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Christmas Eve weather warning- lowest pressure in over 100 years

An amberweather warning for strong winds tomorrow (Tuesday) has been issued by the Met Office.

There is also a yellow warning for winds on Christmas Day.

Stornoway Gazette weather correspondent Eddie Graham is also warning of ‘a very rare and extraordinarily deep low pressure system’ which is forecast to pass very close to the Hebrides late on Christmas Eve.

On his blog he states: “There is disagreement between the various met models on its exact depth (as they don’t get to deal with such unusual weather so often!), but the various forecasts indicate a low of somewhere between 924hPa and 936hPa.

“To put this in perspective,a value of 940hPa has only been recorded once in Stornoway (and nowhere else in the UK or Ireland) since 1886. This was recorded at Stornoway at 0020 GMT on 20 December 1982 when a value of 937.6hPa (or millbar/mbar, the same thing) was measured. So, if Tuesday’s low centre passes close to Lewis, the lowest air pressure in the British Isles for 127 years may be recorded (with thanks to Stephen Burt for this research).

“The lowest authenticated air pressure in the British Isles is 925.6 hPa/mbar, recorded at 2145h on 26 January 1884 at Ochtertyre, near Crieff (Scotland).

“As air pressure is directly related to the mass of the atmosphere, this means that when a low of 920hPa passes over your head, the atmosphere is actually about 10% lighter than the average at sea-level! You might not feel anything special (other than your ears “pop”), but it is equivalent to climbing to the top of a Munro of 3,000 feet or more!

“On a more serious note, lighter air (= lower air pressure) means that there is less weight of air lying on the surface of the sea, and hence the sea “re-bounds” upwards. The effect is equivalent to about a 1-metre rise in sea-level for every 30hPa drop in pressure. Therefore, sea-levels around the Hebrides could rise by more than 2-metres above their usual heights during the passage of the low system. Please take note if you live near sea-level, or work in this area

“Finally - how windy will it be? Well, firstly I hasten to add that there is NOT a direct linear correlation between the absolute depth of a low pressure system and the strength of the wind. But often very deep lows bring very strong winds. Presently, it looks like Ireland, SW Scotland and the southern Hebrides are most at risk from the strongest and damaging winds (60-90mph, possibly 100mph).”

Keep up to date with his weather reports at http://uhi-mahara.co.uk/view/view.php?id=1258

 

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