One can calculate the atmospheric pressure at a given altitude. Pressure varies smoothly from the Earth's surface to the top of the mesosphere. Temperature and humidity also affect the atmospheric pressure, and it is necessary to know these to compute an accurate figure. The graph at right was developed for a temperature of 15 °C and a relative humidity of 0%. As altitude increases, atmospheric pressure decreases. Although the pressure changes with the weather, NASA has averaged the conditions for all parts of the earth year-round.
For higher altitudes within the troposphere, the following equation (the barometric formula ) relates atmospheric pressure p to altitude h.
One atmosphere (101 kPa or 14.7 psi) is the pressure caused by the weight of a column of fresh water of approximay 10.3 m (33.8 ft). Thus, a diver 10.3 m underwater experiences a pressure of about 2 atmospheres (1 atm of air plus 1 atm of water).
where the constant parameters are as described below:
The altimeter setting in aviation, is an atmospheric pressure adjustment.
These variations have two superimposed cycles, a circadian (24 h) cycle and semi-circadian (12 h) cycle. This effect is strongest in tropical zones, with an amplitude of a few millibars, and almost zero in polar areas. Atmospheric pressure shows a diurnal or semidiurnal (twice-daily) cycle caused by global atmospheric tides.
See pressure system for the effects of air pressure variations on weather.
Atmospheric pressure is expressed in several different systems of units: millimetres (or inches) of mercury, pounds per square inch (psi), dynes.
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At heights above 1,500 to 3,000 metres (5,000 to 10,000 feet), the pressure is low enough to produce mountain sickness and severe physiological problems unless careful acclimatization is undertaken. However, over cold air the decrease in pressure can be much steeper because its density is greater than warmer air. Near Earth’s surface the pressure decreases with height at a rate of about 3.5 millibars for every 30 metres (100 feet).
Altitude is calculated from air pressure and vice versa. Using ISA standards, the defaults for pressure and temperature at sea level are 101,325 Pa and 288 K.
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The pressure at the bottom of the layer is determined from the user provided inputs of the pressure and temperature at sea level knowing that the altitude at the bottom of the layer is 11 km; assuming the default pressure was used at sea level, the pressure at the bottom of the stratosphere is 22,632 Pa.
Pounds per square inch absolute (psia) is used to make it clear that the pressure is relative to a vacuum rather than the ambient atmospheric pressure. Since atmospheric pressure at sea level is around 14.7 psi, this will be added to any pressure reading made in air at sea level.
It is used in mechanics for the elastic modulus of materials, especially for metals. The megapound per square inch (Mpsi) is another multiple equal to a million psi.
The kilopound per square inch (ksi) is a scaled unit derived from psi, equivalent to a thousand psi (1000 lbf/in2).
The conversion in SI Units is 1 ksi = 6.895 MPa, or 1 MPa = 0.145 ksi.
Therefore, one pound per square inch is approximay 6894.757 Pa.
Therefore, 1 atmosphere is approximay 14.7 pounds per square inch.
Since atmospheric pressure at sea level is around 14.7 psi, this will be added to any pressure reading made in air at sea level.