boundary separating two masses of air of different densities
A weather front is a boundary in between two masses of air of different densities, and is the main cause of significant weather. In surface weather analyses, fronts are depicted using various colored lines and symbols. The air masses separated by a front usually differ in temperature and humidity.
Cold fronts may feature narrow bands of thunderstorms and severe weather, and may on occasion be preceded by squall lines or dry lines. Warm fronts are usually preceded by layered precipitation and fog. The weather usually quickly clears after a front passes. Some fronts produce no precipitation and little cloudiness, although there is usually a wind shift.