The density of steam at 1 atmospheric pressure is 0.59 kg/m3, against an air density at STP of 1.29 kg/m3.
Under this condition, the weight of air displaced by, say 1 m3 of steam, is 1.29g N, or 12.9 N (if taking g, the acceleration due to gravity, to be 10 m/s2 for simplicity of calculation).
The upthrust given to 1 m3 of steam, by the Principle of Floatation
= 12.9 N
But 1 m3 of steam only weights 0.59g N, or 5.9 N,
there is a net upward force of (12.9 - 5.9) N = 7 N acting on the steam. This 7 N of net upward force is sufficient to uplift the steam against gravity.
The density of steam at various pressure can be found from the web-page below:
http://www.thermexcel.com/english/tables/vap_eau.htm