As air masses move from their source region they are bound to be modified due to temperature and moisture exchanges with the surface over which they pass.
Thermodynamic changes occur when air masses move over cold or warm surfaces. If an air mass hovers along a cold surface e.g over Antarctica the lower layers gets cooler than the upper layers which can create a temperature inversion or if a warm moist air mass gets heated by the ground, it may rise adiabatically (convectional uplift)producing towering cumulonimbus clouds with heavy rains (given air mass has enough moisture). When condensation occurs latent heat trapped in the air mass gets liberated.
Air masses can be forced to rise over barriers such as mountains which can result in orographic rainfall or they may meet together creating frontal or convergence rainfalls.
When air masses hover along water surfaces such as sea or ocean, moisture is added up to the air mass, which when condensation occur torrential rainfalls can occur Subsequently, as air masses move over dry regions such as deserts, moisture content maybe reduced. This explains why tropical storms die over land.