to right of their path in Northern Hemisphere b deflected to left of their path in Southern Hemisphere c regardless of direction of travel d not affected at equator 2 affect on wind flow a changes direction at 90O angle to wind flow b does not affect wind speed c wind speeds affect amount of Coriolis Effect 1 greater speeds more deflection 2 slower speeds less deflection D friction of Earth s surface affects wind flow 1 upper levels of atmosphere not affected by friction a wind flow follows isobars b geostrophic winds 2 slows wind speeds at lower levels of atmosphere a reduces amount of Coriolis Effect pressure gradient prevails b surface winds directed toward low pressure at angle across isobars c surface roughness affects amount of surface friction III High pressure and low pressure A Low pressure called a cyclone 1 northern hemisphere cyclones turn counterclockwise as winds blow inward toward low pressure and are deflected to right by Coriolis effect 2 southern hemisphere cyclones turn clockwise by same effect 3 flow inward results in surface convergence a creating uplift and storminess due to expansion and cooling b consequent divergence aloft may become stronger than surface convergence and intensify cyclone B High pressure called anticyclone the opposite of cyclone 1 winds flow outward 2 surface divergence at center a convergence aloft where air drawn into area of divergence b subsiding air precludes rainfall because it is compressed and warms C these effects are the basis for fair and stormy indications on barometer D isobar maps show high pressure ridges and low pressure troughs IV General circulation of the atmosphere A Greatest heating in tropics creates uplift of rising air 1 flow from poles to equator would occur without Coriolis Effect or friction 2 these break single circulation into smaller cells with surface directions B Idealized global circulation 1 equatorial low created by Sun heating a abundant precipitation b 20 30O N and S of equator c Cooling aloft and poleward flow 2 descending air about 30O N and S of equator a subtropical high pressure b descending air does not rain desert belts across Earth 3 wind flow between equatorial low and subtropical high a affected by Coriolis b creates Trade Winds 4 poleward flow at surface from subtropical high deflected into Westerlies 5 cold dense air from Polar High converges with Westerlies to create subpolar low a Polar easteries occur here b Polar front is interaction of cold polar air and warmer midlatitudes air 6 Jet Streams are geostrophic winds created at the interaction of global circulation patterns a Polar front jet stream at the polar front Rossby waves b Subtropical jet stream between tropical and midlatitudes air 7 continents interfere with idealized global circulation a result is closed semi permanent pressure cells b high pressure 1 Pacific High persistent 2 Azores high seasonal winter 3 Siberian High seasonal winter a Results in offshore winds from Asia to Indian Ocean dry b Summer heating draws air off Indian Ocean wet Monsoon 4 Bermuda High seasonal summer c Low pressure seasonal winter source of storms 1 Aleutian low 2 Icelandic low 8 Westerlies a Coriolis Effect creates wind from west to east b Interrupted by migrating cyclonic systems bringing weather 1 Cyclones driven by upper level wind flow 2 Upper level flow migrates seasonally a Winter months allow storms further toward equator b Summer month storms generally further poleward V Local wind systems created by local temperature and pressure differences A Land and Sea Breezes 1 heating land in daytime causes rising air 2 air drawn in off sea is a sea breeze 3 nighttime cooling of land leaves sea warmer 4 air drawn toward sea from land is land breeze B Mountain and Valley Breezes 1 similar to Land and Sea Breezes due to temperature changes 2 daytime heating of slopes results in a valley breeze more predominant in summer 3 nighttime cooling of upper areas can chill air to descent slopes as mountain breezes most predominant in winter C downslope strong drying warm winds have local names Chinook Santa na Mistral VI Measuring wind A Winds named for the direction from which they come B Prevailing wind describes the usual direction of wind 1 sometimes indicated by slant of tree trunks or branch density 2 US has generally west winds we are in the Westerly Wind Belt 3 interference of migrating cyclonic systems C anemometer measures wind speed D some areas have very reliable predominant winds 1 knowledge of persistent pressure patterns helps predict these 2 Trade Winds are example VII El Nino and La Nina A Interruption of Trade winds and equatorial oceanic current B Consequent see saw of pressure centers in southern hemisphere
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