Some gases are buoyant in air because they have a ensities lower than that of air (about 1.2 kg/m3, 1.2 g/l). Lighter-than-air gases are used to fill craft called aerostats, which include free balloons, moored balloons, and airships, to make the whole craft, on average, lighter than air. (Heavier-than-air aircraft include aeroplanes, gliders and helicopters.)