Warm front
Topic: Air Masses, Fronts, Cyclones and Thunderstorms · NCERT: Class 11, Ch 9 "Atmospheric Circulation and Weather Systems"
Meaning
A warm front forms where advancing warm air meets retreating cold air. Warm air is lighter, so it glides gently up over the cold air along a long, low slope. As it rises, it forms clouds in a fixed order. High, thin cirrus come first, then cirrostratus, altostratus, and finally thick, rain-bearing nimbostratus. These clouds spread over hundreds of kilometres, so steady rain or drizzle falls for a long time ahead of the front. On weather maps, a warm front is shown by red semicircles.
Example
In Britain, wispy cirrus clouds often appear a day before a warm front arrives from the Atlantic. The sky then slowly thickens and greys, and hours of steady rain follow.
Don't confuse with
- Cold front: here cold air pushes steeply under warm air. It brings tall cumulonimbus clouds and short, heavy showers or thunderstorms, not long, steady rain.