Warm front

Fig. 18-2. Surface chart for 06Z Friday, November 20, 1981. Contours are isobars of atmospheric pressure 12 is 1012 mb. Line with triangles, cold front line with semicircles, warm front line with both triangles and semicircles, an occluded front (a cold front that has caught up with a warm front). Wind direction is with the arrow wind speed is 10 knots for 1 barb, 5 knots for one-half barb. Small station circles indicate calm. H, center of high pressure L, center of low pressure.

FIGURE 4-15a A warm front occurs when a warm air mass overtakes an adjoining cold air mass and rises over it. Adiabatic cooling tends to produce condensation, and ultimately precipitation, along the front. In the situation shown, the warm air mass is unconditionally stable. If the warm air mass is conditionally stable, instability is produced when clouds form along the front, and vertical cloud development (often including thunderstorms) occurs. 

FIGURE 4-15b A cold front occurs when a cold air mass overtakes a warm air mass, which it displaces upward. Again, adiabatic cooling of the rising air tends to produce precipitation. Cold fronts are usually faster moving than are warm fronts they are also narrower, with the slope of the interface between air masses typically about 80 1. 

A passing cold front is heralded by clouds, a drop in temperature, and precipitation cooler air and clear skies occur behind the cold front (see Fig. 4-15b). The slower moving warm front is characterized by a more gradual lowering of cloud heights, followed by rain or snow (Fig. 4-15a). As air masses pass, so do their burdens of airborne chemicals. The clouds and precipitation formed along the front act as sinks for certain atmospheric chemicals because of rainout and washout processes. These processes, which remove particles, gases, and dissolved chemicals from the atmosphere and deposit them on Earth s surface, are discussed in Section 4.5. 

Frequently, weather systems involve a close interaction among fronts, cyclones, and anticyclones. The weather system shown in Fig. 4-16 has a large cyclone (low-pressure area) located at 180° west. A warm front, indicated by a line with semicircles pointing in the direction of movement, extends eastward from the cyclone, while a cold front, indicated by a line with triangles pointing in the direction of movement, extends southward. An occluded front extends a short distance northward. Note that clouds and precipitation are associated with this system, especially to the north of the low-pressure area and along the fronts. 

FIGURE 20.16 Vertical cross section of a typical warm front. 

In a cold front, the direction of flow is reversed (Figure 20.17). The warm air is forced ahead by moving cold air, producing a more steeply inclined frontal surface than in the warm-front storm. Presence of low-level warm air creates a relatively unstable situation leading to convective uplifting and formation of clouds and precipitation. 

A similar phenomenon exists when cooler air flows from a colder sea toward an oceanic warm front. For example, over the Gulf Stream when the wind direction is approximately perpendicular to the edge of the stream, a convective IBL may develop. At approximately 70 km downwind from the edge of the Gulf Stream, the height of the convective IBL, hi, was observed at 300 m. This says that over the Gulf Stream when eonditions are right... 

The horizontal Laplacian of temperature advection may be shown to be proportional to the temperature advection itself There is maximum warm-air advection at the ridge axis associated with the warm front and maximum cold-air advection at the trough axis associated with the cold front. If there is warm-air advection, the omega equation implies ox 0 or upward motion. For cold-air advection, downward motion is implied. Therefore, in the typical synoptic situation, temperature advection produces upward motion at the ridge axis and downward motion at the trough axis. 

An idealization of a middle-latitude cyclone is shown in Fig. 7. At the ground or surface of the earth, the cyclone is characterized by a warm front and cold front emanating fi om the central low-pressure area (L). The fronts... 

Occluded Front Front produced when a cold front overtakes a warm front and forces the air upward. 

The migration of an air mass results in transport of its associated heat, moisture, and chemical contents. Along boimdaries between air masses of differing femperatures and moisture contents, called fronts, the warmer, less dense air mass tends to rise over the cooler, denser air because of adiabatic cooling, this frequently results in precipitation. When a warm air mass overtakes a cold air mass, the front is called a warm front (Fig. 4.15a) when a cold air mass overtakes a warm air mass, a cold front results (Fig. 4.15b). A cold front is usually narrower and faster moving than a warm front, but both can produce clouds and precipitation. An occluded front results when a cold front catches up to a more slowly moving warm front, and warmer air becomes squeezed between two colder air masses and is pushed upward, causing extensive cloud development and precipitation (Fig. 4.15c). 

FIGURE 4.15 (a) A warm front occurs when a warm air mass overtakes an adjoining cold... 

Frequently, weather systems involve a close interaction among fronts, cyclones, and anticyclones. The weather system shown in Fig. 4.17 has a large cyclone (low-pressure area) located at 180° west. A warm front, indicated by a line with semicircles pointing in the direction of movement. 

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