Low Level Clouds
There are three genera in the low level (surface to 6,500 ft [2,000 m]). Stratus clouds are usually the lowest of the three genera. Stratus clouds blanket the sky and usually appear gray. They form when a large mass of air rises slowly and the water vapor condenses as the air becomes cooler, or when cool air moves in over an area close to ground level. These clouds often produce mist or drizzle. Fog is a stratus cloud at ground level. Cumulus clouds have flat bases, are vertically thick, and appear puffy. Inside a cumulus cloud are updrafts that create the cloud's appearance. They form when a column of warm air rises, expands, cools, and condenses. Cumulus clouds occur primarily in warm weather. They consist of water droplets and appear white because the sunlight reflects off the droplets. Thick clouds appear darker at the bottom because the sunlight is partially blocked. Cumulus clouds can develop into cumulonimbus clouds. Stratocumulus clouds are large, grayish masses, spread out in a puffy layer. Sometimes they appear as rolls. These clouds appear darker and heavier than the altocumulus cloud. They can transform into nimbostratus clouds.