Front is also characterized by an abrupt change in wind direction and speed across its surface, know as wind shift , which is associated with convergence, leading to upward motion, clouds, and perhaps precipitation. Precipitation associated with the front tends to increase or diminish in intensity as the front strengthens or weakens. It intersects the isobars at the surface in a trough region. Each Isobar kinks as it crosses the front. It occurs when precipitation falls from relatively warm air into the relatively cool air near the surface and evaporates as increasing water vapor concentration to saturation.
Frontal fog is observed just ahead of a warm front or just behind a cold front.
Stationary front is often associated with a wide region of clouds and rain or snow on the cold side of the front. The clouds and precipitation result from overrunning , as warm humid air flows upward over the cooler air mass, more or less along the frontal surface, cools through adiabatic expansion which triggers condensation and precipitation. A warm front in the eastern US separates advancing mT air of Gulf of Mexico from retreating cold mP air of North Atlantic. As the stationary front, warm front is associated with a broad cloud and precipitation shield that may extent hundred of kilometers ahead of the surface front. The type of frontal weather depends on the stability of the warmer air.