It is a spring day in southern B.C., with temperatures of 11 oC. Westerly winds

Question

It is a spring day in southern B.C., with temperatures of 11 oC. Westerly winds push warm moist air off the ocean towards the coastal mountains, which are 1000 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?

b) What will the temperature of the air layer be as it passes over the summit of the range (i.e. at 1000 m)? You may wish to explain your steps.

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).

Orographic lifting

d) Do you think it will be raining or snowing at the mid-station of Whistler-Blackcomb (mid-station is at 1600 m elevation), the ski area north of Vancouver? Be sure to explain your steps.

Explanation / Answer

3.a) Condensation starts when the relative humadity increases to 100% and the air mass becomes saturated. The elevation at which condensation begins is given by the formula :

h = 125 ( T- Td)

where, h= elevation

T= temperature of air

Td= dew point temperature of air

So, h= 125 ( 110- 60 ) = 625 m

b) At 1000m elevation, the temperature of dew point will be the same as the temperature of air i.e. 110c. At this stage, cloud begins to form.

c) After descending to the surface (0m) the temperature of air will be the same as dew point temperature because the descending air is now dry.

d) North America's biggest ski resort is Whistler. Whistler and Blackcomb are the two side-by-side mountains that link the terrain more than 8,000 acres area. Whistler lies at the foot of Blackcomb. At mid-station of Whistler mountain snowfall ocuurs in March ansd April. This snowfall is measured from the Pig Alley Weather Station situated on Whistler Mountain. The average snowfall on these mountains is 11.59m based on the last 10 years record.

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