Why does a rising parcel of unsaturated air cool at a greater rate than a rising parcel of saturated air in which condensation is taking place )?

For the atmosphere, the drop in temperature of rising, unsaturated air is about 10 degrees C/1000 meters (5.5 deg F per 1000 feet) altitude. If a parcel of air is at 24 degrees C at sea level, and it rises to 1000 meters, its temperature will go down to 14 degrees C. If it goes up to 2000 meters, its temperature will go down to 4 degrees C.

4. What will its temperature be at 3000 meters?

The temperature would be minus 6 degrees C.

This rate of temperature change of unsaturated air with changing altitude is called the dry adiabatic lapse rate: the rate of change of the temperature of rising or subsiding air when no condensation is taking place (we’ll talk about the condensation part shortly).

If the air subsides, it also changes temperature. It warms up, and it is warming up at the dry adiabatic lapse rate. So, if the air at 4000 meters altitude has a temperature of -10 degrees C, and it subsides to 3000 meters, its temperature will warm up to 0 degrees C. If it continues to subside, then at 2000 meters, its temperature will be 10 degrees C.

5. What will the temperature of this air be at 1000 meters?

Its temperature would be 20 degrees C.

Make sure you notice that we are talking about moving air (rising or subsiding), not still air. The change in temperature of still air (that is, air that is not rising or subsiding) follows the environmental lapse rate, which varies considerably, but averages about 6.5 deg C/1000 meters (3.6 deg/1000 feet). In still air, if you went up in a hot air balloon, carrying a thermometer and taking the air temperature every 1000 meters, on average the temperature would drop 6.5 degrees C every 1000 meters. The rate of temperature change as you rise in still air is not as great as the rate of change of rising air; that is, the air parcel does not cool off as fast.

For instance, the air temperature at sea level is 28 degrees C. Climb into your balloon, release the tethers, and go up 1000 meters in the still air.

6. On average, what will the air temperature be at 1000 meters?

The temperature will be 21.5 degrees C.

7. If the air is rising, and the temperature at sea level was 28 degrees C, what will the temperature of the air be after it rises 1000 meters?

The temperature will be 18 degrees C, cooler than the still air; the dry adiabatic lapse rate is greater than the environmental lapse rate.

Let’s abandon the still air for the moment, and return to the air which is rising, and getting colder. Remember what happens to relative humidity when air temperature decreases? Ok then.

8. What happens to the relative humidity of a parcel of air when the temperature decreases?

If the temperature of the parcel of air decreases, the relative humidity increases. This is a KEY point. If you did not answer this correctly, you really should go back and review the explanation of relative humidity.

You can maybe see what’s coming next. If the air is rising and cooling at a rate of 10 deg C/1000 meters, (5.5 deg/1000 feet), eventually, it’s going to cool off enough for the relative humidity to reach 100%, and condensation can take place. The dew point is the temperature at which the air becomes saturated and condensation takes place (note: dew point is a temperature, given in degrees C or F). The lifting condensation level is the altitude at which condensation begins (note: lifting condensation level is an altitude, given in meters or feet). You can look up at the windward sides of mountains and see where the lifting condensation level is, because that is where you will see the bases of clouds that have formed.

Here’s where it gets a bit complicated. Remember what happens when water changes state?

9. When water evaporates, is heat absorbed or released?

Absorbed

10. When water condenses, is heat absorbed or released?

Released

So, if condensation is taking place, latent heat is being released to the surrounding air. So you have two opposing trends going on at the same time within this parcel of air. It’s rising and cooling, but it’s also condensing and being warmed. Which one will win out? That is, will the air get colder, or will it get warmer?

Well, what happens is that the air will still cool off, but not as fast. If water vapor in the air is condensing, the adiabatic rate is lower. The air is only cooling off at a rate of about 5 degrees C/1000 meters (2.7 deg per 1000 feet). This is called the saturated adiabatic lapse rate (or the wet adiabatic lapse rate, or the moist adiabatic lapse rate, depending on the textbook you are using). The saturated lapse rate varies with the original temperature of the air parcel, but 5 degrees C/1000 meters is a commonly used value.

So, let’s assume a rising parcel of air reaches the lifting condensation level at 2000 meters, at a dew point temperature of 12 degrees C. At this point, clouds will form. As the air continues to rise, it will continue to decrease in temperature, but more slowly than it cooled off before condensation began.

11. What will the temperature of this parcel of air be at 3000 meters?

The temperature at 3000 meters will be approximately 7 degrees C. The saturated adiabatic lapse rate is given as 5 deg C/1000 meters, so if you go up 1000 meters, the air will cool off 5 degrees. 12-5=7.

12. If air is subsiding (for example, if it has gone over the crest of a mountain range, and is flowing down the leeward side of the mountains), will the temperature increase or decrease? Assuming that all the moisture was removed from the air as it rose up the windward side, which lapse rate would you use to figure out the exact amount of change?

