Question

It is a spring day in southern B.C., with temperatures of 10 oC. Westerly winds push air off the ocean towards the coastal mountains, which are 1400 m high. The dew point temperature of the air mass is 6 oC. Determine what happens as the air is forced to rise by answering the questions below. A temperature-height graph (Graph C) is provided at the end of the question in case you wish to work on the answers graphically.

3 a) At what elevation does condensation begin on the western slope of the range? [1 point]

b) What will the temperature of the air layer be as it passes over the summit of the range (i.e. at 1400 m)? You may wish to explain your steps. [1 point]

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GEOB 102 – Introduction to Climate and Ecosystems W2, 2019-20

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c) What will the temperature of the air layer be after it has descended to the surface (i.e. at 0 m) over an interior plateau? Assume that all condensed moisture fell as rain on the windward slope (i.e. the descending air is now dry). [1 point]
d) Do you think it will be raining or snowing at the top of the Whistler-Blackcomb (at 1800 m elevation), the ski area north of Vancouver? Be sure to explain your steps. [2 points]

Answer 1

3.a.Before getting saturated with the vapour present within itself, the air mass will cool following the dry adiabatic lapse rate.
Value of th this lapse rate=10°C /km
Difference between the temperature at the surface and the dew point temperature
=(10-6)°C=4°C
So, the air mass must cool 4 degrees before being saturated.
So, the height at which the air will be saturated/condense=(4X1000/10) meters
=400 m
3.b. After condensation and being saturated by the vapour, the air mass will cool following the moist adiabatic lapse rate. At this stage some of the provided energy is used for the condensation of vapour into water. So, greater amounts of energy are required to decrease small value of temperature.So, the moist adiabatic lapse rate is lower than the dry one.
Value of moist adiabatic lapse rate=4°C /km
Altitude difference between the summit and the level of condensation
=(1400-400) m=1000 m
For this altitude gain, decrease in temperature will be=4°C
Temperature at the height of condensation=6°C
So, temperature at the summit=(6°C-4°C)=2°C

3.c.After reaching the other side, the air mass will start to descend. This will increase the temperature of the air mass and subsequent increase of pressure due to coming to lower altitudes.
Rate of increase of temperature=dry adiabatic lapse rate
=10°C/km
Total change in altitude to reach the surface on the other side=1400 meters
So, the total increase in temperature=(1400X10/1000)°C=14°C
At the top of the summit temperature of the parcel=2°C
So, the final temperature at the surface on the other side=(2+14)°C=16°C

3.d.After reaching the level of condensation, temperature will decrease following the moist adiabatic lapse rate.
At an altitude of 1400 meters the temperature we found to be=2°C
Difference between the altitude of Whistler-Blackcomb, the ski area north of Vancouver
=1800 meters
Difference in altitude=400 meter
Rate of change=4°C/km
So, the difference bteween the temperatures=(4X400/1000)°C=1.6°C
So, temperature at the given location=(2-1.6)°C=0.4°C
This temperature is very close to 0°C(freezing temperature for ice formation).So,we can get snowfall at this location.
Please rate the answer. Took a lot of time to complete the answer.