The word stratus comes from the Latin word that means "to spread out." Stratus clouds are horizontal, layered clouds that stretch out across the sky like a blanket. Sometimes a layer of warm, moist air passes over a layer of cool air. Stratus clouds often form at the boundary where these layers meet. Where two such layers of air meet, the warm air is cooled. If the warm air is cooled below its dew point, the excess water vapor condenses to form a blanket - like layer of stratus clouds. If the layers of air are very large, the stratus clouds may extend for many kilometers across the sky.