Air masses and fronts

An air mass is a large body of air with fairly uniform temperature and moisture, named for where it formed (polar or tropical, continental or maritime). A front is the boundary between two air masses.

Cold front

Cold, dense air pushes under warmer air like a wedge, forcing it up quickly. Cold fronts are usually fast-moving with a steep slope. If the warm air is moist and unstable, expect cumuliform clouds, showers, gusty winds, turbulence and thunderstorms, sometimes in a squall line ahead of the front. After it passes: cooler temperatures, clearing skies and good visibility.

Warm front

Warm air slides up over cooler air on a gentle slope. Warm fronts move slowly and bring widespread stratiform clouds, low ceilings, poor visibility and steady precipitation that can last a long time, sometimes with freezing rain in winter. The clouds can show up hundreds of miles ahead as cirrus.

Stationary and occluded fronts

• Stationary front: neither air mass is advancing. Weather can hang around for days.
• Occluded front: a fast cold front catches up with a warm front. It can combine the hazards of both, including embedded thunderstorms.

Frontal passage

When a front passes, you'll notice changes in temperature, dewpoint, wind direction (the most reliable sign) and pressure. On a weather map, cold fronts are blue lines with triangles, warm fronts red lines with half-circles.