|ISENTROPIC LIFTING (UPGLIDE)
METEOROLOGIST JEFF HABY
The density varies between cold air and warm air with cold air being relatively more dense. Due to the higher
density, cold dense air sinks to the surface and has a high resistance to being lifted in the vertical. In a
situation, relatively warm air will lift over shallow surface cold air because the warm air is less dense.
The idea of isentropic lifting is air "prefers" to move toward a region with the same density (potential temperature
surfaces (a.k.a. along a constant theta surface)).
Warm air resists undercutting cold air. To stay at the same density
potential temperature) it must override the cold air. The depth of cool air on the cool side of a
warm front generally increases moving to the north of the surface warm front boundary. As warm air advects over
colder air, it advects to a higher altitude above sea level as it moves north of the warm
front boundary. The warm, less dense
air rises gradually in the vertical as it overrides the sloping cold dense air (less potential temperature air).
It must do this to stay at the same potential temperature (same relative density). This is why warm fronts tend
widespread light to moderate precipitation. See diagram below:
The uplift is at a lower angle than uplift that is generally
associated with cold fronts and
thermodynamic thunderstorms. You will often come across this complicated concept
when reading NWS forecast discussions. Terms that they will mention that relate to this concept include (theta
surface, potential temperature surface, isentropic upglide, differential advection, and density perturbation).
For another explanation of this process of isentropic lifting (and diagrams) see Chuck Doswell's article at: