The air pressure at sea level averages 1013 millibars and declines gradually with increasing elevation, dropping to just 12 mb at 30 km. Adiabatic temperature changes occur as a result of the compression or expansion of air. These temperature changes represent the conversion of mechanical energy into heat energy. Compressed air becomes warmer, expanding air becomes cooler. Rising air undergoes adiabatic cooling, resulting in increasing relative humidity and eventual saturation.
Clouds form as a result of condensation driven by adiabatic cooling.
Clouds form when condensation takes place but condensation forms tiny water droplets that condense on minute dust particles in the air. It takes a million or so droplets to form a single raindrop. Precipitation can occur by two processes. Clouds are classified based upon their altitude and appearance. Wind is the horizontal movement of air that flows from areas of high pressure to areas of low pressure.