The sunset effect resulted in a rise of about 500 feet (5 mb) in the floating level of the balloon at 1830 MST. This seems to be due to a change in the effective seal-off pressure of the aneroid capsule of the automatic ballast valve which was the consequence of a decrease in the temperature of the trapped air inside. The rise in altitude experienced corresponds to a decrease of temperature of about 6°C, the superheat of the aneroid, which was lost at sunset. This valve may be compared with the 30°C found on Flight 10. On the earlier flight a black valve was used while on this flight the equipment was polished aluminum, with a highly reflective surface.
-51-