The air thins and they begin to lose the protective weight of the miles of air above them.

The pressure is dropping, when you finally get to the stratosphere, and to the altitude we like to work at, it's only one one hundredth that of it's on the ground, it's a whole different world than down here.

The weight of the miles of air above us creates air pressure, we often think of air as weightless, but all those gas molecules add up, grand central station's main concourse contains over 200 tons of air, as gravity pulls the air down, it compresses the layers below, making them denser, this dense air is what we are used to dealing with on the ground, just a pint of it contains more than 8.5 billion trillion gas molecules, but higher up around the balloon, there was less atmosphere pressing down from above, the air up here is thinner.

It's like being under a thousand blankets, and each blanket is a layer of atmosphere, and we are being pushed under all of these blankets, as you go higher, you go up through the blankets, so it's pushing on you less, until you are at the top above most of the blankets where you don't feel it at all.

That thin air seems alien to us because our bodies and technology are so perfectly adapted to thicker air, we even need it to hear, the gas carries sound through air as pulsing waves of pressure.

Sound is just a wave like a water wave in a pond, but if you take away the water, there is nothing for the wave to go across, and as the atmosphere gets thinner and thinner, it supports that wave less and less.