A warm front is the leading edge of an advancing warm air mass that slides gently up and over the retreating cooler air. Because the warm air rises along a very gradual slope (roughly 1:150 to 1:300), the cloud and precipitation band extends 200–400 km ahead of the surface front — much wider than a cold front.
The classic warm-front cloud sequence, first described by the Bergen School of meteorology, is a reliable natural forecast: high, wispy cirrus clouds appear 24–36 hours before the front arrives, thickening to altostratus and then to nimbostratus, which produces prolonged, steady rain or drizzle. On weather maps, warm fronts are depicted as red lines with semicircles pointing in the direction of advance.
Temperatures rise gradually as the front passes, the wind veers (e.g., from south-east to south-west), and precipitation eases. The warm sector between the warm front and the following cold front often brings mild, cloudy, and sometimes foggy conditions. In mountainous areas, warm fronts can produce prolonged snowfall at higher elevations. Understanding warm fronts is key to predicting the typical sequence of weather associated with Atlantic depressions crossing Europe.