Stratospheric balloons are a type of research balloon that is constructed of polyethylene, a high-grade plastic, with a thickness no greater than the average dry cleaning bag. The balloons can be up to 800 feet (240 meters) long and can hold millions of cubic feet of helium. They are filled only partially at launch so that they will rise to an altitude where their internal pressure is equal to the atmospheric pressure and they can float.
The largest balloons can carry instrument payloads weighing more than two tons. Both the balloon material and the instruments must be able to withstand extremes of temperature. In the tropopause—the boundary between the troposphere and the stratosphere—temperatures can fall to minus 75 degrees Celsius (minus 103 degrees Fahrenheit), yet in the stratosphere, radiant heat from the sun can raise temperatures on sunlit surfaces to 100 degrees Celsius (212 degrees Fahrenheit).
At 20 to 30 miles (30 to 45 kilometers) above the Earth's surface, scientists can measure chemical reactions in the ozone and the effect of human-manufactured pollutants, as well as cosmic radiation, using these balloons.