While leaving your gun in the cold will help it is not always a very practical option. Anti-fog wipes or coatings like rainaway helps a lot, as does having butler creek or similar scope covers and leaving them closed until the gun has come down from inside temperature to ambient.
High humidity is very rare on real cold days. The relative humidity the weather channel reports is actually temperature corrected. Seventy percent humidity at 80F degrees is WAY wetter than 70% at -25F. The colder the air the less water it can hold so while it is still 70% saturated, that is based on its capacity, not the actual water content. The real water content is very low so once it comes down to ambient you should not have much issue unless you breath on the scope.
If you wear a ball cap make sure you face the bill backwards when raising the scope, if you are wearing a Balaclava especially, but even if you aren't, do not breath through your nose and direct your breath down by pushing your upper lip out over your lower lip, if you must breath after bringing the scope up. This is where the anti fog helps.