ES 106 2006 May 19 Air Pressure and Wind, Air masses, fronts, mid-latitude cyclones, Severe Storms 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 pressureA. 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 air7. 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 hemisphereVIII. Air Masses A. Defined: large body of air, 1600 km or more across, with similar temperature and moisture at similar altitudes 1. Brings these characteristics with it as it moves to different areas 2. example a. Canadian continental air mass goes to Mexico b. Air mass temperature changes, but brings its cold
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