Tests of exposure to high humidity and temperature, and resistance to salt spray or to attack by fungi must be performed by trading time for exaggeratedly severe conditions. These are not necessarily fully valid survival tests, but they attempt to give a fair picture of the behavior of material.
Disregarding overtesting where severity is increased while time is shortened, a valid question is how realistic within the limits of normal terrestrial conditions is the frequent range of test conditions from -65F to +160F. The low end of the scale is appreached or even exceeded on airplane transportation at high altitude and in arctic areas. As to the high end, an illuminating study available to the public is by Porter, conducted at Yuma, Arizona. Under the most adverse conditions, a maximum temperature of 152F was encountered under and near the roof of railroad freight cars. While munitions will wherever possible be packed and protected so that ambient maximum temperatures are not reached or built up due to slowness of heat transfer and limits of exposure time under the diurnal cycle, it must also be considered that under emergency conditions in the field the opposite may take place-maximum exposure when munitions are openly exposed to the weather because there is neither time nor the means available for protected storage. A temperature of 215F has been measured on an aircraft wing in the direct sun in Arizona and elsewhere 177F on an aircraft where the internal temperature had reached 167F.
Every time I read this paragraph out of a book on munitions, I can't help but picture a case of 5.56 misplaced on an airplane that has made daily roundtrips from Arizona to Alaska for 10 years. Puts things in perspective