There is a shallow flow of warm air near the ground from the east, called the 'warm conveyor belt', which turns south and ascends before the front. There is also a strengthening of the warm wind (northerly in the southern Figure 133 Idealised cross-section of an active cold front, showing cumulus cloud (Cu), stratus (St), nimbostratus (Ns), altostratus (As) and cirrostratus (Cs).įigure 134 Three distinct airstreams involved in a typical cold front. Cumulonimbus may occur if the atmosphere is unstable. The last brings rain, which may persist for hours, even after the passage of the cold front. The cloud thickens to cirrostratus, then altostratus, followed by nimbostratus. The approach of a cold front is usually heralded by a rapid movement of cirrus cloud across the sky. This is the cold conveyor belt, which slowly subsides under the frontal surface. Similarly, cold air follows the cold front and pushes it eastward. The rising creates clouds, so the warm conveyor belt is often visible from satellites. This flow is called the warm conveyor belt, which carries sensible and latent heat poleward (Section 12.3). It is eventually deflected to the east by the upper westerlies. Moist, warm air is drawn from the north-east ahead of the front and slowly rises over the frontal surface. Such fronts commonly occur along the south-east coast of Australia in summer, when the air is too dry to generate enough instability for cumulonimbus and there is no lifting by a jet stream.ĭetails of the winds around a front are shown in Figure 13.4. Without these stimuli, the front is a passive front or katafront (from the Greek word 'kata', meaning downward), which is only 1-2 km deep and brings cooler air and stratus cloud but little rain. The instability and the jet stream promote the air's ascent, and usually trigger heavy rain and sometimes thunderstorms (Note 7.H). A front is termed 'active' if it is preceded by a warm air mass which is already unstable and being slowly forced to rise by a jet stream above (Note 12.L). The zone slopes at only about 1 km in 100 km, unless there is an active cold front (or anafront-from the Greek word 'ana' for upward) when the zone may be two to four times as steep. There is a tilted frontal zone which is 20-200 km wide, intersecting the ground at the front The half-circles and/or triangles are plotted on the forward side of the front. The result of the cold air's advance is often the pattern of uplift, subsidence, clouds and rainfall shown in Figure 13.3, the classical Figure 13.2 Conventional depiction of the most common types of fronts and a trough on weather charts. The basic feature of a cold front is the insinuation of a heavy, cold air mass under a lighter, warm one.
0 Comments
Leave a Reply. |
Details
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |