| Understanding Weather- not predicting - 22 Aug 08 |
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| Written by John Olszewski | ||||
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The weather world sat up and took notice during the past few days. The saga began a week ago. The build-up over the week saw Ariamsvlei receiving 0.2mm rain this past Thursday evening.
Last Friday morning saw continental haze thick enough to
obscure the horizons: Friday afternoon witnessed the change. The
horizons were now visible with cloud in the middle layers. Since we
have not seen a trace of cloud for some months, this meant weather was
on the march. On the surface, the cold air behind the front surged northward during Friday night and Saturday morning saw frost occur locally across the southern half of the country. The undercutting cold air moved the middle cloud layers away toward the southeast. The expected mobility of the whole system ensured that it departed our skies fairly rapidly. The result is the pleasant day-time temperatures we had since the beginning of the week. The continued presence of cloud throughout the week indicated a considerable amount of moisture being drawn in by the vortex which formed behind last weekend’s cold front. Standing further back, the broad view exemplified just what can happen when weather systems are not quite in their usual place. This picture has been recurrent throughout the winter months of 2008. The ready ability for major anticyclonic cells to build and advance along latitudes between 40South and 50South means that the associated circulation just as readily picks up parcels of air from semi-polar and even polar sources. Such input accentuates cold front activity, likewise that of the associated upper air trough. Colder air is likely to perform antics which displace given forecasts within hours. The usual speed ranges of cold air weather systems does mean that the induced activity will be likely enough short-lived at any one place, but still defying regular prognostic identification. |
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