The density of air is 1 gram per liter (at room temp and press).

The pressure due to the weight of 300 km of atmosphere above is equal to that of:

1 atmosphere=

.. 1 kg (9.8n) per square centimeter

.. 10 meters of water

.. 100 kPa

.. 760 mm of mercury

.. 1013 millibars

The total force on the human body due to the pressure of 1g/cm2 is about 20 megagrams (metric tons).

It is the force of atmospheric pressure that causes the phenomenon commonly called "suction". The atmosphere pushes air into regions of lower pressure.


Capacity of air is the number of grams of water vapor that can be dissolved in 1 cubic meter of air at a given temperature. The capacity increases with temperature.

Absolute Humidity is the actual number of grams of water vapor dissolved in a cubic meter of air at any moment.

Relative Humidity is the ratio of Absolute Humidity to Capacity expressed in percent.

100% Relative Humidity means that the air is saturated as in a cloud or in fog.

Example determination of Relative Humidity:

At 20 C, the Capacity is 18 g/m3. If the Absolute

Humidity is 9 g/m3,

The Relative Humidity is 9 g/m3,/18 g/m3, = 50%

Here is a table of Capacities at various Temperatures:

Temp in degC Capacity in g/m3,

0....... 5

5....... 8

10..... 10

15..... 14

20..... 18

25..... 22

30..... 30

35..... 40


Adiabatic refers to changing the temperature of air by either compressing it or expanding it. Compression warms air, expansion cools air. (An example of adiabatic heating is in the ignition of a diesel engine. The air is heated so hot by compression that it ignites the fuel on contact. Adiabatic cooling is noticed when air is allowed to escape from a tire).

Upslope fog is formed by adiabatic cooling of the air as it is blown uphill. As the air is cooled, its Capacity is lowered until it equals the Absolute Humidity. Then we have 100% Relative Humidity and further cooling will cause fog to form. Fog, like clouds, is composed of LIQUID droplets (NOT water vapor).


A great new video that shows the ten-meter action with water in glass pipes (it boils,too) and the crushing of a steel oil drum is

"Atmospheric Pressure"

Films for Humanities & Sciences

Box 2053 Princeton, NJ 08543-2053

10 mins colour (around $49)

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