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Is there an equivalent to heat index (temp & humidity), which also includes exposure to direct sun, perhaps as given by the commonly reported "UV index" shown on weather sites? In other words, temp measurements are always given for "in the shade," but direct sun & UV exposure is dramatically more meaningful & potentially dangerous to living organisms (especially humans).

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    Interesting question. But I'm not sure it's a great idea to combine such a thing with a temperature measure, because bright sun and UV can be big problems even in cold temperatures, like in snow. Visible light is also kind of a separate issue from UV, in that they have different types of effects. Visible light heats you and makes your eyes hurt, whereas the main issue with UV is that it's ionizing. – Ben Crowell Jun 9 at 21:22
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    Despite what OP says, you cannot get a meaningful temperature measurement in the sun. If you could, then people would already be using it. A mercury or alcohol and glass thermometer will probably break. It is relevant where in the sun you are too, a sun-trap will feel hotter than an open area. You could measure the surface temperature of objects that are in full sun, but that will vary with their reflectivity, i.e. a black rock will get hotter than a white rock. – Weather Vane Jun 10 at 16:55
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@BenCrowell's comment is right. This was going to be a comment agreeing with his, but grew.

I think the best single number you could do is some sort of total sun risk index, which is of limited value as mitigations for the different risks are different (shade is good for both, clothing can increase heat retention while protecting from UV, extra water is needed for heat but does nothing for UV, and sunscreen does nothing for heat).

Dropping the UV consideration, you'd have an enhanced heat index risk including solar gain. The problem is that the amount of heating from direct sunlight depends strongly on clothing- and even skin-colour, and wind offsets the heating to some extent, though if sweat evaporates fast you might not realise how much you're sweating. Still, a solar heat risk index would be useful. It would be based on certain assumptions about exertion though, and you can't arbitrarily limit your exertion (e.g. cycle slower uphill, you fall off, paddle gently all the time on white water, you lose control - but it's hot in a drysuit in the Alps in summer, at least above the chilly water). But the risk index and the mitigations are too closely linked: stopping in every patch of shade for a good drink reduces your average solar gain, thus the index changes.

Overall I reckon you should risk assess heat and UV separately -- and cold, as I hint at in my kayaking example, but also if the area is prone to sudden storms, as many mountain areas are even in the height of summer; deserts can get below freezing if you're out overnight. When I say "risk assess" I don't necessarily mean a formal process as many of us have to do in work, but the informal rolling risk assessment that any leader needs to be thinking of -- and this isn't heavily based on numbers (it would be quite different if a member of the party was already dehydrated or had to cover up because they'd already got sunburnt)

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  • Wind does offset heating to some extent, but it can be dangerously deceptive, causing dehydration, and does not lessen the radiant power of the sun, in other words you burn just as quickly. – Weather Vane Jun 10 at 17:13
  • @WeatherVane arguably wind makes sunburn more likely as you're less likely to feel hot on the surface and seek shade, but I didn't make that point because the main mention of wind was after setting aside UV. I did consider mentioning dehydration, but left it out as I wasn't happy to include it without research. Wind dries sweat from the skin, but that in itself isn't a cause of dehydration - sweating isn't regulated by skin moisture, but heat. Can it cause enough other drying to be a risk? Does the cooling from enhanced sweat evaporation actually help? Or both, depending on exact conditions? – Chris H Jun 11 at 6:06
  • When I visited Tenerife I read warnings that there have been many cases of dehydration among hikers due the stiff sea wind combined with exertion and to be alert to the danger, and here is one reference. – Weather Vane Jun 11 at 6:17
  • @WeatherVane I'm inclined towards the same warning, but not to state it as fact that wind directly causes dehydration (as opposed to not realising how much you're sweating because it dries) without support from the scientific literature. The author of a guidebook can't be assumed to know more about the subject than you or I, and there are plenty of well-meaning, even helpful, but factually incorrect warnings. – Chris H Jun 11 at 9:59
  • ncbi.nlm.nih.gov/pmc/articles/PMC4207053 says that insensible sweat loss varies with wind speed, which is progress, but journals.lww.com/acsm-msse/Fulltext/2007/02000/… states that wind minimises wasted (dripping) sweat – Chris H Jun 11 at 10:02
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Via the NWS:

The WetBulb Globe Temperature (WBGT) is a measure of the heat stress in direct sunlight, which takes into account: temperature, humidity, wind speed, sun angle and cloud cover (solar radiation). This differs from the heat index, which takes into consideration temperature and humidity and is calculated for shady areas. If you work or exercise in direct sunlight, this is a good element to monitor. Military agencies, OSHA and many nations use the WBGT as a guide to managing workload in direct sunlight.

More specifically a wet bulb thermometer has a moistened sleeve over the bulb. Combining the readings of a wet bulb, dry bulb, and black globe thermometer yields an apparent temperature which takes into account all of the above factors. (.7wet + .2black + .1dry for the curious)

Parenthetically AccuWeather offers "RealFeel" which is an apparent temperature which claims to include radiation. Since the formula is proprietary and US weather stations don't measure radiation directly it's most likely using an estimate based on location and sky cover. Buying a wet bulb thermometer is a much better solution.

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