7-Atmospheric Motion-v2011-v2

7-Atmospheric Motion-v2011-v2 - Atmospheric Motion Leila M....

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Atmospheric Motion Leila M. V. Carvalho
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What is wind? Well, wind is air in movement. We can feel the wind and we notice its presence, we can fear, and we can even use its energy to move and generate power
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Isaac Newton (1642-1727) first law of motion states that an object at rest will remain at rest and object in motion will remain in motion (and travel at a constant velocity along a straight line) as long as no force is exerted in the object” So, if wind is air in movement, a force must exist to move air from rest to movement. Let’s examine the forces that act upon the atmosphere Imagine a volume with molecules at a given temperature. They are randomly vibrating and colliding with each other, depending on the temperature Horizontal Wind Wind is therefore the movement of a given volume of air. F = ma (mass x accelaration) , and F=F1+ F2 +…+ Fn
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Low Pressure High Pressure In this condition, what should be the direction of the wind? Here, we observe an horizontal pressure change or horizontal pressure gradient. Gradient measures the spatial variation of a scalar (pressure, temperature, density, etc). Pressure gradient = difference in pressure/distance, PG=Δp/d
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According with Newton second law, a force must act upon an object to change its speed (from rest to a speed V). This force, in this case is the Gradient Pressure Force Low Pressure p2 High Pressure p1 wind direction Pressure Gradient REMEMBER THAT A GRADIENT” ALWAYS POINT TOWARD THE HIGHEST MAGNITUDES OF THE SCALAR . THE PRESSURE GRADIENT FORCE POINTS FROM HIGH TOWARDS LOW PRESSURE Pressure gradient = difference in pressure/distance, PG=Δp/d The Pressure Gradient Force per unity of mass (PGF) is proportional to - Δp/d, Where Δp=p2-p1 In mathematical terms: , where ρ is the atmosphere density at a given level Pressure Gradient Force E W s p Fg - = ρ 1
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Let’s recall the definition of air pressure Air pressure is simply the weight of mass of air above a given level, the force per unit area exerted against a surface by the weight of the air molecules above that surface. P2 P1 P1>P2
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Pressure measurements (pg. 100 text book) Inches or millimeters of mercury Pressure is measured based on the compression of chambers Aneroid barometer Barograph Cylinder moves and pressure is registered along the days Mercury barometer Barometer: literally means measure ‘Bar’, which is one unity of pressure. Bar is a relatively large unity, and because surface pressure changes are normally small, the common unity in Meteorology: millibar (mb), 1mb=1/1000 bar (1mb = hecto Pascal (100 Pa), where Pa=N/m2) STANDARD PRESSURE AT SEA LEVEL: 1013.25mb = 29.92 in Hg = 76cm
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We also learned that pressure and density decrease exponentially with height : rapidly at first and slowly at higher altitudes P2 P1 P1>P2
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4,322m 1000m If you are climbing a mountain from the sea level to about 1000m, you will notice that pressure will decrease about 100mb/km = 10mb/100m (10mb/328 ft) Δp/ Δz = 100mb/km
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7-Atmospheric Motion-v2011-v2 - Atmospheric Motion Leila M....

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