What causes thunder and lightning? - Met Office

Thunderstorms develop when the atmosphere is unstable. This is when warm air exists underneath much colder air.

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Practicing Safety: When Thunder Roars, Watch For Lightning! – NBC

Thunder - Met Office

What causes thunderstorms? Here's how storms (and lightning) form.

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Why do thunderstorms happen after hot weather? - BBC Newsround

What Causes Thunder And Lightning? - The Fact Site

What Causes Thunderstorms? - Weather Tomorrow

Met Office issues first thunderstorm warning - BBC News