Boyle's Law

• Boyle's Law states that, for a fixed amount of an ideal gas kept at a fixed temperature, pressure and volume are inversely proportional. This means that, as pressure decreases, the volume of a confined gas--like that in the gas bag of a weather balloon--increases.
  
  

Atmospheric Pressure Decreases with Altitude

• As altitude above the Earth increases, the atmospheric pressure decreases. At sea level, atmospheric pressure is 14.7 pounds per square inch. As a balloon rises, the weight of the air directly above it decreases, and the number of air molecules touching it per square inch decreases; thus the atmospheric pressure drops.
  
  

Volume of Gas Expands

• As the atmospheric pressure around the balloon drops, the volume of the gas inside the balloon will increase. If the balloon is half-full of gas when it is launched, the expanding gas in the balloon fills it up as it rises.
  
  

The Balloon Will Burst

• When a weather balloon's gas bag expands, it can burst just like a child's balloon that's overfilled. A weather balloon can climb more than 20 miles before it bursts and its instruments parachute back to Earth.
  
  

Reaching the Right Height

• If the balloon had been filled to capacity when it was launched, its gas would make it burst prematurely, before its instruments get the desired data. Launching the balloon only partially inflated keeps that from happening.