A weather balloon containing helium is released into the atmosphere. As it rises atmospheric pressure and temperature both decreases. Explain why the size of the balloon increases.

What do YOU think.
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Here is your answer. PV=mRT... where p is pressure, V is the volume of the balloon, m is the mass of gas in the balloon (constant) and R is universal gas constant divided by mean molar wt of air (about 28 g/mol) and T is thermodynamic temperature (T in Kelvin; T=273 +t (in deg C). Think pressure.