As the surface front approaches, CI, CS, AS and finally NS (ST) clouds appear. The cloud sequence ahead of a warm front with significant overrunning follows a normal routine. If the colder air mass is retreating slowly, but the warmer air mass is advancing rapidly, then the warm air ascends the frontal surface and widespread precipitation occurs. If the colder air mass is retreating as rapidly as the warmer air mass is advancing, then there is little lift along the frontal surface, thus little or no precipitation occurs. The movement of the warm front and the strength of the upward vertical motion caused by warm air gliding up the frontal surface (overrunning) are closely related. The speed with which the colder air mass retreats determines the speed at which the warm front advances. Warm fronts generally move more slowly than cold fronts. (Recall: The wind veering with increasing height suggests warm-air advection). Shallow fronts normally extend only up to 850 mb, not above.Īn upper-air sounding originating on the cold air side of the surface front will exhibit veering winds in the vertical within the stable layer. Fronts with steep slopes generally are visible up to 700 mb, but may be reflected up to 500 mb. The slope of a warm front is typically between 1/100 and 1/300. Weak fronts are normally best defined at 850 mb and below. Strong fronts are clearly defined up through 700 mb. Above the 850-mb level, a well-defined frontal trough may not be found. Appreciable cyclonic turning is usually found in this region. The upper front at 850 mb marks the intersection of the frontal surface with the 850 mb constant- pressure surface. The frontal surface is identified as the warm side of the transition zone. Vertically, the frontal surface slopes, over the colder air mass. Winds turn cyclonically across the frontal trough and veer with frontal passage. The pressure behind the surface front may fall slowly, rise slowly, or remain steady because of weak temperature advection. A slower change in pressure occurs after FROPA. This is caused by the warm- air advection in the transition zone.

Strong pressure falls (PRESFR) are observed ahead of the surface front. The surface and 850mb fronts are located in frontal troughs. While the warm air behind the front holds more moisture, it is farther from saturation. So, the relative humidity decreases with FROPA. However, the air ahead of the warm front is normally nearer to saturation than that behind the warm front because fallen precipitation is evaporating. In other words, the dew point increases with FROPA. Air behind the warm front typically is more moist (in terms of an absolute measure of moisture) than air ahead of the warm front. The surface front is located on the warm side of the transition zone.Ī moisture discontinuity exists across the frontal surface. A front in which a warmer air mass is advancing and replacing a retreating colder air mass is a warm front.Ī horizontal temperature discontinuity exists across the frontal surface, with colder air ahead of the frontal surface and warmer air behind it.