Lifting Mechanisms

Even though the atmosphere at a given time may not be showing a great display of activity, you cannot be too sure that the atmosphere is stable. A bucket of water balanced on top of a slightly opened door (waiting for you to walk through) may appear to be stable. However, with just a small nudge you quickly understand how unstable that bucket really is. You can think of an unstable atmosphere in much the same way as the bucket balanced on the door. The term stable means that the atmosphere is in a state that is resistant to change. An unstable atmosphere may be in balance but not at all resistant to change. A little push in the right direction (in this case, up) may produce all the change necessary to sustain vertical motion, causing turbulent mixing and the production of storms. Here, we will review a few mechanisms which serve as catalysts that can bring about that change.

Orographic Lifting

When air in motion reaches a barrier that it cannot go through or around, it often goes over it. We see this in nature when air lifts over a mountain. This process of a parcel or layer of air rising as a result of the topography is referred to as orographic uplifting. If you remember from a previous reading, the equation of state and Poisson's equation illustrate the relationship between the temperature and the pressure of an air parcel. As air lifts over a mountain, the temperature decreases according to the dry adiabatic lapse rate until reaching the lifting condensation level (LCL). Above the LCL the temperature decreases according to the moist adiabatic lapse rate. If the temperature of the rising air decreases faster than the lapse rate of the air around it (the environmental lapse rate), then the parcel will continue to rise only as long as it is forced from below. A parcel of rising air that cools at a slower rate than the environmental lapse rate will continue to rise as a result of its buoyancy. We shall see a little later that this comparison of lapse rates is the process we use in determining the stability of the atmosphere. Click on the image below to view an animation of orographic lifting.