Relative humidity is the amount of water vapor in the air, expressed as a percentage, compared to the amount of water vapor it can hold at that temperature.
As air is heated, it expands. With this expansion, the amount of moisture it can hold also increases as there’s more space for the water to be placed within.
Example: If current relative humidity reads 52% at 70°F, that means the air is 52% saturated with water vapor at 70°F. Think of the dry sponge below as a 70°F pocket of air. When we add a shot glass of water to the sponge, and that represents 52% of water vapor.
If you were to heat that same air to 90°F and read a relative humidity of 28%, this would mean that the same air is now holding 28% of the moisture that it can potentially hold instead of 52% due to the air expanding in volume. We’re going to use a slightly larger sponge to represent the increased space between the molecules allowing for more water vapor to be held between them. The amount of actual water vapor in the air has not changed.