Balloons are a type of aircraft, a subclass of aerostat, that rely predominantly on lighter-than-air liftgas buoyancy to provide lift. The liftgas is usually helium, hydrogen or hot air. Theoretically vacuum or lowered density air can also be used but the containers for such are weight prohibitive. Balloons may be tethered to the ground or untethered. Balloons are borne on the wind, and do not propel themselves in an particular direction. Balloons should not be confused with airships which are able to do this.
Two styles of balloons that rely on inherently buoyant liftgas predominate, the ambient pressure balloon and the superpressure balloon. Ambient pressure balloons will burst when the volume at ambient pressure exceeds the volume allowed by the balloon envelope, unless the weight of the payload prevents breaching this operational ceiling altitude. Superpressure balloons have tougher envelopes that contain the liftgas at a higher pressure than the atmosphere, so will lift up to an operational altitude and not further, when the buoyancy provided by the pressurized liftgas equalizes with the density of ambient air, and thus will not burst.