# Why can air accommodate more water when the air has a higher temperature?

The amount of water(g) that air can accommodate depends on temperature. The absolute humidity then is defined as the percentage of water to air and decreases when the temperature decreases. The relative humidity is the ratio between the current and the maximum humidity, which increases with decreasing temperature. The temperature at which the relative humidity reaches 100% is called the dew point.

Anyway, why can air keep more water when it is warmer?

The liquid-vapor coexistence line of the phase diagram of water has a positive slope $\mathrm{d}P/\mathrm{d}T$. Humidity in air is a partial pressure of water vapor. If partial pressure of water is increased starting from $<100\%$ humidity, it will reach the vapor-liquid transition and any additional water vapor will simply condense into droplets ($=100\%$ humidity).
An increase in temperature corresponds to an increase in the saturation vapor pressure, thus at warmer temperatures there will be a higher partial pressure of water vapor in the air at a given $\%$ humidity.