The density varies between cold air and warm air with cold air being relatively more dense. Due to the higher density, cold air sinks to the surface and resists being lifted in the vertical. In a differential advection situation, relatively warm air will lift over cold air because it is less dense. The idea of isentropic lifting is air "prefers" to move toward air with the same density. Warm air resists undercutting cold air. To stay at the same density it must override the cold air. The depth of cold air on the cool side of a warm front generally increases moving to the north. As warm air overrides colder air, it is forced to a higher elevation as it moves north of the warm front boundary. The diagram below shows the process of air rising over a warm front and staying at the same density level. Density 5 represents the highest density while density 1 the smallest. The warm less dense air rises gradually in the vertical as it overrides the sloping cold dense air. It must do this to stay at the same density. This is why warm fronts tend to bring widespread light to moderate precipitation. The uplift is at a lower angle than uplift that is generally associated with cold fronts and thermodynamic thunderstorms.

exercise questions

1. What process in the atmosphere lead to isentropic descent?

Answer: Cold air advection

2. Why is the moisture content of the air in the warm sector important to the process of isentropic lifting?

Answer: Higher values of moisture will lead to higher rainfall when the air is lifted. Moist air releases more latent heat than less moist air when lifted at the moist adiabatic lapse rate.

3. Why is isentropic lifting in association with cold fronts more vertically inclined than that associated with warm fronts? In order words, why is lifting more intense with cold fronts?

Answer: The change in density across a cold front is greater than the change of density across a warm front. A larger density gradient leads to a larger frontal slope and more vertically inclined convection in association with a cold front. Also, a cold front moves more quickly and tends to be more deep.

4. What can you infer will happen if positive vorticity advection overrides a region of isentropic lifting?

Answer: Large upward vertical velocities can be inferred. Two lifting mechanism will combine together to amplify the amount of uplift.

5. Why is it important to be able to see the atmosphere is 3-D when analyzing isentropic lifting?

Answer: It is because isentropic lifting does not occur on a constant pressure surface.