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