Air which is subsiding will be increasing in temperature. If we assume that there is no moisture left in the air (which may not always be the case), the applicable rate is the dry adiabatic lapse rate.

Notes on “Air Temperature and Clouds With Flow Over a Mountain”

Air Parcel à an imaginary small body of air only a few meters wide that is used to explain the behavior of air

Lapse rate à the rate at which temperature changes with height

Environmental lapse rate à the rate at which the temperature changes with height. This is measured by a rawinsonde (weather balloon) and is mapped out on a sounding diagram (Stuve or Skew-T diagram). It is the temperature line on sounding diagram.

Dry Adiabatic Lapse Rate (DALR) à The rate of change of temperature in a rising or descending DRY (unsaturated) air parcel. DALR ~ 10°C/1km. So if a DRY air parcel is ascending it is cooling by 10°C per kilometer. If the DRY parcel is descending it is warming by 10°C per kilometer (aka compressional warming)

Moist Adiabatic Lapse Rate (MALR) à The rate of change of temperature in a rising (and only rising) MOIST (saturated) air parcel. MALR ~ 6°C/1km. So if a saturated (i.e., RH=100%, VP=SVP and T=Td) air parcel is rising, and then it is cooling by 6°C per kilometer.

Dew Point Lapse Rate à the rate of change of the dew point temperature in a DRY (unsaturated) rising or sinking air parcel. The dew point lapse rate = 2°C per 1km. Once a parcel is saturated, the dew point lapse rate is equal to the MALR.

Lifting mechanism à any force that causes a parcel of air to ascend.  Examples = mountains and fronts

Lifting Condensation Level à The level where the T = Td and a cloud begins to form.

Adiabatic à A process that takes place WITHOUT the exchange of heat with the environment. In other words it is the internal temperature change of an air parcel due to rising or sinking in the atmos. When a parcel raises it cools due to expansion and when a parcel sinks it warms dues to compression.

IMPORTANT NOTES:

1.   The MALR only applies to parcels that are ascending (rising). An air parcel ALWAYS warms by the DALR when descending.

2.   If a parcel is going up in the atmos, then it is COOLING (by expansion). If it is going down, then it is WARMING (by compression.

3.   A parcel that is sinking can never* make a cloud. Why? Because as a parcel sinks, its temperature increases by 10°C/km while its dew point only increases by 2°C/km. This means that as soon as the parcel starts to descend, the temperature is growing faster than the dew point temperature. SO, T and Td no longer equal each other (therefore, the RH <100% and VP<SVP). NO CLOUD CAN FORM IN THESE CONDITIONS. (*One exception are mamatus clouds…look this up to find out why.)

4.   The reason why the DALR and MALR are different is because of condensational warming. As the parcel ascends, it eventually reaches saturation (RH=100%, VP=SVP and T=Td) and a cloud forms (at the LCL). The parcel is still continuing to ascend, but the lapse rate changes due to condensational warming. In other words, as the parcel continues to rise and cool, the excess water has to be condensed out. As it is condensed out, it releases heat to the parcel thus offsetting the cooling rate and reducing it from 10°C/km to 6°C/km.

HOW TO GET A CLOUD…

To form a cloud, a parcel of air must be lifted (by a lifting mechanism) until its temperature equals its dew point temperature (the air parcel is then saturated). The reason why the parcel becomes saturated is because the ascending parcel is becoming cooler at a rate of 10°C per kilometer (while it is still dry). This means that we are lowering the SVP as the parcel ascends (in other words, the ascension and cooling is reducing the total amount of water the parcel can hold). Once the parcel reaches saturation at the lifting condensation level (where RH=100%, VP=SVP and T=Td) the lapse rate changes because once the T = Td, the parcel is at capacity (or saturated) and cant hold any more water. The parcel is still rising however because it is being forced up by a lifting mechanism and because it is still rising and cooling, the amount of water that parcel can hold is still going down. All the excess water has to go somewhere and it is condensed out into cloud droplets. As a result the water being condensed out, latent heat is released to the parcel. This release of latent heat is what reduces the lapse rate to 6°C per kilometer (in other words, the release of heat due to condensation is added to the DALR (10°C per kilometer) and the result is about 6°C per kilometer (or the MALR)).

HOW THIS PERTAINS TO OUR EXERCISE…

We are going to force a parcel air over a mountain, which acts a lifting mechanism. We are going to see how changing the temperature and dew point temp of an air parcel changes the height of lifting condensation level (LCL).

        DEFINITIONS:

-         Windward à The side of the mountain that faces into the wind

o      This side of the mountain is the side that gets all the rain.

-         Leeward à the opposite windward, or the side of the mountain that DOESN’T face into the wind

o      This side often much drier and usually corresponds to the region commonly called the rain shadow (or the region that is anomalously dry).