High temperatures can be dangerous regardless of where you are. The dangers are two-fold; dehydration and the loss of electrolytes, mostly sodium and potassium, through sweating. However, the dangers aren’t quite the same if you are in an area with very high humidity as they are if you are in a place with very low humidity.
The body’s normal response to the heat that approaches or exceeds body temperature is to sweat. This is increased if a person is doing something active because they are generating additional body heat. Sweating causes dehydration. Sweat also contains both sodium and potassium, and if either of these becomes depleted, a person can suffer heat exhaustion or heat stroke. These two chemicals are exceedingly important for regulating body temperature.
If they become unbalanced, the body temperature can soar rapidly, which is the heat stroke or heat exhaustion. This is why the military gives soldiers salt pills if they are deployed in a hot region.
All of this is true whether you are in a high humidity or low humidity environment. So what is the difference and which is more dangerous?
The difference is sweat.
In high humidity areas, where the humidity is above 80%, the sweat doesn’t evaporate very quickly because the air is already filled with moisture. It tends to be the most uncomfortable because the sweat just rolls down your body, leaving you sticky and wet. However, profuse sweating is an obvious cue that you should drink fluids with electrolytes added. People who are even mildly observant should know that they need to rehydrate when they are dripping large amounts of sweat.
A strange thing happens when the humidity drops below 15%, though. People sweat just as much as under high humidity conditions, however, the air is so dry that the sweat evaporates the moment it gets to the surface of the skin. Even people who are quite observant may never realize that they are sweating, dehydrating, and losing electrolytes. There are literally no visible signs of it.
By the time a person starts feeling the first effects, usually dizziness, it can already be too late, if they are somewhere where they can’t rehydrate and take electrolytes. This makes dry heat substantially more dangerous than wet heat.
In dry areas, and this includes Montana in the summertime, the only thing that can be done is to drink fluids and take electrolytes even if you don’t feel like you need either. If you don’t, you could pass out and die without ever knowing that there was a problem in the first place.