Altitude Limitations for Hot Air Balloons
What is the maximum altitude a hot air balloon can reach?
65,000 ft is the high altitude record for a hot air balloon set in 1988
by Per Lindstrand flying his Stratoquest balloon over Laredo, Texas.
Why is there a maximum altitude for hot air balloons?
Hot air balloons have a limit on the altitude they can reach. As they rise
in the atmosphere, the surrounding air gets thinner and less dense so the
higher the balloon goes, the less buoyant they are. Eventually, they will
reach a point where they cannot displace enough air to off set their weight
and they will begin to sink.
Gravity pulls air to the ground causing a build up of air pressure close
to the earth, allowing balloons to be more buoyant.
What role does air density play and why does it decrease as
the you go higher in the atmosphere?
Density of air decreases as the balloon goes upwards. This is because the
atmosphere is mainly concentrated in a belt of 6 km in thickness, held there
by the force of gravity. Gravity tries to pull all the air down to the
ground but as more of the air is pulled down, the pressure and density at
ground level builds up and creates an opposing upwards force. Thus in equilibrium
in the atmosphere reaches a density profile which is approximately exponentially
decreasing with height and the buoyancy force on a balloon decreases with
height. For hot-air balloons, this sets a limit to how high a balloon can
go. For gas filled balloons, such as those used to gather wind speed and
direction data, the skin will rupture when it has expanded sufficiently at
large heights. The pressure due to air at ground level is about 100,000 N/m2
(or ``Pa" after Pascal). (http://staff.science.nus.edu.sg/~parwani/htw/c2/node20.html)
Buoyancy is what makes a hot air balloon rise. It rises in air because the
density of the hot air is less than the density of air. This produces a buoyant
Fb=(pair – phot air)Vg which
means you can have a mass of m=(pair – phot
air)V for a balloon of volume V. As the air pressure decreases
in the atmosphere the air pressure in the balloon has to keep decreasing as
well. Eventually there will be a point were the air pressure cannot decrease
anymore in the hot air balloon.
I wonder which aircraft can go higher? http://www.photohome.com/photos/hot-air-balloon-pictures/balloon-39.html