Page "Cloud" Paragraph 6

All weather-related clouds form in the troposphere, the lowest major layer of the Earth's atmosphere.

This generally happens when one or more lifting agents causes air containing invisible water vapor to rise and cool to its dew point, the temperature at which the air becomes saturated.

The main mechanism behind this process is adiabatic cooling.

Atmospheric pressure decreases with altitude, so the rising air expands in a process that expends energy and causes the air to cool, which reduces its capacity to hold water vapor.

If the air is cooled to its dew point and becomes saturated, it normally sheds vapor it can no longer retain which condenses into cloud.