Sometimes, in the minutes that immediately precede a heavy shower, there may be an anomalous burst of wind to send a little shiver down your spine. This is a squall, a term which describes a sudden temporary increase in wind strength. In meteorological terminology, it is distinguished from an instantaneous gust of wind by the fact that it may last for several minutes.
Heavy showers are associated with cumulonimbus clouds - those very tall, dark, threatening clouds like giant cauliflowers which tower upwards for many miles into the high atmosphere. Cumulonimbus clouds have their origins in strong vertical currents of air, these in turn very often triggered by the heating of the ground by the sun on a hot summer's day, or by the advection of cold air over a warm sea or land surface. The upward currents are accompanied by a drop in the temperature of the ascending air, which ultimately results in condensation and the formation of a cloud. But there are also downward currents.
The downdraughts are created initially by the drag exerted by millions of raindrops or hailstones as they fall earthwards, pulling the air with them. As the cold air, dragged down in this way, reaches a lower level, it finds itself cooler than the surrounding atmosphere, and it therefore continues to sink for many thousands of feet - accelerating rapidly towards the ground. These powerful currents of air, descending vertically, continue right down to the Earth's surface and then fan out radially beneath the cloud - becoming superimposed on the existing wind at ground level.
By this mechanism, in otherwise calm conditions, a heavy shower is commonly preceded for a few minutes by a gentle drift of wind at ground level directed towards the shower cloud - feeding the initial updraught. This is abruptly followed, just before the rain begins, by a much stronger and distinctly turbulent wind in the opposite direction - radiating outwards from the cloud - which marks the commencement of the downdraught.
If there is already a light breeze blowing before the arrival of the shower, the downdraught reinforces the existing breeze downwind of the shower-cloud. Upwind of the shower-cloud, on the other hand, the force of the radiating downdraught is often sufficient to reverse the existing wind direction for a time.
It follows, of course, that flags and other wind sensitive objects are often seen to behave erratically in the vicinity of a shower or thunderstorm. This may well explain Byron's moving but meteorologically enigmatic lines from Childe Harold's Pilgrimage:
Yet, Freedom! yet thy banner torn, but flying,
Streams like the thunderstorm against the wind.