Vapor pressure is the amount of pressure in millibars exerted by the water vapor in the air. There is a maximum amount of moisture that can be in the air for a given temperature. This maximum amount of moisture is known as the saturation vapor pressure. At the saturation vapor pressure the relative humidity is 100%. The relationship between temperature and saturation vapor pressure is not linear which means that the rate that saturation vapor pressure increases is not a constant rate as temperature increases. The saturation vapor pressure increases more rapidly as temperature increases.

The table below shows the value of saturation vapor pressure for temperatures in increments of 10 C from -10 C to 40 C. An example of how saturation vapor pressure change increases with an increase in temperature, compare the change in saturation vapor pressure when going from 0 C to 10 C as compared to going from 20 C to 30 C. From 0 C to 10 C the change is (12.2 6.1 = 6.1 mb) while from 20 C to 30 C the change is (42.1 23.1 = 19 mb). The change is a 10 C interval in both cases, but the amount of change of the saturation vapor pressure is over 3 times as much when going from 20 C to 30 C as compared to going from 0 C to 10 C.

The implication for this is that warm air has the capacity to contain much more water vapor than cold air. Thus, warm air thunderstorms have the capacity to produce much more rain than rain and snow that occurs in cold air. This has an important implication for climate also. Warm and humid locations tend to have much more yearly rainfall than cold and dry locations.