1) Sufficient atmospheric moisture - Clouds form when water vapor condenses into cloud droplets or ice crystals. This requires enough moisture to be present in the atmosphere for the droplets to form and grow.

2) Cooling of the atmosphere - For water vapor to condense, it must cool below its saturation temperature, the point at which the air is unable to hold any more water vapor as a gas. This cooling can occur through various processes such as rising air due to convection, radiative cooling, or through contact with a cold surface.