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Cumulonimbus cloud. Cumulonimbus cloud over the sea at sunset. The largest clouds formed in the Earth's atmosphere, they cause thunderstorms and can be taller than Mount Everest. Atmospheric instabilities allow strong updrafts of warm, moist air to keep rising and condensing, up to altitudes of 10 kilometres. A natural temperature barrier, the tropopause, prevents further uplift. Spreading at the tropopause and high-altitude winds form an anvil shape, as seen. Photographed off the Baltic coast of Finland. (KEYSTONE/SCIENCE PHOTO LIBRARY/PEKKA PARVIAINEN)