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A cloud is a visible mass of condensation droplets or ice crystals suspended in the atmosphere above the surface of the Earth or another planetary body. The branch of meteorology that studies clouds is nephology. On Earth, the condensing substance is water vapor, which forms small droplets of water (typically 0.01 mm of ice crystals) that, when surrounded with billions of other droplets or crystals, are visible as clouds. Clouds reflect all visible wavelengths of light equally and are usually white, but they can appear grey or even black if they are so thick or dense that sunlight cannot pass through.