Clouds are essentially a collection of tiny droplets of water or ice crystals in the atmosphere. They are formed when rising air cools to its dew point, at which the air becomes saturated. Further cooling allows water vapour to condense into cloud droplets.
Cumulonimbus clouds are one of the most recognisable cloud types, characterised by their threatening anvil-shaped tops and the torrential rain, hail, thunder and lightning that they often produce. They are the tallest clouds we see, and can extend through the entire height of the troposphere. They are often isolated, but can also develop along fronts.
In the UK, these clouds are associated with thunderstorms, most commonly in summer. Single cell thunderstorms form through local convection as a result of instability. Air rising rapidly can form a towering cumulonimbus cloud. Single cell storms usually last less than an hour, but multicell or supercell storms can last longer.
A “Spanish plume” can bring an increased risk of thunderstorms to the UK. Very warm air from Spain travels northwards meeting cooler air at height. This instability allows the hot air to rise and can result in the formation of cumulonimbus clouds. This setup generated many thunderstorms in July 2014, with 43mm recorded within just one hour in Essex.