One of the annoying phrases often heard in weather forecasts is “clouds bubbling up” (letters, March 29). But of all the many clichés used by forecasters this one has some science to it.

April brings longer days and stronger sunshine, which also delivers a sharp increase in showers. The extra solar energy heats up the land until it launches bubbles of warm air above, rather like hot-air balloons lifting off the ground. As these bubbles rise up they gradually cool off and grow cold enough for their moisture to condense into puffy, cotton-wool cumulus clouds. These are notorious for short, sharp downpours of rain, hail, sleet or even snow. And because the seas around our shores are still cold, the April showers are usually restricted to the land.

However, such showers tend to be localised, and they cause endless trouble for forecasters trying to predict when and where rain will fall. And if the air high up is particularly cold, the rising bubbles of warm air can reach the base of the stratosphere, miles above the Earth’s surface, and create towering cumulonimbus clouds that produce thunderstorms. All of this helps the springtime growth of plants, as the folklore saying goes: “April showers bring forth May flowers.”