2. 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……
3. 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.
4. 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?
It decreases
5. 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
__D_A_L_R_____ .
6. 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 _1_6_o_C_.
7. The Mountain
If the air continues to rise to 2000
meters, its temperature will be
_____ 6oC
degrees C.
The temperature of 6 degrees C is
dew point. That means that the
relative humidity in the parcel of air
is now __________ 100
%.
And that means that the water
vapour in the air will start to
____________.
condense
8. 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 ___________ latent
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 _____________ Saturated
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 __________ .
1oC
9. 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? ___________
Increases
If the temperature is rising, will condensation take
place? No
____ 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? ________________.
DALR
Then, at 2000 meters on the leeward side of the
mountain, the temperature will be _________ 11
oC.
At 1000 meters, the temperature will be ___ 21
oC.
And at sea level on the leeward side of the mountain,
the temperature will be ________ 31
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.
10. 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?
11. 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
