Why?

* Molecular motion: Water vapor is simply water in its gaseous state. When air is heated, the molecules of the air and the water vapor move faster and spread further apart. This increased spacing allows more water vapor molecules to fit into the same volume of air.

* Saturation point: The amount of water vapor that air can hold is determined by its saturation point, which is the maximum amount of water vapor it can hold at a given temperature. Warmer air has a higher saturation point because the molecules are moving faster and have more space between them.

* Relative humidity: Relative humidity is the amount of water vapor present in the air compared to the amount it could hold at that temperature. Even if the amount of water vapor in the air doesn't change, warm air will have a lower relative humidity because it can hold more water vapor.

Example:

Imagine two containers of air, one at 10°C (50°F) and one at 25°C (77°F). Even if both contain the same amount of water vapor, the warmer container at 25°C will have a lower relative humidity because it can hold more water vapor.

In summary, warm air can hold more water vapor than cold air because the increased molecular motion and higher saturation point of warm air allow for a greater density of water vapor molecules.