The base height of stratus is normally in the lowest 2,000 ft (600 m) of the atmosphere. Often stratus shrouds the tops of hills or sea Cliffs, and it is then known as stratus nebulosus (from the Latin word for "mist"). It forms in stable air, which has little or no turbulence-so the cloudy air does not mix with the clean air above or below it. As a result an airplane flying through stratus has a relatively smooth ride. If stratus forms in air that is lifting over hills or along a front, it may be followed by rain. People who live in valleys know to expect rain when stratus appears on the hilltops. Stratus can also form at night when moist air moves over land that is cooling. The following morning is cloudy, but if the sun evaporates the water droplets the cloud will soon clear and the day will be fine.
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