Why does humidity make heat feel hotter?

Summary: How hot is hot? The type of humidity indicates how your body will react to the heat and how fast it will cool off.

Back in the late 80s I took a trip to Phoenix, Arizona with a friend who grew up there. Nothing noteworthy really, except that the trip was to take place over the 4th of July holiday. I kept thinking to myself, “The desert in July? 105F at 7 a.m.? 5 a.m. tee times just to beat the heat? Am I nuts?” Clearly, the answer was a resounding, “Yes!” (although my friends will tell you it has little to do with my vacation choices). I kept thinking to myself that I had to be mad, and yet all of my coworkers chided around me, mimicking each other like mina birds: “But it’s a dry heat.” Dry heat or no, when the central air’s pumping out at full force 24/7 and you’re still sweating like a farm animal, hot is hot. Nonetheless, by the time my ten days of arid temperatures were up (never once dropping below the upper 101F in the shade) I had come to learn a few things about humidity and what all of those do-gooders were trying to make me feel better about.

Humidity is a relative thing – that is to say that it’s not absolute humidity that concerns us when we step outside in the dead of summer, but relative humidity. Absolute humidity is simply the amount of water vapor contained in the air; relative humidity is a measurement of water vapor in the air taking into account air temperature and pressure. This provides us with a much more useful reading – one which not only measures the amount of vapor, but does so in respect to the air’s water vapor capacity. So unlike absolute humidity that simply tells us how much moisture is in the air, relative humidity is a ratio that basically says, “This is how much water vapor the air is capable of holding and this is how much water vapor is in the air right now.”

For example: say we have a cubic foot of air and that air can hold 20g of water vapor yet contains only 4g. The relative humidity would be 20%. If the air is then warmed and can contain 40g or water vapor and yet still contains only 4g then the relative humidity would be 10%.

So what does this mean to you, me, Florida, and Arizona? Well, our bodies exercise their internal air conditioners via evaporative cooling – we sweat when we’re hot and the heat causes the sweat to evaporate leaving us cool as a cuke (ideally anyway). But if we’re in super humid environments like the southeast we sweat, but the evaporative part doesn’t happen as fast leaving us hot and sticky and anything but cool. In more arid climates, however, our natural cooling systems get to shine. In that excessive desert summer heat our bodies perspire and the dry heat causes the moisture on our skin to evaporate almost instantly keeping us cooler. So, at least in theory, as long as you have a full water bottle on hand at all times, you’re golden.

But of course, again I have to point out that 105F is hot no matter what. So if you’re thinking about a summer vacation destination, I hear Vancouver’s beautiful this time of year.

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