Remember that there is a difference in pressure of any fluid between the bottom and top of an object in it given by Pbottom = Ptop ρgh. Even though the density of a gas, ρ, is much smaller than that of a liquid, there is still a pressure difference, and therefore a buoyancy. There is another difference between a liquid and a gas. A gas can be compressed, and so the density of the atmosphere decreases as you rise. When a hot air balloon is launched the air within it is heated and it will rise. As it rises the density of the air decreases, and so the buoyant force decreases. At some height the weight of the balloon and basket equals the weight of the air displaced and the balloon stops rising. The operator of the balloon can go higher by making the gas in the balloon hotter, and thus less dense.