The effect of thermal conductivity in controlling the temperature distribution of the high atmosphere is considered. The energy absorbed in theFregion is conducted downward into a denser region of the atmosphere, where it is dissipated by infrared emission. The calculations indicate that the atmosphere is isothermal above about 250 km, and that between 100 and 200 km there is a very strong positive temperature gradient. The temperature in the isothermal region is assumed to be 1100°K, in order to provide enough atmosphere near 300 km to support anF2 region. The low temperature at 80 km is due primarily to the lack of absorbed energy there rather than to the presence of a strongly emitting layer at that level