Adiabatic Lapse Rates

Dry Adiabatic Lapse Rates

Dry adiabats are drawn as solid lines, drawn diagonally towards the upper left. These lines represent the change in temperature that an unsaturated parcel of air (imagine a balloon) would experience as it moves up the atmosphere (i.e. to a lower pressure level). These lines appear as light blue curved lines, curving upward and to the left. The temperature markings are on the bottom (the same legend as for the isotherms). On this diagram, the dry adiabat on the far left should be -50 degrees C. The farthest (unmarked) dry adiabat on the right is 100 degC.

A more common name for these lines is the Dry Adiabatic Lapse Rate, or DALR. "Dry" air is defined as any air parcel that has less than 100% humidity. As the parcel of air rises, the pressure becomes less, so the parcel expands. As it expands, it cools without exchanging any heat with the atmosphere (thus, it cools adiabatically). The dry adiabatic lapse rate is around 9.8 degrees C per km of height in the atmosphere.

If you can print out the diagram, you might want to try this exercise. Alternatively, you can "follow along" with the graphic on the right. Suppose you have an air parcel that has a temperature of 10 degrees C at the surface, where the pressure is 1013 mb. What would the temperature be at a pressure of 700 mb? Start the parcel at the surface on the 10 degree dry adiabat curve. Follow this curve up to 700 mb. Then, read the temperature from the dark blue isotherms that skew up to the right. You should have a value of about -18 degrees C.

Let's see how close that is to the calculated value. Using the lapse rate calculator, use these values:

1. Intial temperature of air parcel: 10 degrees C
2. Initial height of the parcel: 156 m (based on a surface pressure at 1013 mb)
3. Final height of the parcel: 3143 m (the height at 700 mb)

What value do you get? You should get -19 degrees C, pretty close to the "eyeball" value of -18 degrees.

Saturated Adiabatic Lapse Rates

You should also notice a series of curved dotted lines (green on this particular diagram). These are the saturated adiabatic lapse rates, or SALR (also known as "moist"). These represent the path of a parcel that has 100% humidity (i.e., saturated) will take as it rises in the atmosphere. The SALR is around 6 degrees per km (actually 5.8). Again, an example is helpful (but we won't work this one for you!). Given a saturated parcel starting at 10 degrees, what is its temperature at 700 mb? If we follow the saturated adiabat up to 700 mb, we read off that the temperature of the parcel is about -7 degrees. A calculation using the same starting values as before, and the SALR of -5.8 degC/km, we calculate the same value.