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| Saturday December 05, 2009 | Accessibility help | Text only | Print | Send to a friend | Make this my homepage | Contact Us | Help |
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Orographic Uplift |
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I imagine almost every schoolgirl and schoolboy realises that the western part of Britain is wetter than the east. They would also know, I am sure, that most of our clouds and rain come in from the Atlantic where the majority of the depressions form. But that is only half of the story - the other has to do with orographic effects which means how hills and mountains change our weather. Most of our highest mountains are in the west...
Most of our highest mountains are in the west, such as those in north Wales, or in central areas like the Pennines and the Scottish mountains. As the rain-laden air comes in from the Atlantic the mountains force the clouds upwards making the rain heavier and more persistent.
As the air then descends on the lee side of the mountain, it has less moisture in it and so as it pushes eastwards, the rain is often lighter. In fact, places to the lee of high ground are said to be in a rain shadow. Exeter in the lee of Dartmoor gets considerably less rainfall each year than Princeton in the centre of the moors. ...lenticular clouds ... do not move because they form as the air rises and dissolve as it descends...
Hills and mountains can also produce clouds by themselves. In certain atmospheric conditions clouds form as the air ascends the high ground. The air cannot continue to rise and winds then 'bounce' downstream of the mountains. The clouds that form this way are known as lenticular clouds. Unlike other clouds that are blown along by the wind, lenticular clouds do not move because they form as the air rises and dissolve as it descends in the 'bounce'.
Many years ago, as a weather observer, we used to play a trick on new recruits using this type of cloud. Outside in the weather enclosure, as well as a screen full of thermometers, there was an instrument called a nephoscope. It looked a bit like an upturned rake. What you had to do was align its spikes with some high clouds, measure the time it took for the clouds to cross the spikes and then you could calculate the speed of the clouds and hence the wind at that level. The new recruits would be asked to do this on some lenticular clouds, which obviously didn't move, and we would then see how long they stayed out there measuring. I think our record was close to two hours! When there is an isolated peak, a banner cloud can form, which is a type of orographic cloud. One of the most famous is on the Matterhorn and another over the rock of Gibraltar. In these cases as the air flows up the mountain peak, an eddy forms in the lee of the summit and the reduction in pressure can form this lenticular cloud. All these orographic effects have to be taken into account by the forecaster when deciding on the amount of rain any weather front might bring. Indeed, sometimes the mountains and hills will make the clouds drop all of their rain on the windward side leaving little or no rain as the system moves across the rest of the country. This is why modelling the topography of the ground in as much detail as possible is so important in numerical forecasts. | |||||
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