Atmospheric pressure

On Earth, atmospheric pressure is equal to the weight of the air column that ‘weighs’ on the Earth’s surface. This is true at any altitude, but 99% of the atmosphere’s mass is concentrated in the first 32 km.
Pressure variations at sea level do not usually exceed 4% of the normal average value (that is 1013 millibar): lower values (up to 900 millibar) can be registered in the eye of tropical cyclones. With the exception of some small local variations, atmospheric pressure and density decrease with altitude following an exponential curve up to a level of about 100 km, where they reach a value equal to one millionth of the value at sea level.
Atmospheric pressure is not distributed uniformly in the atmosphere because it depends on different factors, among which temperature (warm air expands and is therefore lighter) and humidity (since water vapour is lighter than air, damp air is lighter than dry air). Hence, atmospheric pressure proves to be higher in polar regions, where the air is colder and dryer and lower in equatorial regions, where the temperature and the humidity of the air are greater. Moreover, atmospheric pressure undergoes daily variations that can be compared to the tide phenomenon.
Unit of measurement of atmospheric pressure
At sea level, the density of air is about 1.3 g/dm3 and decreases exponentially with altitude. Furthermore, at sea level, atmospheric pressure, and therefore the weight of the air above it, is equal to 1,033 g/cm2. This weight is equivalent to the pressure of 1 atmosphere (atm), i.e. to the weight of a 760 mm high mercury column with a cross-section of 1 cm2. In meteorology, this ‘historical’ unit of measurement is not used any more. The millibar (mb) is the unit that is generally utilized and it corresponds to a force of 1,000 dyne/cm2: a dyne is the force required to impart an acceleration of 1cm/sec2 to a mass of 1 gm. One million dynes/cm2 correspond to one bar, and the millibar is a thousandth of the bar. Recently, with the adoption of the International System in 1974, pressure really ought to be expressed in Pascals (Pa). 1 Pa corresponds to 1 Newton/m2, 1 atm is equal to 100,000 Pa, i.e. to 1,013 millibar: since the Pascal is a very small unit, in meteorology hectopascals are used (hPa), equal to 100 Pa.

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