| Understanding Rainfall and its occurrence |
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| Written by John Olzsewski | ||||
Page 2 of 2 Again, looking at the history of daily falls, it became necessary to flesh out the definition. Based on historical figures, it is reasonable to add 20mm per day, hence 80mm over 4 days, or 100 mm over 5 days, I describe as an expansion of the basic wet spell range. When I designed this definition of a wet spell, the productive value of the measured rainfall was the guiding line. Three consecutive days with 60mm are generally more productive than 5x12mm over three weeks. In our unique climate, where rainfall is usually very erratic, it was necessary to clearly define the benchmark for those spells that really make a difference to grazing, farming conditions and the state of underground water reserves. But why has this kind of rainfall pattern remained unidentified? In the entire temperate world, the focus is always on divisions of ten days, the so-called dekads. The next timeframe is monthly totals. In Namibia however, productive falls are lost within these conventional divisions. Worse, there is the tendency to discard fluke occurrences because they are well beyond the normal ranges. This average conformity is at home in the temperate rainfall belts of the world where rainfall is much higher but also much more consistent. So when I realised the importance of our local wet spells, it made sense to record and identify these occasions because they are an integral part of our rainfall regime. With station analysis is at an advanced stage, it shows that a Wet Spell at one station can be recorded across a range of adjacent stations. To my mind, this represents the wet culmination of a major synoptic pattern on the weather maps for those days. When there is a tendency for the same group of stations to have a level of conformity across the years where these events are concerned, the outline of a rainfall district looms. This key topic continues next week with some recorded examples. |
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