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In June, Seattle had a major heatwave with temperatures reaching 41C with 30-40% humidity. It was a bit cooler in the mountains but still quite unpleasant during the day. This made me wonder - at which temperature is it no longer safe to go hiking? Now, I do understand that this varies based on cloudiness, weight in your backpack, what clothes you're wearing, how much wind is blowing, etc, but I presume someone like the US military would have a nice chart going into detail on this topic, due to numerous military campaigns in desert areas.

I did try to search "temperature limit for hiking" but all the search results were opinion based rather than linking to solid scientific research.

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  • humidity is a factor.
    – ab2
    Jul 26 '21 at 3:51
  • Take a look at this. Gives data on Army training in the heat. nbcnews.com/news/us-news/…
    – ab2
    Jul 26 '21 at 4:35
  • Quoted temperatures are usually shade temperatures, and there's not a lot of shade in the desert. But there's also not a lot of humidity
    – Chris H
    Jul 26 '21 at 9:15
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The limit isn't temperature, it's heat index. Since your body cools itself by sweating, you need to take the humidity into account when figuring out if you're going to overheat. As an example, 10% humidity and 41°C (what you might find at Arches National Park) will heat you about the same as 100% humidity and 28°C (which you might find in the Everglades), both of which have a heat index of 38°C (100°F).

The absolute limit for a human is a heat index of around 54°C (130°F): above this point, you risk heat injury just lying around in the shade. For most people, the practical limit is somewhere between 30°C and 40°C (85°F and 105°F), depending on personal fitness and heat adaptation. But variation outside this range is possible: I once encountered someone suffering from heat exhaustion at a heat index of just 26°C (79°F).

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It’s not just about the temperature, it’s also about the level of difficulty ( easy, moderate, strenuous, etc.), distance , elevation, humidity and how hydrated the hiker is. From personal experience a 1 mile hike with 99 Fahrenheit , around 10% humidity while staying hydrated is doable. Not comfortable but doable.

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