Skip to content

How Do Thunderstorms Form?

One of the main features of British summertime is thunderstorms. Generally occurring between May and September, they will nearly always form in the wake of hot and humid weather. People respond differently to thunderstorms – some are terrified because storms feature heavily as a trope of horror films. Some people sense the electricity in the air and get a positive energetic feeling from them. For migraine sufferers, they can bring on particularly bad episodes. As children, no doubt we told each other scary stories while our parents explained how “God is moving his furniture around” to make it seem less scary.

ThunderstormHow do they start, what is thunder and lightning?

The Build up to a Thunderstorm

Thunderstorms are simply the formation of a cumulonimbus cloud. These are the enormous anvil-shaped clouds often seen throughout spring and summer and sometimes into unusually warm autumns. If you’re seeing one, the chances are there is a thunderstorm going on inside it or beneath it, or somewhere behind it a thunderstorm has just finished. Cumulonimbus is also the only type of cloud able to produce hailstones. In fact, hail – even when it does not hit the ground – is a key element in triggering a thunderstorm.

This cloud type begins through a process where warm air at ground level meets much cooler air above it. As the warm air rises, the liquid contained within it condenses and freezes to form ice crystals. Some of these ice crystals compact together, get too heavy and then fall towards the ground as hail. The hail may evaporate before it gets anywhere near the surface, but the effect of creating a thunderstorm is the same.

Thunder and Lightning are the Same Phenomena

As the hail falls, it creates friction in the form of a negative charge as it rubs against the positively charged ice crystals still forming within the cloud. The hail collects near the base of these enormous clouds creating a pool of negatively charged material attracted to the ground. At the top of the cloud, the positively charged ice crystals continue to accumulate. Something must cancel out these two charges when they become too strong. That is how the effects of a thunderstorm are released.

  • Lightning: This is the actual effect of the discharge of energy between the positive and negative charge existing within the same cloud
  • Thunder: This is an after-effect of the lightning. The air in the cloud heats and expands rapidly, creating a sonic boom effect

How is it We Sometimes See One and not the Other?

This may surprise you considering that under some conditions, we will see thunder and not lightning or vice versa. Humid Mediterranean climates tend to experience lightning without rain fairly frequently, particularly in North Africa and the islands. Lightning strikes light up cloud cover but there are no other effects such as rain or even thunder. There are two simple explanations: the first is that you may be too far away to hear the thunder. Secondly, the weather conditions mean the sound bends away from the ground instead of towards it.

Similarly, you may sometimes hear a rumble of thunder but not see lightning. As thunder is an after effect of lightning, it’s not possible for thunder to exist without lightning. There are several possible explanations for why you might hear thunder in isolation. Firstly, the lightning may have been too dim for you to notice, perhaps due to extensive cloud cover. The second explanation is that the storm is so far away that it’s been some minutes between the two. You either dismissed the lightning or simply didn’t notice it. Finally, it may not have been thunder at all.