They can form when very weak, upward vertical air currents lift a thin layer of air high enough to initiate condensation . Stratus clouds also form when a layer of air is cooled from below to its dewpoint temperature and water vapor condenses into liquid droplets.
Stratus clouds look like a layer of fog that never reaches the ground.
Precipitation rarely falls from true stratus clouds since the upward vertical motion needed for precipitation is very weak, but light mist and drizzle can sometimes accompany stratus clouds. Cirrostratus clouds are so thin that the sun and moon can be seen clearly through the cloud. Sometimes these clouds are so thin that the only clue to their presence is a halo around the sun or moon. Ice crystals in cirrostratus clouds bend the light rays from the sun or moon as they pass through the cloud, which forms the halo around the sun or moon. Thick cirrostratus clouds give the sky a bright white appearance and often signal rain or snow within 24 hours, especially if they are followed by mid-level clouds.