Thunderstorms form by the uplifting of warm and moist air. When humid air is elevated, the air will be cooled and moisture will condense to begin the formation of cumulonimbus clouds. Upon further uplifting, the cloud will extend higher and water droplets in the cloud continue to grow in size. A cumulonimbus cloud results when it grows to a height of 10 to 20 kilometres. The pilot who took this picture was flying over the UK south coast heading north-west towards London with this massive line of thunderstorms filling the entire window.