The
Mountain
Ok, here's the
mountain. It is exactly 3000 meters in elevation, rising from sea level, since it is
located right on the coast. It is in the middle latitudes, and the prevailing
westerly winds blow from the ocean on to the shore. The air temperature at sea level
is 26 degrees C. The moving air strikes the mountain, and is forced to rise along the
windward slopes until it gets to the top. Then, it can subside down the leeward
slopes.
The first question is, what happens to the temperature of the air as it rises up the side of the mountain?
Go on, answer the question: 1) the temperature: a) increases, or b) decreases. (Answer 1).
The next question is, how much will the temperature change? To answer this, you have to first decide what lapse rate to use. We know that the air is rising, and we will also state that condensation is not taking place in the rising air (that is, the temperature of the air has not reached dew point). Therefore, the lapse rate we will us is the __________ (Answer 2). Note: do not go on until you answer this question. Going on without thinking about how all this works will give you very bad luck on your next exam.
The next thing we need to do is give some lapse rates you can work with. Here they are:
Normal Environmental Lapse rate: 6.5 deg C/1000
meters (about 3.6 deg/1000 feet)
Dry Adiabatic Lapse Rate: 10 degrees C/1000 meters (about 5 ½ deg F per 1000
feet)
Saturated Adiabatic Lapse Rate: 5 degrees C/1000 meters (about 3.3
deg per 1000 feet)
Also, we will state that the dew point temperature for this parcel of air is 6 degrees C.
So, since we have already determined that we will be cooling the air parcel off using the dry adiabatic lapse rate, let's see what happens to this air.
The air starts at sea level with a temperature of 26 degrees C. By the time it rises to 1000 meters altitude, its temperature will be _______________. (Answer 3).
If the air continues to rise to 2000 meters,
its temperature will be _______ degrees C. (Answer 4).
The temperature of 6 degrees C is dew point. That means that the relative humidity in the parcel of air is now __________ %. (Answer 5). And that means that the water vapor in the air will start to ____________. (Answer 6).
For this particular situation, 2000 meters is the altitude at which condensation takes place. This is called the lifting condensation level. Please note the difference between dew point and lifting condensation level. Dew point is a temperature, and will be given in degrees C or F, while the lifting condensation level is an altitude (the altitude at which dew point is reached), and will be given in meters or feet.
Once the temperature reaches dew point, and condensation begins, the condensing molecules of water will release ___________ heat. (Answer 7). So, as the air continues to rise, it cools to a lower temperature (because it is rising), but it doesn't cool off as rapidly (because latent heat is being released). Therefore, the lapse rate changes. Once condensation begins in rising air, the air cools at the _____________ adiabatic lapse rate. (Answer 8). Notice that condensation is taking place, so clouds (composed of tiny droplets of liquid water) will form, though it may or may not rain.
So, at 3000 meters, the top of the mountain, the air temperature will be __________ C. (Answer 9).
Once the air reaches the top of the mountain, it can begin subsiding down the leeward slopes. It does this because it is cooler than the surrounding air. Once the air begins to subside, what happens to its temperature? ___________ (Answer 10).
If the temperature is rising, will condensation take place? ________ (Answer 11). Let's assume that no more moisture remains in the subsiding parcel of air. So, what lapse rate will we use to figure out the temperature of the air parcel? ________________ (Answer 12). Then, at 2000 meters on the leeward side of the mountain, the temperature will be _________ degrees C (Answer 13). At 1000 meters, the temperature will be _______ degrees C (Answer 14). And at sea level on the leeward side of the mountain, the temperature will be ________ degrees C (Answer 15).
Actually, the air is warmer at sea level on the leeward side of the mountain than it was when it started at sea level on the windward side. It is also drier. Do you see why the leeward sides of mountain ranges are in the "rainshadow" of the mountains? There are many rainshadow deserts as a result of this process.
Note that we did not use the normal environmental lapse rate in this example. That's because the environmental lapse rate applies to still air, and the air around this mountain is either rising or subsiding.
That's it! Try testing your knowledge with a quiz on Lapse Rates and Adiabatic Processes, or one on Atmospheric Moisture (which also covers cloud types).
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mailto:jthorn@mail.sdsu.edu
03/08/02
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