This time a question from myself: I was on a video call the other day with a pilot friend in the Southwest and, naturally enough, the conversation rolled around to weather. I made the comment that, as a new resident to the Northeast, I found that the humidity made the cold seem more bone-chilling, which came as a surprise to me.
I knew that, with heat, humidity “makes it worse.” A dry 95℉ in the Western deserts feels altogether different than a humid 95℉ in the South. But I didn’t make the mental connection that the same phenomenon might exist at the other end of the thermometer.
That comment led to a request for a current temperature and humidity report. A quick check of Weather Underground showed that at the time of our call, it was 21℉ outside with a relative humidity of 78%.
Then, as I saw that, I had a case of brain freeze.
How can you have humidity below freezing? Doesn’t water freeze at freezing? I instinctively knew that water vapor was a special case, but I couldn’t put my mental finger on the mechanism.
Neither could my pilot friend. We both quickly Googled, and the AI-dominated search results assured both of us that the freezing point of water vapor is, in fact, freezing. Additionally, there was no shortage of information from books, articles, posts, and self-proclaimed experts saying the same thing: That water vapor freezes at 32℉.
But that can’t be true, or all temperatures below freezing would always have zero humidity (can you imagine how chapped your lips would be?) and, as it turns out, zero humidity is regarded as a scientific impossibility on our planet, where it is usually very much higher, even when “dry.”
Our least-humid state in recent years is Nevada, with its average annual relative humidity of 38.3%. (Although the lowest humidity ever captured came close to zero: On June 20, 2017, Safi-Abad Dezful in Iran hit 0.36% relative humidity at 115.7℉ — but measurements like that are exceedingly rare).
So what’s going on here? I suspect the problem is one of careless word usage and a lack of critical thinking. Let’s put on our thinking caps and review how water functions in the atmosphere.
As pilots, we all learned that there’s always some water vapor in the air. How much water vapor a parcel of air can “hold” is a variable that relates to its temperature and its pressure. We measure “humidity” — the volume of water vapor — as a percentage of the maximum that a parcel can hold at a given time.
When this relative humidity reaches 100%, the parcel of air is fully saturated. It’s holding as much water vapor as is physically possible for the temperature and pressure. Add one more molecule of water vapor and it will be forced out of its gaseous state. If the temperature is above freezing, the excess vapor condenses as a liquid. If the temperature is below freezing, the excess vapor skips the liquid state and goes straight into a solid. This is called deposition.
This direct freezing, when it happens, is when water vapor changes states of matter. But what about the vapor itself? Does it have a freezing point independent of changing states of matter? A point where the vapor just — Boom! — becomes ice?
It turns out the answer is yes, theoretically — but not for reals in our universe.
Upon further research (thank you University of Arizona!), in the vapor state water isn’t some sort of gas cloud. Instead, each individual H2O molecule is independent and separate from the others — the reason water vapor is invisible to the naked eye. The units are just too small.
For water vapor to freeze, then, it would require the atoms in the molecule to stop moving and that happens at absolute zero, which is around -459.67℉. Outer space, by the way, averages about 5° warmer than that, so water vapor isn’t frozen, even in deep space.
But back to how wet air feels differently than dry (or drier) air.
At high temperatures, there’s something called the “Heat Index,” which the National Weather Service defines as the combined biological impact of air temperature and humidity. It’s basically a measure of how hot the ambient air “feels” to our bodies. The human-apparent temperature, as it were.
The actual math isn’t for cowards: Heat Index = -42.379 + 2.04901523T + 10.14333127R – 0.22475541TR – 6.83783 x 10-3T2 – 5.481717 x 10-2R2 + 1.22874 x 10-3T2R + 8.5282 x 10-4TR2 – 1.99 x 10-6T2R2 where T is the air temperature in Fahrenheit and R is percentage relative humidity. Don’t worry, if you’re a math coward like me, there’s an online calculator.
I tried entering numbers below freezing, but it gave me no answer. Then I noticed the little note that says: “Please note: The Heat Index calculation may produce meaningless results for temperatures and dew points outside of the range depicted on the Heat Index Chart.”
This chart starts at 80℉, so that lovely math can’t deliver a cold index, nor does there appear that there is any sort of cold index beyond Wind Chill math, which combines the effects of air temperature with wind speed for a measure of human-apparent coldness from wind-cooling.
Heat Index and Wind Chill, by the way, both deal with our bodies’ temperature regulation systems. Well, actually, interference with that regulation system. When it is hot and humid, our sweat-based evaporative cooling system is less effective. Likewise, cold wind can strip temperature away from our minimally-insulated skin.
Both make sense, but neither explains why on a calm winter day, 25℉ feels nastier when humid than when dry. And it’s not just me. Many people feel colder in damp conditions. And while there’s no shortage of self-professed experts espousing about it online, I couldn’t find anything scientific enough to comfortably hang my hat on as to why this is, or of any way to quantify it, either with heroic math or cowardly calculators.
Chime in readers: Let’s hear your theories on why we feel colder when it’s damp and how we might be able to put a number on our misery.
