Keith, Many thanks for the explanation. Saves me having to picture the lss in the atmosphere which is what I was doing before.
I'll work through the examples when I can keep my brain working long enough, and I'm not drugged up on Day-Nurse / Night-Nurse or whatever it is Mrs QNH keeps giving me; I'm suffering a stinking cold at present. At least I should be over it by exam day.
Good luck to all of you sitting this month.