Convection

The last 24 hours have seen thunder, lightning, torrential downpours, very gusty winds and pea-sized hail. This is quite a contrast to the calm, still and increasingly cold conditions traditional here at this time of year, but they are linked.

There is now much less sunlight than there was in the summer. The atmosphere is cooling, and rather more quickly than the sea and (to a lesser extent) the land which have been warmed all summer and take longer to radiate away their heat.

Often at this time of year, the cooler air settles into high pressure areas over us. The lack of cloud and long nights allows the land surface to radiate heat into space and rapidly cool in the evening and stay cool all night, so we have dew, mist and fog forming and even some frost. 

In the last 24 hours, low pressure has been in charge. Warm air from the land and sea has been rising into very much colder air above. Warm wet air from ground level will cool, but remain warmer than the cold air around it and so will keep rising. When it has cooled enough, the moisture it is carrying condenses into clouds, which actually warms that air causing it to continue to rise. We say that the air mass is unstable, in comparison with the stable air in high pressure areas, where cloud will tend to disperse or form gloomy layers, rather than growing vertically.

In unstable conditions, we have clouds that form fairly low in the atmosphere and extend to great height, with currents of rising air within them. This creates conditions in which a great deal of rain can be generated quickly, which will all fall at once as upward air currents collapse. it is almost as if the cumulonimbus clouds will continue to grow until too heavy, when everything will fall at once giving a shower or even a downpour. Rain that is lifted again (often many times) into the cloud can freeze into hailstones and even if they melt back into rain before they reach the ground, the friction between ice rising and falling within clouds separates electrical charge (like a van de Graaf generator) which eventually reconnects as lightning between different parts of the cloud or from the cloud to the ground.  All these processes are energetic and the resulting weather can be lively and even damaging.

Even so, the root cause is the cooling of the atmosphere in autumn as nights grow longer and the sun provides less heat. Often that gives calm and crisp weather, especially in the mornings and evenings but at the moment it is giving us lively convective weather.

This article was updated on Friday, 18 October 2019