Cold Fronts

The basic feature of a cold front is the insinuation of a heavy, cold air mass under a lighter, warm one. 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 half-circles and/or triangles are plotted on the forward side of the front.

'Norwegian' or 'Bergen' model. There is a tilted frontal zone which is 20-200 km wide, intersecting the ground at the front

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. 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 instability and the jet stream promote the air's ascent, and usually trigger heavy rain and sometimes thunderstorms (Note 7.H). 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. 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.

Details of the winds around a front are shown in Figure 13.4. Moist, warm air is drawn from the north-east ahead of the front and slowly rises over the frontal surface. It is eventually deflected to the east by the upper westerlies. This flow is called the warm conveyor belt, which carries sensible and latent heat poleward (Section 12.3). The rising creates clouds, so the warm conveyor belt is often visible from satellites. Similarly, cold air follows the cold front and pushes it eastward. This is the cold conveyor belt, which slowly subsides under the frontal surface.

The approach of a cold front is usually heralded by a rapid movement of cirrus cloud across the sky. The cloud thickens to cirrostratus, then altostratus, followed by nimbostratus. The last brings rain, which may persist for hours, even after the passage of the cold front. Cumulonimbus may occur if the atmosphere is unstable. 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).

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Figure 134 Three distinct airstreams involved in a typical cold front. 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. A mid-level flow, called the 'cold conveyor belt', approaches from the south, turning eastwards as it subsides behind the front. A jet stream blows from the north-west at high levels, either slowly descending or slowly ascending, depending on the strength of the frontal disturbance. The coastline of south-east Australia indicates the scale.

Figure 134 Three distinct airstreams involved in a typical cold front. 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. A mid-level flow, called the 'cold conveyor belt', approaches from the south, turning eastwards as it subsides behind the front. A jet stream blows from the north-west at high levels, either slowly descending or slowly ascending, depending on the strength of the frontal disturbance. The coastline of south-east Australia indicates the scale.

hemisphere) and thus a rise of temperature by a degree or two, causing an elevation of the freezing level just ahead of the front. In addition, there is a fall in atmospheric pressure, due to the front lying in a trough of low pressure, caused by the lightness of the air in the warm conveyor belt. The front lies like a stream in a valley, with opposite-facing pressure gradients on the two sides, this being the cause of the opposing winds and differing air masses. The low-pressure centre at the end of a front is like the lake at the end of the stream.

The immediate proximity of a front is shown by a pressure minimum, an overcast sky, backing of the wind in the southern hemisphere from north-westerly to south-westerly, and a fall of dry-bulb temperature by several degrees. It fell 10 K in 20 minutes on one occasion in Melbourne, for example. There is also a change of dewpoint, according to the kinds of air mass. Commonly, there is a pulse of strong wind (e.g. over 28 m/s) and maybe rain, as shown in Figure 13.3. The rainfall eases once the front has passed, and then low cloud tends to clear, visibility improves and the weather becomes bright and cool. In addition, the tropopause lowers as the cold front passes (Figure 13.3), in accordance with the Palmen-Newton model (Figure 12.16).

The situation is different at sea, where the incoming cold air mass over a surface of warmer temperature creates an unstable atmosphere in which cumulonimbus clouds can arise, with showers and gusts of wind. This is a dangerous time for small boats.

Rain and thunderstorms can occur well ahead of a cold front, as shown by the pattern of clouds in Figure 13.3. For instance, a cold front crossed Western Australia on 8 November 1995 and the associated band of cloud is easily seen in Figure 13.5. There was a trough ahead of the front, called a prefrontal trough, where warm air converges and may trigger severe thunderstorms. So it is of concern to weather forecasters. There was another front to the south-west (Figure 13.5), and the cloudiness over New Zealand indicates a non-frontal trough.

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  • matthias aachen
    What are common climates at cold fronts?
    3 months ago

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