ThunderheadI took this photo while on a voyage in the south pacific a number of years ago. It was for the most part a beautiful partly cloudy day with calm seas. But off in the distance you could see this thunderhead of cumulonimbus clouds start to form. Towering cumulonimbus clouds are typically accompanied by smaller cumulus clouds. The cumulonimbus base may extend several kilometres (miles) across, or be as small as several tens of metres (yards) across, and