If the warm sector air is maritime tropical, the dew point shows a further increase. The cirrus clouds may appear 700 to 1,000 miles or more ahead of the surface front, followed by cirrostratus clouds about 600 miles ahead of the surface front and altostratus about 500 miles ahead of the surface front. Precipitation in the form of continuous or in-termittent rain, snow, or drizzle is frequent as far as 300 miles in advance of the surface front. However, when the warm air is connectively unstable, showers and thunderstorms may occur in addi-tion to the steady precipitation. This is especially true with a cyclonic flow aloft over the warm front.
Fog is common in the cold air ahead of a warm front.
Clearing usually occurs after the passage of a warm front, but under some conditions drizzle and fog may occur within the warm sector. Vertical cross section of a warm front with stable and unstable air. The amount and type of clouds and precipita-tion vary with the characteristics of the air masses involved and depending on whether the front is active or inactive. Generally, with warm fronts, an increase of the wind component with height perpen-dicular to the front gives an active front. This produces strong overrunning and pro-nounced prefrontal clouds and precipitation.