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.
Last edited by BackPacker; 9th Dec 2010 at 14:04.
Reason: Must learn to do multiplications properly.