Warm fronts move slower than cold fronts. Their average speed is usually between 10 and 20 knots. They move with a speed of 60 to 80 per-cent of the component of the wind normal to the front in the warm air mass. 

The troughs associated with warm fronts are not as pronounced as those with cold fronts and sometimes make location difficult on the surface chart. The pressure tendency ahead of the front is usually a rapid or unsteady fall with a leveling off after frontal passage. A marked decrease in isallobaric gradient is noticed in the warm sector except when rapid deepening is taking place. The wind increases in velocity in advance of warm fronts because of an increase in pressure gradient and reaches a maximum just prior to frontal passage. The wind veers with frontal passage, usually from a south-easterly direction to a southwesterly direction behind the front. This shift is not as pronounced as with the cold front.

Temperature generally is constant or slowly rising in advance of the front until the sur-face front passes, at which time there is a marked rise. This rise is dependent upon the contrast between the air masses. Dew point usually increases slowly with the approach of the front with a rapid increase in precip-itation and fog areas. If the warm sector air is maritime tropical, the dew point shows a further increase.

WEATHER 

A characteristic phenomenon of a typical warm front is the sequence of cloud formations (fig. 4-4-1). They are noticeable in the following order: cirrus, cirrostratus, altostratus, nim-bostratus, and stratus. 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. Sur-face precipitation is associated with the nim-bostratus in the warm air above the frontal surface and with stratus in the cold air. 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. Warm fronts usually move in the direction of the isobars of the warm sector; in the Northern Hemisphere this is usually east to northeast.

Figure 4-4-1.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. Inactive fronts, characterized by broken cirrus and altocumulus, are produced by a decrease with height of the wind component perpendicular to the front.

When the overrunning warm air is moist and stable, nimbostratus clouds with continuous light to moderate precipitation are found approxi-mately 300 miles ahead of the front. The bases of the clouds lower rapidly as additional clouds form in the cold air under the frontal surface. These clouds are caused by evaporation of the fall-ing rain. These clouds are stratiform when the cold mass is stable and stratocumulus when the cold air is unstable.

When the overrunning air is moist and unstable, cumulus and cumulonimbus clouds are frequently imbedded in the nimbostratus and altostratus clouds. In such cases, thunderstorms occur along with continuous precipitation. When the overrunning warm air is dry, it must ascend to relatively high altitudes before conden-sation can occur. In these cases only high and middle clouds are observed.

Visibility is usually good under the cirrus and altostratus clouds. It decreases rapidly in the precipitation area. When the cold air is stable and extensive, fog areas may develop ahead of the front, and visibility is extremely reduced in this area.

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