Altitude

Year 13

The dry adiabatic lapse rate (DALR)

In dry air the lapse rate is 10 degrees Celsius per 1000m altitude.

This is dry air so condensation will not take place – but in wet air……

The saturated adiabatic lapse rate (SALR)

In wet air the lapse rate is 5 degrees Celsius per 1000 metres altitude.

Our parcel of air becomes saturates at 2 degrees C and the rate of cooling reduces because condensation is releasing latent heat into the atmosphere above 3000 metres.

The MountainOk, 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?  It decreases

The Mountain

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 use is the __________ .DALR

The Mountain 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 oC per 1000 meters• Dry Adiabatic Lapse Rate: 10 oC /1000 meters• Saturated Adiabatic Lapse Rate: 5 oC per 1000 meters

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 _____.  16oC

The Mountain

If the air continues to rise to 2000 meters, its temperature will be _____ degrees C.

The temperature of 6 degrees C is dew point.  That means that the relative humidity in the parcel of air is now __________ %.

And that means that the water vapour in the air will start to ____________.  

6oC

100

condense

The Mountain

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.   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. 

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 __________ .

latent

Saturated

1oC

The Mountain 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?   ___________ If the temperature is rising, will condensation take place?   ____  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? ________________.  Then, at 2000 meters on the leeward side of the mountain, the temperature will be _________ oC. 

At 1000 meters, the temperature will be ___ oC.

And at sea level on the leeward side of the mountain, the temperature will be ________ oC.   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.

Increases

No

DALR

11

21

31

IMPLICATION – Fohn winds

Moist air from the Pacific has been warmed because it has lost its moisture – can you draw a diagram to show the impact of SALR and

DALR on this air mass?

Fohn winds are common -Here are some examples of local names for Fohn winds from around the world

•Autan from France •Bohorok from Sumatra •Chinook from North American Rockies •Föhn from European Alpine regions •Ghibli from Libya •Koembang from Java •Maloja from Switzerland •Puelche from the Andes •Reshabar from the Caucasus Mountains •Santa Ana from Southern California •Warm Braw from New Guinea •Yama Oroshi from Japan •Zonda from Argentina