When two air masses with different temperatures and levels of humidity meet they form weather fronts. This can include cold, warm, occluded or stationary fronts depending on the characteristics of the air masses.
An air mass is a large body of air that has a consistent temperature and level of humidity throughout. Air masses are classified based on temperature, humidity and region of origin. Air masses typically form over a large area with an even surface, such as an ocean, plain or desert.
All weather fronts can result in precipitation, with warm front precipitation more widespread and continuous, and cold front precipitation, such as thunderstorms, more quick and localized.