The only way the Earth receives the energy that keeps the surface warm is through sunlight. When the surface and atmosphere warm, they emit heat, or thermal energy, to space. If the radiation budget is in balance, then, the surface of the Earth should be neither warming nor cooling, on average. In this image, the dark and unreflective ocean is colored blue, while the more reflective land is colored green to grey.
Clouds are very reflective.
We see them standing out from the dark ocean. This image shows high emission from warm surfaces in yellow. Where the surface is colder, the image appears blue or white. Over the Sahara, we see through to the very warm surface of the Earth. Nearer the poles, clouds cover the surface and we see them as cold, blue areas in this image.