Behaviour of Gases

Surprisingly, the viscosity of a dilute gas behaves exactly opposite to what you might expect for liquids. Liquid viscosity generally decreases as temperature is lowered. The viscosity of a dilute gas increases as and when you raise its temperature. This counter-intuitive behaviour was clearly established experimentally and is explained by the kinetic theory.

It's because gas molecules constantly move around at super high speeds. They randomly bump into each other constantly. Gravity does pull on them. The pulling effect is not much, however. When we consider the distance from the floor to the ceiling, gravity's pull is overcome by the molecular kinetic energy. So they just spread out and move from one place to another. They can't settle down.