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kilotesla t1_j0lawaq wrote

Reply to comment by TonyJPRoss in How does high humidity affect perceived temperature in hot and cold environments? by MindTheReddit

>Moist air is a better conductor than dry air, so moist cold air feels colder.

There are two problems with this explanation.

  1. If we are talking about temperatures below about 50 F or 10 C, the amount of water vapor in the air, even at 100% humidity, is very small, a fraction of a percent, and so the impact it can have on heat transfer is very low.

  2. According to this analysis, the impact, which is only about 3% at 50 F, is in the other direction: higher heat transfer with dry air. Perhaps that is not the best high quality source, so I would be open to revising this point if people have better sources.

I think we still need someone to come and provide a better explanation for the common perception that a cold day feels colder when the humidity is high. There may be other correlations. For example, it may be that dry, cold days often have more solar radiation, making surfaces around you warmer as well as warm and you directly, so you feel warmer because of heat transfer by radiation even while the heat transfer by convection is very similar to what it would be on a wet day. And on a wet day, the wet ground may be cooled by evaporation, making that difference even bigger. I hope someone can cite a study that includes that affect—the mean radiant temperature.

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s0rce t1_j0lgxsm wrote

I've looked into this and haven't found good data but agree with you it's not the air thermal conductivity. I think the issue is largely the effectiveness of most clothing insulation is reduced significantly by humidity

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kilotesla t1_j0m3ajh wrote

Here's some data, a regression model based on a large data set of actual comfort surveys in different conditions. The results are summarized in figure 8, where you see that the highest temperature considered comfortable is dramatically affected by humidity, but the effect on the coldest temperature considered comfortable is tiny, and perhaps not statistically significant.

This is all indoors, however, whereas I think the question is more focused on outdoors. It's worth noting that the temperature considered there is the operative temperature including the effect of the mean radiant temperature as well as the air temperature.

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kilotesla t1_j0lv6y5 wrote

Yes, the effect of humidity on insulation is a very real effect, perhaps better documented for building insulation then for clothing, although it's well known among outdoors enthusiasts that cotton loses its insulation value rapidly when it's wet to the point of being soggy. Of course, that's complicated by the fact that the local environment between the shell of your outer layer and your skin might have different humidity than the outdoor air, especially when you first step outside and form the impression of how cold it feels.

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