Swelling cumulus are clouds with flattish bottoms and rounded tops, having a vertical development. They may precede cumulonimbus clouds that bring rain. Clouds are are made up of tiny droplets of water that are floating in the air and look white because they reflect all the colours the exact same amount.
The picture was created with a Nikon D40X camera and Nikon 55-200mm VR AF-S f/4-5.6G ED lens, using these settings: exposure time – 1/200 seconds, focal length – 75 mm, ISO – 100, F- number – 5,6.