Hot AND Humid

One of our nephews recently relocated to the southeast, just in time for the June 2022 heat wave (timing is everything). His rental didn’t offer any air conditioning (no surprise), but when he looked to open a window he discovered that they had all been painted shut (also probably not that much of a surprise).

So… why try to open a window? Conventional wisdom tells us that moving air will trick our body into thinking that it’s actually cooler than it really is. But that chicanery can only work because of the magic of sweat (yeah, it may be stinky at times, but it’s what keeps us from dying of heatstroke — to quote the Moody Blues: “Breathe deep the gathering gloom…”).

Anyway, the only way the cooling works is if the perspiration can evaporate from our skin and enter the atmosphere. A bit of scientific clarification may be helpful right about now (sorry).

It takes one calorie of energy to evaporate one gram of water — yeah, me too: I spent years thinking that calories are what makes food taste good, not realizing until almost too late that a calorie is actually something very much different (and far more important). The change of phase from liquid to vapor requires energy, and in this case it gets the heat from our skin to evaporate the sweat, which results in the cooling. Just another gift from the earth!

But the evaporation (and cooling) can only happen if there is enough room in the atmosphere to accept the vapor. This leads us to the daunting (and generally underappreciated) concept of humidity — the amount of water vapor in the air. (BTW: Water vapor is invisible. The “steam” we see coming off a teapot is actually water in its liquid form, as are clouds in the sky and fog near the ground.)

So what the heck is the “relative” humidity that is usually mentioned on the weather report? This one is pretty easy— it’s the percentage of vapor in the atmosphere relative to how much the airmass could hold, based upon its temperature.

Warmer air can hold more vapor than colder air. This is why the windows in your car get steamy on a cold winter’s evening, and why clouds often have flat bottoms: the distance above the ground where the clouds abruptly start is called the “dew point” — that elevation where the temperature has dropped enough for the relative humidity to reach 100% and condensation to begin.

This actually makes sense, and again is all too easy to understand. Imagine a dry sponge. You can trickle water onto it and it will gladly soak up the liquid… until it gets saturated. At that point it simply cannot accept any additional water.

The atmosphere works exactly the same way: it will happily evaporate the sweat from our skin, as long as the “relative humidity” is low enough to leave some space for more vapor molecules. And the lower the RH, the easier it is to evaporate the perspiration and gain the cooling. This is why a 110° day in the arid southwest can feel so much more comfortable than a 90° day along the humid and muggy Gulf Coast.

(As an aside, dogs do not have sweat glands (other than on the pads of their paws), so the only way they can make use of this cooling process is to pant. It may seem a bit crass, but this is just how nature does it.)

This relationship between temperature and relative humidity leads to what is called the “heat index” — a very specific description of how hot it feels, depending on how full of water vapor the atmosphere is. Here’s a chart supplied by the National Oceanic and Atmospheric Administration (NOAA) that details this unfortunate reality.

Note that a 90°F day with only 80% relative humidity actually feels more like 113°F. Bump the humidity to 95% and the heat index jumps to a sweltering 127°F! These values are the reality for much of the Southeast during most summers, so it’s no wonder that many of the locals (including my nephew) need to change their shirts so often.

Toss in a flock of hungry mosquitos and a typical summer day in that part of the nation can feel pretty miserable for many of us.