Weather and climate - 7.26.07 High vs. Low Pressure o High...

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7.26.07 High vs. Low Pressure o High Pressure – air blows around a high in a clockwise manner in the northern hemisphere; counterclockwise in the southern hemisphere (cross section) Surface Divergence (spread apart)/Upper air convergence (coming together) Dryer, more stable o Low Pressure – air blows around a low in a counterclockwise manner in the northern hemisphere; clockwise in the southern (cross section) Surface convergence/Upper air divergence Low pressure in dry air but no condensation Surface Pressure Charts o With corrections made to surface pressure readings, surface pressure charts use isobars to depict lines of equal pressure Upper Air (constant height) Charts o Upper air charts – constant height charts Difficult to measure exact pressure…instead finding the height helps to indicate the pressure o Constructed to show height variations alone a constant pressure height surface o Higher heights = higher pressure o Lower heights = lower pressure o On upper air (constant height) charts, height contour lines depict pressure o The lines on the chart are not isobars, they indicate lower or higher height o Remember, 1013.2 millibars is standard surface atmospheric pressure….so… o 850 millibars chart = approx 4,000 feet ASL ( above sea level) o 700 millibars chart = approx 9,800 feet ASL o 500 millibars chart = approx 18,500 feet ASL Most important because it is half of the average atmosphere, a lot of weather develops, the weather is steered by the wind, half the mass of the earth’s atmosphere o 300 millibars chart = approx 30,000 feet ASL o 250 millibars chart = approx 34,250 feet ASL o 200 millibars chart = approx 38,500 feet ASL Introduction to Wind o Newton’s first law of motion : an object at rest will remain at rest and an object in motion will remain in motion ( and will travel at a constant velocity along a straight line) as long as no force is exerted on the object o Newton’s second law of motion : the force exerted on an object equals its mass times the acceleration produced (F=ma) o Newton’s third law of motion : for every action, there’s an equal and opposite reaction Atmospheric Forces that cause Winds to Blow
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o Pressure Gradient Force : causes air move from high to low (high pressure vs. low pressure) o Coriolis Force: deflection caused by a rotating object CF only affects wind direction not speed not existent at the equator, most greatly felt at the poles mainly found in large scale wind systems hurricanes will never form 5 above or below the equator and never cross the equator Amount of deflection depends upon… Rotation of the earth The latitude The objects’ speed o Friction : wind blowing across the earth’s surface and against other air molecules and tends to slow things down.
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Weather and climate - 7.26.07 High vs. Low Pressure o High...

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