Have you ever noticed how more thunderstorms seem to occur in the summer than at any other time of year? This is because they thrive under specific conditions, requiring two basic elements – moisture and rapidly-rising warm air.

Consequently, it makes sense that there are more likely to be thunderstorms in spring and summer, in particular in humid regions. High humidity, combined with warm temperatures, will create large amounts of warm, moist air. It can easily form a thunderstorm as it rises into the atmosphere.


How do storms begin?

In summer, the number of hours of sunlight is higher than during the winter. This means the sun is heating the earth for a longer period of time. The basic law of science states that warm air will rise. This is an important factor for thunderstorms, as the warm air can rise for miles, creating a towering cumulonimbus cloud.

Nicknamed the ‘king of clouds’, cumulonimbus clouds are also known as thunderclouds. They are usually characterised by their anvil-shaped, icy top and can exist throughout the entire height of the troposphere.

The troposphere is the lowest region of the earth’s atmosphere, extending to a height of around 6km to 10km from the earth’s surface, as far as the lower boundary of the stratosphere. Cumulonimbus clouds are the only clouds that can create hail, heavy rain and thunderstorms

Thunderstorms develop when the atmosphere is unstable. When warm air exists underneath colder air, it cools and condenses as it rises, forming small droplets of water. When the upward surge of warm air is rapid, the resulting water vapour quickly forms a cumulonimbus cloud. This type of cloud can form in less than one hour.

Why does lightning form?

When the warm air continues to rise, larger droplets are formed as the smaller ones combine. They will then freeze to form crystals of ice. When the droplets get heavier and can no longer be supported by the up draught of air, they will fall as hail.

When the hail moves in the cloud, it rubs against smaller, positively-charged ice crystals, thus picking up a negative charge. This leads to a negative charge forming where the hail collects at the base of the cloud and a positive charge forming at the top of the cloud, where the lighter ice crystals gather.

The negatively-charged hail is attracted to the earth’s surface and when the attraction is too great, the negative and positive charges will come together, resulting in a flash of lightning – also known as a lightning bolt or a lightning strike.

Dangers of thunderstorms

The accompanying loud thunderclap is caused by the rapid expansion and heating of the air that the lightning bolt has produced. The UK Met Office describes the tall cumulonimbus cloud as a “huge powerhouse” that stores the same amount of energy as 10 atomic bombs, equivalent to the size of the one used in the Hiroshima attack during World War II.

Thunderstorms can be very dangerous, not only because of the power in the bolt of lightning – which can cause death, serious injury, destruction of property and fires – but also because the storm can cause rainfall rates to increase.

If the rain carries on for a long period or is heavy enough over a short space of time during a severe thunderstorm, flash floods can also occur. This can cause floods on highways, and rivers or streams may burst their banks, causing flooding in the surrounding area and damaging neighbouring properties.

Consequently, a thunderstorm can be serious even in the summer. Although individual cumulonimbus cells can dissipate within one hour when the rain starts to fall, causing heavy but short-lived showers, supercell or multicell storms can contain numerous cumulonimbus clouds, causing intense rainfall to last much longer.

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