A balloon is to be used to carry meteorological instruments to an elevation of 18000 feet. The balloon is filled with helium at the ground level temperature and pressure ( 70 degrees Fahrenheit, 1 atm). The balloon material weighs .001 lb per square foot. If the instruments weigh 12 lbs, what should the diameter be of the spherical balloon ? Assume the temperature to drop 10 degrees Fahrenheit for every 4000 feet height.