Cloud Formation

Favorable conditions for the formation of clouds are:

• High environmental lapse rate in the troposphere; this means air at moderate altitude significantly colder than air at the surface.
• High surface relative humidity; this means condensation will begin after slight cooling.

Let's first examine the conditions that don't favor clouds forming: atmospheric stability. A weather balloon is launched to measure the air temperature aloft:

The balloon is launched in the morning. The temperature is found to be not much cooler aloft than it is at the surface. The Sun will increase the surface temperature without affecting the air aloft right away. When the surface air is hot enough, convection will begin causing some of the surface air to rise:

The rising air cools by adiabatic expansion as it enters regions of lower surrounding pressure. Note that the rising air does not cool by conduction with the cool surrounding air. In this example, the dew point temperature of the surface air is low: considerable cooling of the rising air is necessary before it reaches 100% relative humidity and begins to form a cloud. As a result, the cloud base is at a high altitude:

Since the air at middle altitude is not very cold, the rising air expands and cools to a temperature lower than the surrounding air. Cold air is denser than warm air, and tends to sink. The lack of buoyancy and the lack of moisture combine to form a high, thin cloud if a cloud forms at all.

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