When a large body of air sits over a uniform surface for days, it takes on that surface’s temperature and moisture. The resulting air mass can stretch hundreds of kilometers and move as a coherent unit.

Meteorologists classify these masses by their source region. Continental polar air is cold and dry; maritime tropical air is warm and humid. As an air mass travels, it slowly modifies, yet its core properties often persist long enough to dominate regional weather for several days.

Fronts form where different air masses meet. The contrasts drive cloud development, precipitation, and wind shifts. Tracking air-mass boundaries remains a basic step in daily forecasting.

• Forms over uniform source regions
• Classified by temperature and moisture origin
• Moves and modifies gradually
• Creates fronts at boundaries