Adiabatic Lapse Rates:

On the second half of the worksheet for this week there are questions relating to the Moist

Adiabatic lapse Rate (MAR) and the Dry Adiabatic lapse Rate (DAR) as they relate to elevation

changes and Relative Humidity. So, let's take these apart and look at how they work.

First, the Dry Adiabatic lapse Rate (DAR) is 10 degrees per 1000 meters elevation change.

Dry, unsaturated air has wider swings in temperature because there is less water to moderate the

energy in the atmosphere. So, when an air parcel is rising (unstable air) it will cool as it rises at

the DAR, and when the air parcel is cool it will sink or subside at the DAR.

This changes when the air parcel reaches the Dew Point. This is the point where condensation

occurs, and the Relative Humidity is 100%. At this point a great deal of energy is expended in

the air parcel as water condenses, and the Adiabatic lapse Rate changes from Dry to Moist. This

Moist Adiabatic lapse Rate (MAR) is 6 degrees per 1000 meters elevation change. The water

in the atmosphere is moderating the temperature change.

So, if we start at the beach, and the temperature is 20 degrees C, with specific humidity

(content) of 10 g/kg, we go to the table and see that the absolute humidity (capacity) is 20 g/kg.

10g/kg / 15 g/kg x 100 = 67% Relative Humidity

Now, we're going to go inland and up Mt. Palomar, which is 2,000 Meters high. When we

reach 500 meters, the temperature will be decreasing at the DAR, since the air parcel is dry.

20-5 = 15 degrees C at 500M

and the air parcel now has a Relative Humidity as follows:

10g/kg /10 g/kg x 100 = 100 % Relative Humidity

Now we have to change and continue up the mountain decreasing by the MAR, since the air

parcel is saturated. So, we continue up the mountain, remembering that we have an additional

1500 meters to go. It would look like this:

15-(6+3)= 6 degrees C at 2,000 M

At 6 degree C, the air parcel is still at 100% Relative Humidity, however now it is colder and the

air parcel cannot hold much moisture. It is raining, to be sure. We go back to our table, and we

see that at 6 degrees C, the air parcel can hold about 6 g/kg of water. It's rained a lot.

Now, we are crossing the Air Mass Boundary on the mountain, and heading east to Anza

Borrego desert. Since the air is now cooled by all that rain and increase in elevation, it is heavy

and will subside (sink) back down toward the desert. The air parcel will compress as it descends,

and get warmer at the DAR. By the time we get to Anza Borrego, the air parcel will look

something like this:

6 + 20 = 26 degrees C

At 26 degrees C, the air mass has an absolute humidity of 26 g/kg. So, now the Relative

Humidity is much different than when we started out at the beach.

6g/kg / 26g/kg x 100 = 23 % Relative Humidity

As you can see, the air parcel is both warmer and dryer in the desert than it was when it was near

the ocean.

When you do the worksheet for this week, remember that the last 4 questions all pertain to the

same set of data, and will look much like this example.