Scurrying home from a rather bleak and windy pontoon at Brighton Marina the other night I was thinking about the weather advice that one 
of the old salts had been giving me over a warming glass of wine (yes, not rum!)…
Given the strength of the depressions that are passing over the UK at the moment I had asked how he would tell if a depression was going to bring strong or gale force winds – given of course that he wouldn’t know a grib file from a grab bag – and he advised me that as a rough guide a fall in the barometer of 6mb in 3hrs expect a force 6; 8mb in 3hrs expect a force 8. Given that we first discovered the connection between atmospheric pressure and the weather in the 17th Century you would have thought we would all be accustomed to using the barometer well instead of treating it like a piece of brass furniture – or is it just me?
In a BBC weather forecast when the barometer is described as “falling quickly” or “rising quickly”, it means a fall or rise of 3.6 to 6mb in a 3hr period. If the words used are “very quickly” then that means a movement of 6mb in a 3hr period.
February 6th, 2008|
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Martin Waller lives in Chelmondiston – a small village in Suffolk, England, on the south bank of the River Orwell, and to the east of the Ipswich.
Great tip from your salty sea dog friend. But take it with a pinch of his sea salt. I suspect it can only be considered locally.
Here’s why:
wind strength is dependent on many factors, 3 MAIN are factors:
1)pressure gradient (which your friend gave you)
2)degree of curvature of the isobars.
3)latitude
Your salty sea dog friend was giving you the pressure gradient and its resulting wind speed at a specific latitute ie. the latitude he spent most of his time at sea and noticed the pressure gradient drop and associated wind speed.
Without knowing the degree of curvature of the isobars his advice can never be totally reliable. If the isobars have high angle of curvature the wind is far less strong than if the isobars where almost parallel with each other (here I assume the distance between isobars is the same ie. pressure gradient).
Now, unfortunately, the degree of curvature of the isobars of the weather system could never be worked out from one position (where your salty sea dog is standing reading the barograph and noticing the wind speed) and so this important factor is not taken into account.
His experience averages these 3 factors and basically saying, on average the weather systems passing over where I spend most ofmy time have on average isobars with x degree of curvature.
What I’m saying is that his advice is average, literally. It could be get windier than the old salty sea dog pressure gradient rule tells you, if the isobars in the weather system are almost parallel the wind very high.
I only mention 3 factors. There are lots more affecting wind speed.
Experience has taught me never to trust people with experience.
Neil Smith
Hi Neil, of course you are right – if you were not then I suspect the Met Office boffins would be scurying down to the local souvenir shop for a barometer and pulling the plug on their NEC SX-6/8. With this supercomputer the Met Office aims to improve on its 86% accuracy rate for 24-hour forecasts…I would love to see a table of forecasts from the Met Office benchmarked against “salty sead dog” forecasts…:-)
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