Why Exteriors of Riflescopes Fog Up
Aside from rain, fog, slop and goo that may fall directly on scope lenses in the field, hunters are additionally plagued by "fogging" on exterior scope lenses. Fogging is simply the condensation of water on the scope lenses and body for precisely the same reasons as forms the morning dew. The "dewpoint" of air is simply the temperature at which it can no longer hold water as vapor, condensing from vapor to liquid water.
Humidity is the weight of water vapor contained in a defined weight of air. Warm air can hold more water vapor than cold air. When a surface such as the ground, or an auto windshield, or your scope lense cools the air touching it to the point that the air can no longer hold vapor, the water vapor condenses onto the cold surface as "fog" or frost.
What makes your scope surfaces cooler than the surrounding air? Interestingly, all earthly objects both absorb and radiate heat (IR) radiation. Dark colored objects absorb and radiate faster than light colored objects. The great majority of riflescopes are beautiful, basic black. Thus our black scope body will absorb heat energy rapidly from the sun, but will radiate that energy back into space at night or on dreary rainy or snowy days much faster than does the air surrounding it. (Most air being clear rather than black!) This faster loss of heat energy by the riflescope compared to the air around it, in addition to the cooling effect of physical evaporation of rain or snow water on the scope, may drop the scope temperature below the dewpoint, and, presto, everything gets covered in "fog".
What to do? The first step is to attempt to physically separate ambient air from the scope surfaces. Flip-up type lense covers will provide a barrier over the lenses, but allow the whole surface of the scope body to cool by evaporation, which in turn will cool the lenses. Lenses cooler than the dewpoint will fog. The more effective approach is the full neoprene cover, which keeps the ambient air and physical moisture away from the scope surfaces. This sort of "insulation" slows evaporative and radiational cooling, as well as keeping physical water from the lenses.