Cumulonimbus cloud, ISS image. This large anvil-shaped mass of roiling clouds formed as strong upward drafts of air during storm cloud formation hit a natural barrier in the atmosphere called the tropopause. This prevented further vertical motion and caused the cloud to flatten and spread outwards. Several cumulonimbus towers are seen underneath the main cloud (one at far right). At centre left, the cloud is casting a large shadow on the land below. This cloud formed over western Africa near the Senegal-Mali border, on 5 February 2008. Photographed from the International Space Station (ISS) by an Expedition 16 crew member.