Clouds form when air rises and cools to its dew point, causing water vapor to condense into droplets or ice crystals. The World Meteorological Organisation recognizes 10 cloud types but they can be simplified into two main categories - sheets and heaps. Sheet clouds like stratus clouds appear as an overcast layer, while heaped cumulus clouds vary in size from small puffs to large cumulonimbus storm clouds that bring violent weather.