Was this a normal squall?
QI was on an early-season cruise in the North Sea this year when it got very squally. There was bright sunshine between the clouds, with a Force 4 or 5 from the north-west, and for late April it was freezing! Gradually the clouds became more threatening before huge, angry-looking anvil clouds. Under one of them, the wind picked up considerably and I was forced to shorten sail. There was no lightning, but I was surprised by the force of the squall. What weather should I expect with such clouds, and how can I avoid the worst of it? Michael Stowe
ASimon Keeling, managing director of Weather School. co.uk, replies: That sounds like quite an eventful day! There isn't really an easy way of predicting exactly what winds will do in such systems. However, there are a few pointers that can ease the shock of being hit by such squally winds.
In order for large clouds to produce the gusts you describe it's necessary for the airmass to be unstable. This means that temperatures fall quickly with height and such conditions frequently occur when the wind come from the northwest (in the UK and Europe); we call this a Polar Maritime airmass. Towering cumulus clouds form and the turbulent air within them create areas when cold air sinks rapidly through the cloud, we experience this sinking cold air as strong gusts.
Although you didn't experience them directly there were likely to be showers within the vicinity of your sailing. The shower clouds can group together into more persistent bands of rain; these are called troughs. You can spot troughs on frontal charts as dashed lines. These denote areas where showers are grouped together and where winds are likely to be gusty. I always warn that if a trough passes overhead then you should expect gusts of three to four forces above the average wind speed.
There are clues to these conditions in the Shipping and Inshore Waters Forecasts. Listen out for phrases such as ‘squally showers’ and ‘hail showers’. But even with a forecast of ‘showers’, gusts of two to three forces above the average speed should be expected.
So, with wind in the north-west, a cool airmass, large cumulus clouds and showers being mentioned in the Shipping Forecast, you should have enough clues to prepare and to be less surprised by these gusts next time..