Oleum "fumes"

I'd like to know the physical mechanism of oleum "fuming" on its surface. Dissolved SO3 reacts very exothermically with some air humidity. Why does the mist occur instead of dissolving created H2SO4 in the solution?

 

No, that is too simplified.
The reaction takes place on the liquid's surface. The heat of reaction heats up surface's surroundings.
What does there vaporize? Air humidity? Then why does it condensate 1 cm above the surface? Or sulfuric acid with (~300°C, p(atm)) boiling point? Generally, why the sulfuric acid is being carried away if it can dissolve?

 

That makes me more sense, thank you.

When water putted into acid or conversely acid putted into water, the liquid may spit around due to gas expansion, as I see. Also that is different physical process than oleum fuming.