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Download Air Pressure and Winds - Southington Public Schools
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10/8/2011 Air Pressure • Pressure exerted by the weight of the air above • Exerted in all directions • You have felt this pressure your entire life Air Pressure and Winds Chapter 6 • Measuring air pressure • Millibars • (100 Newtons per square meter) • Newton = force needed to move an object • Sea Level = 1013.25mb • Inches of Mercury • Mercury barometer • Sea Level = 29.92 mb Measuring Air Pressure • Aneroid barometer • Without liquid • Metal chamber Measuring Air Pressure • Aneroid barometer • Connects to barograph • Continuous record of pressure • Sensitive to air pressure • Expands and compresses • Corresponds to dial • Good predictor of weather • Pressure (barometric) tendency • Not necessarily present weather • Decreasing pressure • Clouds, precip., and warm • Increasing pressure • Clear sky's and cool Measuring Air Pressure • Comparing pressure readings from various weather stations • Compensation for elevation • Convert to sea level equivalents • Ideal gas law Pressure Changes with Altitude • Increase in altitude = decrease in pressure • • • • Sea Level – highest pressure Entire atmosphere above Denser air – higher pressure The pressure at any given altitude in the atmosphere is equal to the weight of air directly above that point. • Less dense with height • Decrease in pressure 1 10/8/2011 Pressure Changes with Altitude • Vertical distribution of atmospheric pressure. • Standard atmosphere • Pressure decreases • One-half every 5km Horizontal Variations in Air Pressure • Pressure differences relatively small • Typical range • 1040 – 950 mb • 31 – 28 inches of mercury • High Pressure • Cold dense Air • Low Pressure • Warm, less dense air • Humidity • humidity density • Water vapor is not as dense as Nitrogen and Oxygen Airflow and Pressure • Convergence • • • • The coming together of air Piles up Air is denser Pressure increases Factors Affecting Wind • Wind • Horizontal movement of air • Results from • Horizontal differences in pressure • Flows from High to Low • Unequal heating of the surface • Causes high and low pressures • Solar radiation energy source • Divergence • Air flowing away from an area • Less dense • Pressure decreases Factors Affecting Wind • Pressure-Gradient force • Pressure differences cause the wind to blow • The greater the differences the greater the wind speed • Isobars • Lines connecting places of equal pressure • Controls on Wind • Pressure-gradient force • Coriolis force • Friction Factors Affecting Wind • Pressure gradient • • • • The amount of pressure change occurring over a given distance Change in pressure / change in distance Closely spaced lines = greater pressure gradient = strong winds Widely spaced lines = weak pressure gradient = light winds 2 10/8/2011 Factors Affecting Wind • Horizontal Pressure Gradients and Wind • • • • • Coastline Over night No difference in pressure No pressure gradient No wind Factors Affecting Wind • Vertical Pressure Gradients Upward • Gravity pulls air toward the surface • Hydrostatic equilibrium • Upward pressure-gradient force is balanced by the downward force of gravity Factors Affecting Wind • Horizontal Pressure Gradients and Wind • • • • • Coastline Daytime Land heats faster than water Warm air rises, Cold air sinks Surface winds High to Low (Sea Breeze –from the sea) Factors Affecting Wind • Coriolis Effect • All free-moving objects, including wind, are deflected to the right (in the path of motion) in the Northern Hemisphere (left in the Southern Hemisphere) • Understanding the Coriolis Effect Low Pressure Vertical Pressure Gradient Force High Pressure Factors Affecting Wind • Coriolis Effect • Rocket from North Pole to Equator • By the time the rocket gets there • Earth moved (15o) Misses Target • Higher Wind = More Deflection • Strongest at Poles • Weakest at Equator Factors Affecting Wind • Friction 3 10/8/2011 Winds Aloft and Geostrophic Flow • High altitude winds • Very little friction • Pressure gradient force Winds Aloft Curved Flow and• the Gradient Wind • Isobars Low Pressure • Curved lines • Form loops • High and Low pressures • Very little friction • Perpendicular to isobars • Coriolis force • Gradient Winds • From rotation of the Earth • Causes right turns • Cyclone • Cyclonic Flow • (Troughs) • High Pressure • Constant speeds • Parallel to curved isobars • Anticyclones • Anticyclonic Flow • (Ridges) • Geostrophic winds • Equal Forces • Pressure gradient force • Coriolis force • Winds parallel to isobars Winds Aloft Curved Flow and the Gradient Wind Surface Winds • Friction • Slows the movement of air • Reduces Coriolis force • Winds cross isobars at an angle • Low Pressure • In and counterclockwise • High Pressure • Out and clockwise Winds Generate Vertical Air Motion • Anticyclone • • • • High pressure (stable conditions) Horizontal convergence aloft Air sinking Horizontal surface divergence • Cyclone • • • • Low Pressure (unstable conditions) Horizontal surface convergence Air rises and piles up Horizontal divergence aloft Winds Generate Vertical Air Motion • Other Factors • Friction • More friction • Decreases speed • Convergence (sea to land) • Stormy • Less friction • Increase speed • Divergence (land to sea) • Clear • Mountains • Windward • Convergence • Pile up of air • Leeward • Divergence 4 10/8/2011 Wind Measurement • Direction • Direction from which they blow • Wind vane • Multiple directions • Prevailing wind From Northeast • Wind rose • Percentage of time wind blows in that direction • Speed • Cup anemometer • Recording both • Aerovane • Wind sock • Satellites 5
