Meteorology question

[eugeniun]

Hi

Trying to solve a simple problem from meteorology and still can’t figure out which is the most correct way. Here is the example:

Cloud is forming with a base of 2000 ft. If the surface temperature is 20 degrees Celsius, what would you estimate the approximate dew point to be?


A 11 degrees Celsius
B 14 degrees Celsius
C 18 degrees Celsius
D 19 degrees Celsius

Three possible ways to solve it with three different results:

1. Cloud Base Altitude = ((temperature - dew point) / 4.5) * 1000 (for reference - http://www.csgnetwork.com/estcloudbasecalc.html (http://www.csgnetwork.com/estcloudbasecalc.html)). Result - 11°C – answer A.

2. Cloud Base Altitude = (temperature –dew point) * 400
(for reference - OAT Meteorology CD) Result 12°C – very close to answer A

3. Cloud base can be calculated also using DALR (3°C/1000ft): Cloud Base = (temperature – dew point)/ 3 * 1000. Result 14°C - answer B.

Thanks a lot for help.

[Dick Whittingham]

Insufficient data! Does not compute!

If the sutface temp is 20deg and we assume the cloud has formed from convection, then the cloud base temp is 14deg. At that point the dew point temp, wet bulb and air temp are all the same - 14deg. If you want the surface dew point you need to remember that the dew point there must be more than 14deg, and must be less than the surface wet bulb. The surface wet bulb is 17deg, so the surface dew point has to be between 14 and 17deg.

But the question does not tell you enough to decide which dew point you want, unless you assume it is looking for the dew point at cloudbase, in which case the answer is 14deg. If the question is looking for the dewpoint on the surface - which any ordinary reading would suggest - none of the answers is right. It would be about 15.5deg in standard conditions.

Now this is using the accepted data for the JAR exams. Other sources or perhaps FAA question banks might give a different answer. My best advice is to ask your School to explain.

Dick W

[Turbine King]

If you go up at DALR you reach +14 oC at 2000'

So the dewpoint at the cloud base is 14 oC

The dewpoint lapse is 0.5 oC so at surface the dewpoint is 15 oC

Looking at the answers:ugh:

I guess you have to go for a tall ladder and answer B)

and using your equation:

2. Cloud Base Altitude = (temperature –dew point) * 400
2000 = (20 - dew point) * 400
5 = 20 - dewpoint
dewpoint = 15 :ok: