Air expands, and cools adiabatically. It cools and when it reaches its dew point, condensation occurs. That is, clouds form. Clouds Clouds are important because they are a visual indication of the conditions in the atmosphere. The three basic cloud forms are cirrus, stratus, and cumulus.
Fog is a cloud on the ground.
It is measured with an aneroid barometer . This is the height to which the air pressure will force a column of mercury into a tube immersed in a pool of the liquid metal. Another common way to express the pressure is in millibars. Normal atmospheric pressure is 29.92 inches of mercury or 1013 millibars. Air moves from regions of high pressure to regions of low pressure.