Fronts provide a mechanism to lift air.
The upward force that lifts air to form clouds must overcome the downward force of gravity. As noted in Chapter 2, an atmosphere that is unstable has a temperature and moisture structure that is favorable for lifting air parcels and thus forming clouds and possibly precipitation. A stable atmosphere inhibits rising motions. Because of its importance to storm development, it is useful to revisit the concept of atmospheric stability and describe how to determine if an atmosphere is stable or unstable. To observe the vertical structure of the atmosphere we attach thermometers and barometers to helium filled balloons.