Relative Humidity is a function of temperature. While the RH here in the mountains can be quite high (right now it's 89%), the moisture level feels to be quite low, as the temperature is 28° F (Dew Point = 100% RH is 25°). In fact, at this level, it feels very dry outside, because the body's temperature of 98.6° F is so much higher, and thus no moisture can collect on it. When it's snowing, it's 100% RH, but the actual amount of water vapor in the air is actually quite small. Yet, condensation will occur at the dew point, no matter what the temperature (think of a cool glass or mirror when taking a shower) if the surface is at the same or lower temperature as the air, and this will lead to corrosion of unprotected or susceptible surfaces. And the corrosion rate will also depend on the actual level of oxygen (the partial pressure, not the percentage), which is altitude dependent.
The actual corrosion of an ordnance casing depends on the history of that particular device, starting from its point of manufacture - not the location that the vehicle happens to reside in. Furthermore, once the corrosion chemical reaction begins, it will continue, at lower levels of the amount of water vapor in the air to which it is exposed.
Today's weather, by the way-
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