Warm, cold and occluded fronts

Warm front

Warm air rides up over colder air on a shallow slope (about 1 in 150). Cloud thickens ahead of it: Ci, Cs, As, then Ns, with a wide band of rain or snow up to a few hundred kilometres ahead of the surface front. In winter there is a risk of freezing rain under the warm air.

At the passage (northern hemisphere): the wind veers, the temperature rises, the pressure stops falling. Behind it, in the warm sector, low stratus, drizzle and poor visibility are common.