Clouds are both a sunshade and a blanket, and their rival effects shape Earth's temperature. On balance, they cool the planet today. Their reflective surfaces send sunlight back to space, while their water droplets and ice crystals absorb and reemit infrared radiation that would otherwise escape.
Height is an unusually powerful referee. Low, thick clouds tend to cool because their high albedo reflects substantial sunlight, while their tops are warm enough to emit infrared radiation at rates not far below those of the surface. High, thin clouds often have the opposite effect: they let much of the sunlight through, but their cold tops emit less infrared radiation to space than the warmer surface below. Darkness changes the contest. At night, reflection stops, but the heat-trapping effect remains.
The balance is less settled than the familiar shapes overhead suggest. Cloud thickness, particle size, location and the brightness of the surface beneath all matter; a cloud above dark ocean can reflect sunlight that the water would otherwise absorb, while one above snow has less additional brightness to offer. Climate models must also account for cloud feedback: as the atmosphere warms, shifts in cloud cover and properties can either amplify or restrain that warming. The uncertainty cuts both ways. Above the ocean, a white deck can be a shield against the sun; after sunset, the same sky becomes a lid on escaping heat.
