Chapter 6 Basics Pressure force per unit area Force mass X acceleration Force molecules above X gravity Air density is constant with height Effect of Temperature on Pressure Add molecules pressure rises Remove molecules pressure falls Cool the air it becomes more dense Warm the air it becomes less dense Measuring air pressure Air blows toward the lows it rises in lows and sinks in highs Barometers measured in Inches of Mercury 29 92 in and Millibars mb a metric unit 1013 mb Mercury barometer uses mercury in which the height of the mercury column is a measure of atmospheric pressure Aneroid barometer instrument designed to measure atmospheric pressure but contains no liquid Barograph a recording barometer is a type of aneroid barometer Horizontal change in pressure causes wind Station Pressure vs Sea Level Pressure If pressure changes greatly in horizontal strong wind If pressure changes little in horizontal weak wind Station Pressure barometer reading of actual air pressure computed at the observing station after being corrected for temperature gravity and instrument error varies by location and elevation Altitude corrections are made so that a barometer reading taken at one elevation can be compared with a barometer reading taken at another Station pressure observations are normally adjusted to a level of mean sea level the level representing the average surface of the ocean The adjusted reading is called sea level pressure Pressure changes 10 mb per 100 meters altitude change Isobars lines connecting points of equal pressure are drawn as solid dark lines at intervals of 4 mb with 1000 mb being the base value Charting pressure data With isobars and weather phenomena charted it becomes known as a surface map Constant pressure surfaces 500 mb For example at this height a surface of constant pressure rises in warm less dense air and lowers in cold more dense air This is because of the changes in air density These changes in height of a constant pressure 500 mb surface show up as contour lines lines that connect points of equal altitude above sea level on a constant pressure isobaric 500 mb map Air moves wind blows because air is acted on by several forces the net force of all 4 forces determines wind speed and direction from which the air is blowing Pressure Gradient Force the force due to differences in pressure within the atmosphere that causes air to move and hence wind to blow It is directly proportional to the pressure gradient the rate of decrease of pressure per unit of horizontal distance Pressure gradient force is perpendicular to isobars and directed toward lower pressure The closer the isobars the stronger the pressure gradient force and winds are stronger Coriolis Force an apparent force observed on any free moving object in a rotating system Named after Mr Coriolis Not a push or a pull Not a real force Accounts for the earth s rotation Pulls bends to the right of motion in the Northern Hemisphere and to the left in the Southern Hemisphere force is 0 at the equator a maximum at poles force is 0 for calm winds strong for strong winds Coriolis force causes it to bend to the right in the n hemisphere left in s What happens when you release a parcel balloon of air The Frictional force Slows the wind Wind blows into lows and out of highs Effects of friction a The effect of surface friction is to slow down the wind so that near the ground the wind crosses the isobars and blows toward lower pressure b This phenomenon at the surface produces an inflow of air around a low and an outflow of air around a high Aloft the winds blow parallel to the lines usually in a wavy west to east pattern Both diagram a and b are in the Northern Hemisphere ALOFT SURFACE Watching clouds above 10 000 feet can disregard friction with Buys Ballot s Law a law describing the relationship between the wind direction and the pressure distribution in the northern hemisphere if you stand with your back to the surface wind then turn clockwise about 30 lower pressure will be to your left In the southern hemisphere stand with your back to the surface wind then turn counterclockwise about 30 lower pressure will be to your right To further explain this drawing of a simplified upper level chart is based on cloud observations Upper level clouds moving from the southwest a indicate isobars and winds aloft b When extended horizontally the upper level chart appears as in c where lower pressure is to the west and higher pressure is to the east Centrifugal force when air follows a curved path What else happens when you release a parcel balloon of air 1 Above the level of friction air initially at rest will accelerate until it flows parallel to the isobars at a steady speed with the pressure gradient force PGF balanced by the Coriolis force CF Wind blowing under these conditions is called geostrophic Geostrophic wind a theoretical horizontal wind blowing in a straight path parallel to the isobars or contours at a constant speed results when the Coriolis force exactly balances the horizontal pressure gradient force Is a crude approximation to the real wind because it only considers pressure gradient and Coriolis forces 2 1 2 Motion of air Divergence an atmospheric condition that exists when the winds cause a horizontal net outflow of air from a specific region Convergence almost like divergence but the winds cause a horizontal net inflow of air Rising sinking is slow about 1 mile per day Why don t we lose our atmosphere Hydrostatic equilibrium the state of atmosphere when there is a balance between the vertical pressure gradient force and the downward pull of gravity Pressure gradient force is upward H to L but gravity almost perfectly balances the pressure gradient force Wind rose tells how often wind is coming from each direction This wind rose represents the percent of time the wind blew from different directions at a given site during the month of January for the past ten years The prevailing wind is NW and the wind direction of least frequency is NE Measuring wind direction and speed Wind vane an instrument used to indicate wind direction and cup anemometer Aerovane a wind instrument that indicates or records both wind speed and wind direction Doppler Radar determines the velocity of falling precipitation either toward or away from the radar unit by taking into account the Doppler shift A radar pulse is sent then listened to Some pulse returned Amount returned describes precipitation intensity Frequency is shifted
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