The air's stability helps determine what types of clouds form and what kinds of precipitation fall from them. During winter, most precipitation across the USA is caused by warm air overrunning cold air. Such a situation with relatively warm air above colder air is stable. The gentle cooling of the rising warm air forms nimbostratus clouds, which are relatively thin and produce continuous light to moderate precipitation over a large area.
Visibility is often very poor as low clouds and fog usually form under the main cloud deck.
Since the warm air is gently rising, turbulence is usually minimal to light. In an unstable environment with cool air over warmer air, the lighter, less dense warm air forms strong updrafts that often lead to heavy showers and thunderstorms . Showers that fall from cumulus clouds are usually much heavier than precipitation from nimbostratus clouds. However, such showers tend to be isolated and usually last for an hour or less whereas rain from nimbostratus clouds can last all day. Visibility is usually fair to good in an unstable environment away from the showers, but turbulence can be extremely violent as strong vertical air currents often form.