Hotter air can hold more water
than cooler air. Suppose that you have liquid water in a boiler. Half of it is
liquid and the other half is gas. They are in equilibrium. If you heat the
liquid water, the molecules in the liquid water tend to separate from its neighboring
molecules and go into the gaseous phase of water. The liquid in water has
higher pressure and greater volume. More molecules of liquid water will enter
into the gaseous phase. There will now be more molecules of water in the hotter
air because it has greater volume (the distances between the particles is high)
and faster molecules tend to go in the gas. Also, its kinetic energy is high
due to the increase in temperature. However, when it is in colder air, the kinetic
energy of the molecules is low because the temperature is low.
