What must occur for clouds, fog, or dew to form?

The formation of clouds, fog, or dew is primarily dependent on the condensation of water vapor. As air cools, it can hold less moisture, causing the water vapor to condense into tiny water droplets. This condensation occurs when the air reaches its dew point, the temperature at which air becomes saturated with moisture.

When water vapor condenses around tiny particles in the atmosphere, it forms clouds. In the case of dew, this process happens directly on ground surfaces or objects during cooler conditions, leading to the accumulation of water droplets. Fog is essentially a cloud that forms close to the ground, resulting from the same condensation process.

In contrast to condensation, the other options explore aspects of water vapor that do not directly lead to the formation of clouds, fog, or dew. Water vapor expands under certain conditions, but this does not result in cloud formation. Freezing occurs when water vapor becomes ice, which is a different process that can lead to different types of precipitation, and heating water vapor generally involves energy that can prevent condensation by allowing it to remain in a gaseous state.