A warm front brings a gradual increase in temperature and more humid air as a warm air mass advances and slides up over a colder air mass. This lifting of warm, moist air over the cooler air causes widespread clouds and steady, light to moderate precipitation, often in the form of rain or drizzle. Ahead of the front, clouds typically start as high cirrus and cirrostratus, then thicken to altostratus and nimbostratus, producing persistent rain. Fog can also occur before the front passes. After the warm front moves through, temperatures rise and the weather usually clears, with improved visibility and warmer, more humid conditions
. If the warm air mass is unstable, thunderstorms may occur before and after the front passes, but severe weather is generally less intense than with cold fronts. In winter, snow may initially fall and then transition to rain as the warm front passes
. In summary, a warm front typically brings:
- Gradual warming
- Increased humidity
- Widespread clouds (stratiform)
- Steady, light to moderate rain or drizzle
- Possible fog before the front
- Clearing and warmer weather after passage
- Occasional thunderstorms if the air is unstable
