We have just come off the back of yet another sweltering heatwave in London, and with every good heatwave comes a thunderstorm. The Met Office predicts that storms will hit the capital for three days in the coming fortnight, with thundery showers forecast to cover the city this weekend.
Thunderstorms are certainly inconvenient, and often seem to come out of nowhere. As rumbling clouds travel over the city, they take all of your summery plans with them. Your picnic in the sun on Primrose Hill turns into a rush for cover, and your train home grinds to a standstill. But why do they happen, and why have so many plagued us this summer?
The science behind the storm
The onset of summer brings warm temperatures that heat up the air in the atmosphere, causing it to rise. As the air rises, it cools, condensing the water vapour that travelled along with it.
The water vapour condenses back into water droplets, and these droplets are formed on miniscule particles that float in the air. The floating droplets form the dreaded cumulonimbus clouds that cause thunder and lightning.
Within the newly formed cloud, the water droplets freeze together into ice crystals that fall as hail once the cloud can no longer hold their weight. As the ice crystals fall, they collide with graupel, soft hail, pellets transferring tiny amounts of positive charge into the crystals.
As a result of air currents, the positively charged ice crystals rise to the top of the cloud, creating an electric potential difference with the negatively charged graupel at the bottom. As a result of this electric potential difference, the air breaks down, allowing for the rapid expulsion of electricity that we know as lightning.
The rumbling noises that we associate with a thunder storm results from the heat produced by a lightning strike. The heat causes the air around it to expand rapidly, creating a shock wave that creates the sound of thunder.
Thunderstorms in London
Thunderstorms are especially prevalent in London due to the city’s microclimate, a so-called urban heat island. The air in urban areas, such as London, is significantly hotter than the air found in rural areas, and this is due to the increased capacity of urban land surfaces that can trap heat.
The land in the capital is covered by roads and buildings in a way that the land in other areas are not. Central London is especially populated by skyscrapers that act as urban canyons, further retaining hot air.
The increased heat in London can contribute to the creation of cumulonimbus clouds, the clouds to blame for the number of storms that have hit the city this summer. The distribution of temperature in London (above) means that Central London is likely to be hit the hardest by the oncoming storms.
Climate change and thunderstorms
As our climate continues to change, the UK is expected to experience warmer weather all year round. This increase in temperature is likely to lead to a rise in the number of extreme weather events, such as thunderstorms.
A warmer atmosphere provides the perfect environment for the development of storms, as it is able to hold a greater volume of water vapour. Along with increased moisture, additional atmospheric instability could create the ideal conditions for more violent thunderstorms.
As London heats up, both from natural summer warmth and the city’s own urban heat island effect, the perfect conditions for thunderstorms fall into place. Busy city roads and high-rise buildings lock in heat, pushing warm air upward, triggering the menacing cumulonimbus clouds behind our forecast storms.
With climate change steadily turning up the thermostat, this is not just a seasonal quirk, it’s a sign of summers to come. Hotter air holds more moisture and fuels further unstable weather, meaning thunderstorms like those we’ve seen this season may become a more familiar fixture of London’s forecast.
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