MET question

[wilky]

Hello,

I was wondering if someone could oblige and clear up a MET question I keep getting on PPLquiz, I have still not worked out how they get the answer.

"Given a surface temperature of 21C and a dew point of 7C what height would you expect the cloudbase to be?"

The way I have tried to work it out as follows.

DALR 3C/1000'
To cool to 7C from 21C will take 4600'

the answer they are giving is 5500', where am I going wrong?

Many thanks if you reply.

[BackPacker]

For each degree (C) spread, the cloud base rises approx. 400' in the ISA atmosphere.

21-7 = 14 degrees spread. Times 400 is 5600'.

The explanation I forgot, but it had to do with the intersection of the Temperature/DALR and Dewpoint/WALR line. It's in my PPL book somewhere.

[wilky]

Backpacker,

Thank you for the reply, I have not seen that statement in my Met book yet, will have to go back for a thorough re-read, must have missed it. Handy to know thank you.

[BackPacker]

A quick search for DALR gave me a link to this thread:

http://www.pprune.org/private-flying/418772-another-meteorology-question.html

Contains a good explanation.

[RODF3]

dew point changes with height. Reduces by 0.5 degree/1000. this gives a cloud base of 5600.

[BackPacker]

Become a caver. It gets warmer the deeper you go - although I don't think the ISA rule of 2C per 1000' applies.

Any you get warm climbing out too.