With the weather in April being distinctly showery so far, what exactly causes this changeable weather and why do some showers give thunder and lightning?
Thunderstorms are normally associated with convective clouds which form from rising air warmed by the Sun. At this time of year we have longer days and therefore more heat reaches the Earth’s surface giving a greater chance for convective clouds to form. The air is continuously moving within the cloud in a very disorderly fashion, allowing the cloud to grow and water droplets or ice crystals to form. Given enough time and growth, the cloud may develop into a Cumulonimbus cloud and give quite heavy bursts of rain or hail for short periods of time, and possibly thunder and lightning.
Hail forms when ice crystals or frozen raindrops within the cloud get thrown about with the rapidly circulating air. As they ascend they grow as water freezes on the surface of the droplet or crystal. Eventually the droplets will become too heavy to be supported by the updraughts of air and they fall to Earth as hail.
As hail moves through the cloud it picks up a negative charge as it rubs against smaller positively charged ice crystals. A negative charge collects at the bottom of the cloud where the heavy hail collects, while the lighter ice crystals remain near the top of the cloud and create a positive charge. The negative charge is attracted to the Earth’s surface and other clouds and objects and when the attraction becomes too strong, the positive and negative charges come together, or discharge, to balance the difference in a flash of lightning. The rapid expansion and heating of air caused by lightning produces the accompanying loud clap of thunder.
Thunder and lightning facts:
- A bolt of lightning lasts on average for about one 10,000th of a second.
- The average speed at which the lightning cuts through the air is 270,000 mph.
- There are several types of lightning, the most common being “sheet lightning” in which the discharge of positive and negative charges occurs within the cloud.
- The risk of being struck by lightning is minimal and ninety percent of lightning travels from cloud to cloud. Lightning takes the shortest and quickest route to the ground, usually via a high object standing alone.
- The average annual frequency of lightning is less than 5 days in western coastal areas of the United Kingdom and over most of central and northern Scotland, and 15 to 20 days over the east Midlands and parts of southeast England
Get more facts from our thunderstorms fact sheet.