Chapter 5- Atmospheric Pressure and Wind

# Chapter 5- Atmospheric Pressure and Wind - The atmosphere...

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The atmosphere contains a tremendous number of gas molecules being pulled toward Earth by the force of gravity. These molecules exert a force on all surfaces with which they are in contact, and the amount of that force exerted per unit of surface area is pressure.

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The standard unit of pressure is the pascal (Pa). Meteorologists in the U.S. use the millibar (mb), which equals 100 Pa. Canadian meteorologists use the kilopascal (kPa), equal to 1000 Pa, or 10 mb. Air pressure at sea level is roughly 1000 mb (100 kPa) or more precisely, 1013.2 mb.
The enclosed air molecules move about continually and exert a pressure on the interior walls of the container (a). Pressure can increase by increasing the density of the molecules (b) or increasing the temperature (c). If the air in the container is a mixture of gases, each gas exerts its own specific amount of pressure, referred to as its partial pressure . The total pressure exerted is equal to the sum of the partial pressures. This relationship is known as Dalton’s law .

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Surface pressure is the pressure actually observed at a particular location, whereas sea level pressure is the pressure that would exist if the observation point were at sea level. Sea level pressure allows us to compare pressure at different locations taking into account differences in elevation. To correct for elevation, add 1 mb per 10 meters . For high-elevation sites, this method is unreliable because we must account for compressibility of the atmosphere.
Pressure does not decrease at a constant rate. It decreases most rapidly at low elevations and at greater heights. Pressure decreases with altitude by about half for each 5.5 km.

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The Equation of State (Ideal Gas Law) p = ρ RT where p is pressure expressed in pascals, ρ (rho) is density in kilograms, R is a constant equal to 287 joules per kilogram per kelvin, T is temperature in kelvins. The equation tells us if the air density
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Chapter 5- Atmospheric Pressure and Wind - The atmosphere...

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