The temperature data obtained was of considerable value, especially to determine the effect of insulation of the battery pack. It was found on most flights where reasonable thermal insulation was applied that the temperature of the pack remained above $ 0^{\circ} \mathrm{C} $ after several hours of exposure at nighttime. The extreme observed was $ -10^{\circ} \mathrm{C}. $ Daytime flights had the added advantage of heating from the sun.
The temperature of the lifting gas at the ground was ordinarily found to be somewhat below the temperature of the air. This is due to the extreme cooling encountered in the expansion of the compressed gas as it was fed from the tanks into the balloon. During the rising period, in daytime, the gas gained heat, since it cools adiabatically less rapidly than does air (also less than the normal tropospheric lapse rate); at the floating level a differential of about $ 1 0^{\circ} \mathrm{C} $ was common. A typical temperature trace is shown in Figure 35.
Figure 35. Typical temperature record.
To permit the transmission of both temperature and pressure data by one radio channel, a pair of programming switches have been designed and flight tested. The first is the temperature switch (Figure 36),
Figure 36. Temperature programming switch.