Occluded 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 4-15c An occluded front occurs when a warm air mass rises under the combined influence of two colder air masses, causing extensive cloud development and precipitation. 

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 4-21 shows a major low-pressure system over the Great Lakes and the eastern coast of the United States, with a cold front located to the southeast and an occluded front to the northeast. In this late winter storm system, surface winds are counterclockwise and blowing somewhat toward the center of the low, as would be expected from Fig. 4-16. By convention, the tails of the wind arrows extend in the direction from which the wind is blowing, and the barbs on the arrow tails indicate wind speed, which is generally greater for stations closer to the center of the low. The various precipitation symbols, and the filled-in or partially filled-in station circles that reflect the fraction of sky covered by clouds, show that precipitation and clouds are associated with this system, especially near the fronts. Southward, behind the cold front, clear skies and cooler temperatures prevail RH also is lower, as shown by the larger differences between temperature and dew point.

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—ContM (c) In an occluded front, the tendency of a warm air mass to rise is accentuated by the presence of two converging, colder air masses extensive cloud development and precipitation result. 

Both materials have similar masses and therefore similar reflection coefficients for hydrogen. But, Ta is an exothermic hydrogen occluder and W an endothermic one. Thus, different hydrogen release properties are expected. Figure 13.7 shows the temperature-dependent light intensity in front of Ta... 

At intermediate growth rates, the dispersed drops will first be pushed along, but due to an increase of the amount of droplets at the solidification front, they will coagulate and subsequently be engulfed. This results in a spherulite center consisting of pure crystaUine material, and an outer layer in which dispersed particles are occluded. 

One of the cues used by the visual system for perception of depth or relative distance of two objects from the observer is occlusion. That is, if one object occludes a second object, it is perceived as being in front of that second object. Many vaginal marks of hamsters are deposited as linear streaks or lines, and we hypothesized that hamsters might be able to use olfactory occlusion to determine which scent was on top. That is, in an over-mark the top scent is a continuous streak if it masks the bottom mark in the region of overlap, the underlying scent mark will appear to be discontinuous (interrupted). If hamsters can keep track of the spatial layout of the two scents they could potentially use this continuous versus interrupted information as a means to determine which individual s scent is on top, and thus preferentially remember the top scent. 

---