Fronts and Air Masses

About this topic

An air mass is a large body of air with uniform temperature and moisture, taking the properties of the region where it forms. The boundary where two meet is a front. A cold front (cold air undercuts warm air) brings storms and a sharp temperature drop; a warm front (warm air rides over cold) brings steady rain and gradual warming; stationary and occluded fronts also exist.

Fronts are marked on weather maps with triangles (cold) and semicircles (warm).

Think of a cold front like a snowplough: dense cold air wedges under the lighter warm air and shoves it upward fast, so towering cumulonimbus clouds, a narrow band of heavy showers and sometimes thunderstorms form, followed by clearer, colder, drier air and a pressure rise. A warm front is gentle because warm air slides over a wedge of cold air with a shallow slope, giving cirrus, then thickening cloud, then hours of steady rain. An occluded front happens when a faster cold front catches a warm front.

Mid-latitude weather is largely about fronts and the depressions that form along the polar front, which move west to east. In the UK or the north-eastern USA the winds change direction when a front passes, which helps you spot it. Air masses are named for their source, such as maritime tropical (warm, moist) or continental polar (cold, dry). Knowing fronts helps pilots, farmers and event planners. A common misconception is that cold fronts always bring cold weather for days; the change may be brief. Another is that the front is a thin line; it is really a sloping zone.