No mandatory compensation as far as I'm aware as its a situation beyond the airlines control...though I'm sure some of the stupid angry types would insist the airline could get a very big fan to blow the fog away...
What the airline does for you in the event of a delay depends on the T&C's & attitude of the individual airline. Its a very mixed bag really - I've had food vouchers from Jet2 after looooong weather and tech delays in AMS, but not at MAN, bugger all from KLM/BA/BMI after any type of delay, long or short, and the usual brush off from Easyjet after a 7 hour delay (flight time - 1 hour, I could've driven LPL-AMS quicker!) caused by crewing problems...
