Final answer:
Air masses form based on humidity and temperatures. Warm and moist air rises, cools, and forms condensation and clouds. Cold and dry air can also form air masses when it moves over warmer land or water.
Step-by-step explanation:
Air masses form based on humidity and temperatures. Humidity refers to the amount of water vapor present in the air. When warm air containing moisture rises and cools, it can form condensation and clouds. This process leads to the formation of air masses, which are large bodies of air with similar temperature and humidity characteristics.
For example, when a warm and moist air mass from the tropics moves over cooler land or water, it can result in the formation of a humid air mass. On the other hand, when a cold and dry air mass moves over warmer land or water, it can result in the formation of a dry air mass.