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Be your own weather forecaster, and do a better job

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ARE you responsible for a forthcoming outdoor event? Or are you planning to do the Southwest Coast Path (630 miles), the Scottish National Trail (537 miles), or a saunter to the park?

Summer in the UK usually brings a lot of showery weather, which means anything from a sprinkle to a flood. In these times of apps for anything, you consult the forecast the night before.

You have already made a wrong move. If showers are a likelihood, not one of the 20 or so forecast organisations can tell you what time it will rain tomorrow. They’ll give you useless information like ‘some showers possible’. Many will give you a forecast even more ridiculous: among symbols for each hour you discover that at 10am there is a 20 per cent risk of a shower. What use is that? The forecasting organisations, of course, can say they were right no matter what happens.

The reason for this irritating vagueness is that shower clouds are like islands in the sky, driven by the wind. You might get soaked on your walk, but just along the path the sunshine has not stopped all day.

You might think that surely the Met Office’s Cray XC40 super-computer, capable of 14,000trillion operations a second, can tell you whether it will rain tomorrow? No, it won’t. There is only one way to find out: you have to wait until the morning of your venture, then do it yourself.

You need to access the Met Office’s network of 15 rain radar stations, from Druim a’Starraig on the Isle of Lewis to Predannack in Cornwall, plus two in Eire and one in the Channel Islands. If you search for ‘UK rain radar’ you will find over a dozen, the Netweather site being a good example of the different variations (cloud cover, lightning strikes etc) you can call up. I cannot recommend the Met Office’s own version because the software people have got at it and ruined what should be a simple picture of the radar echoes.

A radar picture gives you a piece of real weather information before the giant computers get at it and turn it into surreal forecasts. You can also turn back time so you can watch what the rain has been doing in the last hour or so, notice its direction and speed of movement, and make your own forecast as to whether the shower will pass by or hit you directly.

Showers are a ‘convective’ type of rain: they form from rising currents of air (otherwise called convection). In the days when you were allowed to have bonfires you could see this happening as the smoke and bits of burnt leaf floated high into the air and went off to annoy the neighbours. Buzzards use these rising currents (thermals) to soar without flapping their wings, and they help glider pilots climb to great heights.

A shower-type weather situation is the only one that may produce hail, thunder and lightning. Hail is a result of raindrops being swept up and down inside the cloud, each upward move through the freezing level adding another thin layer of ice to each hailstone. When they are too heavy to stay up they will fall, maybe on your greenhouse.

The other rain is the sort that often arrives with Atlantic depressions, from those warm and cold (red and blue) fronts on weather charts, so it’s called frontal rain. (Parts of meteorology are surprisingly logical.) It often goes on for a long time.

A radar website can be very useful for this other type of rain but be aware that it does not always show drizzle; rain can appear in the picture but be drying up before it reaches the ground; and if you try several websites you will find inexplicably different opinions as to the rain’s extent. Some are in fact satellite pictures, not true rain radar.

Once you have seen what you can learn from these pictures of reality, you may never want to consult a weather forecast again.

PS The BBC has caused some merriment and much scorn after their website on Thursday (29th June) showed that the UK and Europe were suffering the coldest summer ever recorded. There can be no surprise that now and then computers have glitches, but I am astonished that such incredible temperatures went out unnoticed by the forecasters. Didn’t they know what it was like outside?

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Ivor Williams
Ivor Williams
Ivor Williams is a freelance writer and has been a fellow of the Royal Meteorological Society since 1984.

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