Could only find a Portuguese clip, but according to The Archdemon Dumbledore created a storm over London (according to the canon screenplay).
London is 1572 km^2, assuming stratus clouds since fog is usually similar to those, which are 1000 m thick.
1572*1e6*1000 = 1.572e12 m^3
Liquid density of fog is 0.05 g/m^3.
1.572e12*0.05 = 7.86e10 g, or 7.86e7 kg
Condensation is 2 264 705 J/kg
7.86e7*2264705 = 1.78005813e14 Joules, 42.54 kilotons, Town level
Edit: DontTalkDT suggested using the height of Big Ben instead, which was seen to be covered in fog. The height of that is 96 m, so it makes the result 0.096 the original.
42.54*0.096 = 4.09 kilotons, Small Town level+
Scaling[]
Scales to Prime Dumbledore with and without the Elder Wand and Grindelwald with the Elder Wand.
Old Dumbledore and Voldemort scales to 8-A, while Grindelwald without the Elder Wand would be at least 8-C I think, since he's still a powerful Dark Wizard and should be superior to average Aurors.