If it’s not a squall, then what is it?

Yachting Monthly - - THIS MONTH’S BIG QUESTION -

QWe re­cently ex­pe­ri­enced a dra­matic weather change on an oth­er­wise beau­ti­ful day. We had al­most three hours of rain, hail and winds gust­ing well over 25 knots. How­ever the barom­e­ter never moved from 1010. By 1630 it had all passed and we had a lovely sail back to Porth­madog har­bour. Can you of­fer any ex­pla­na­tion for this dra­matic bout of weather and what signs we should look out for in the fu­ture? Mike Cooke

ASi­mon Keel­ing, owner of weath­er­school.co.uk replies: On the day in ques­tion (Satur­day 28 April 2018) a deep trough was sit­u­ated to the west of Ire­land, shown by the sharp ‘V’ shape on the 500mb chart for mid­day. This high­lights an area of cold air at 18,000ft with tem­per­a­tures of -32°C – the key to the heavy showers on the day.

Mov­ing into spring, sur­face tem­per­a­tures rise be­cause the sun is get­ting stronger. This sharp­ens the tem­per­a­ture con­trast be­tween low and high lev­els in the at­mos­phere, cre­at­ing in­sta­bil­ity. The greater the in­sta­bil­ity, the higher the risk of heavy, squally showers.

Mike de­scribes how all was go­ing nicely un­til about 1200 when, look­ing back across the bay from Aber­soch to­wards Snow­do­nia, he could see a big black cloud sit­ting over the moun­tains and it started head­ing his way.

The morn­ing had be­come sunny in­land so the ground heated quickly, re­leas­ing ther­mals into the at­mos­phere. These bub­bles of air stayed warmer and lighter than their sur­round­ings so con­tin­ued to rise, gen­er­at­ing a huge cu­mu­lonim­bus shower cloud.

Down-draughts of cold within the clouds reach the sur­face and that’s what Mike ex­pe­ri­enced as gusts over 25 knots. You’d have ex­pected the barom­e­ter to fall but, as Mike says, there was no change in pres­sure, a state­ment cor­rob­o­rated by nearby sta­tions. The pres­sure flow on the day was ex­tremely slack, but it’s likely that there was a pres­sure fall in­land where the shower cloud de­vel­oped, but only a very slight one.

I sus­pect the at­mo­spheric con­di­tions on the day meant that it took very lit­tle per­tur­bance for dra­matic changes to take place. Be­cause of that there would have been very lit­tle Mike could do to bet­ter fore­cast the weather on the day.

A deep trough to the west of Ire­land shown on the 500mb chart for the day in ques­tion

Squalls are not al­ways linked to pres­sure changes

Newspapers in English

Newspapers from UK

© PressReader. All rights reserved.