It’s no secret that the British are obsessed with the weather but in July there's been something relatively rare to talk about – an extended period of warm, sunny conditions! And, inevitably, that has meant some spectacular summer storms when the weather broke at last.
Summer storms can be particularly violent and spectacular: along with the thunder and lightning, these weather systems dump vast quantities of rain over a short period because warm air can hold more moisture. This raises the prospect of localised flooding as the rain hits the dry ground and tends to run off rather than soaking in.
So what causes summer storms? There’s a great explanation of what’s been happening in the UK of late in this video from a BBC weatherman: the science behind summer storms.
Of course, storms here on earth are caused by changes in the atmosphere. The brief flash of light and the enormous release of concentrated electrical energy that is lightning has its origins in the water cycle. We all know that heat rises so, as the sun’s energy heats the planet, moisture evaporates and takes to the skies in the form of vapour. Eventually this water vapour will form clouds.
Look out for what we call cumulonimbus clouds: these are massive clouds which stretch high into the atmosphere and are indicative of an approaching thunderstorm. These turn into thunder clouds which are charged like giant capacitors in the sky: due to a combination of updrafts, downdrafts, freezing and particle collisions, the upper portion of the cloud carries a positive charge while the lower portion is negative. This charge differential goes hand in hand with an electric field so, as the charge separation grows stronger, so too does the associated electric field.
We’ve all seen a Van de Graaff generator in a school science experiment where the spark jumps the gap between the two spheres. Just like that Van de Graaff generator, eventually an intense electric field can cause air around a thunder cloud to become ionised, enabling current to flow through the ionised air or plasma to potentially neutralize the charge separation. The path of ionized air is called a ‘step leader’.
While all this is going on aloft, the positive charge is increasing on the Earth’s surface below, and objects (including people) respond locally to this strong electric field by sending out so-called ‘positive streamers’.
The situation is now primed for a lightning flash. When a ‘streamer’ and a ‘step leader’ meet, they can form a complete path for lightning to travel from the cloud to the ground… and we have our lightning strike!
Then, wait for it, we usually here a clap of thunder. As the air around the strike heats up and expands greatly, this causes a shock wave to radiate away from the strike path. We hear this as thunder.
Have you ever tried working out how far away a lightning strike is during a storm? You’ll have noticed that there is often a gap between seeing the lightning and hearing the thunder. We can use this phenomenon to calculate how far away the lightning is. Because sound travels through the atmosphere so much more slowly than light (we can assume light arrives instantaneously over the distances involved on earth) all we have to do is start counting in seconds (or use a stopwatch) as soon as we see the lightning.
Although the speed of sound in earth’s atmosphere changes according to temperature and humidity, we can assume it’s roughly 350 meters per second (1,200 feet per second). Thus, sound travels 1km in around 3 seconds and a mile in about 5 seconds. So, if you count less than five seconds before hearing the thunder, the lightning flash was less than a mile away. Get under cover quick!