Archived from groups: alt.games.microsoft.flight-sim (More info?)
Two possibilities that I can think of:
1. Do you have dynamic weather turned on in the World | Weather
dialogue box? If so, try turning it off or slowing down the rate of
change. If you stop getting clouds when you don't expect them, that's
it. Not necessarily a bug, mind you -- a feature of the sim so that you
don't have the same weather the whole flight.
2. What is the weather like at the weather station you are flying
toward (as opposed to from)? Would that explain the new weather you're
getting 5 mins. after take-off? Again, you can explore this from the
World | Weather
Tell us what you find out -- we'll all learn something, and perhaps
your experience will help others.