A brief lesson on humidity, because I'm a nerd · 7:46pm Aug 31st, 2020

Disclaimer: I have no professional education in meteorology, just a weather nerd. I spend hours of my free time per day (wish I wasn’t joking) reading about weather and meteorology.

You know what annoys me as a weather nerd?

"Percent humidity" is a lousy way of measuring humidity.

Allow me to explain.

Why is humidity uncomfortable?

The evaporation of sweat absorbs heat from our bodies, which keeps us cool. Water vapor in the air makes it harder for our sweat to evaporate. So if sweat can't evaporate, it can't cool us. This is why humidity makes us feel hot and uncomfortable.

Why is percent humidity a lousy way to measure humidity?

Consider two situations. Which is more humid?

· 95F with 30% humidity
· 55F with 100% humidity

Your intuition may be to choose the second option.

This is wrong! It is actually MORE humid at 95F with 30% humidity, compared to 55F with 100% humidity.

Percent humidity is also known as relative humidity. And it's just that, relative. The percentage shown is relative to the temperature of the air. Warmer air can hold more water vapor than cooler air. So the 95F air has more water vapor at 30% “capacity” than the 55F air does at 100 percent capacity.

Here is a good visualization. Same percent humidity, but very different amounts of moisture.

Source

Our bodies feel uncomfortable when the absolute amount of water vapor in the air is high. NOT when the percent humidity is high.

So, how do we measure this “absolute amount of water vapor” in the air?

We use the “dewpoint temperature.”

The dewpoint temperature is the temperature at which the air would be at 100 percent humidity. So, if you ask yourself, “How much do I need to cool this parcel of air for it to get to 100 percent humidity?” that’s what the dew point temperature is.

· Low dewpoint temperature = very little water vapor in the air = comfortable
· High dewpoint temperature = a lot of water vapor in the air = sweating and uncomfortable

If you’ve ever noticed your windows fog up, it’s because the air on the other side is colder than the dewpoint. At temperatures below the dewpoint, the water vapor in the air starts to condense. This is why during the winter, there is condensation on the windows. The relatively humid heated air was cooled below the dewpoint by the cold window surface.
In extremely humid places (I’ve seen this in Singapore), the reverse happens. The air conditioning inside the building is colder than the dewpoint, so the humid outside air condenses on the OUTSIDE of the window.

Understanding dewpoints

Here are some general guidelines for dewpoints, although everyone’s comfort level is different.

· Below 55F—dry or comfortable. This is pleasant for almost anyone.

· 55F-65F—humid. A little sticky, but all right. This is about as humid as it gets in the northern US and central Europe on a typical summer day.

· 65F-70F—muggy. This is about as humid as it gets in most of the continental US and central Europe.

· 70F-75F—very sticky, tropical-like humidity. Regularly gets this humid in the southern US, or the rest of the US during heat waves.

· 75F-80F—oppressive tropical humidity. It regularly gets this humid on the US Gulf Coast, South Florida, southern Japan, Hong Kong, SE Asia.

· >80F—deathly humidity. Usually only occurs in the Persian Gulf (Dubai, Qatar) during the summer.

· 95F—highest dewpoint ever recorded, in Dhahran, Saudi Arabia. The heat index was 174F.

So how can we use dewpoints to better understand humidity?

On August 3, 2020, in New York City, NY, at 2:51pm, it was 92F with a dewpoint of 60F.

This is a percent humidity of 34%.

If we just look at the percent humidity, we would say it is “dry.” But the dewpoint of 60F shows that is still “humid.”

TL;DR takeaways?

· Our bodies feel uncomfortable when the absolute amount of water vapor in the air is high. Not when the percent humidity is high.
· The dewpoint temperature is a measure of the absolute amount of water vapor in the air. High dewpoint temperature = a lot of water vapor in the air = humid and uncomfortable.
· Use the dewpoint temperature to measure humidity. It’s a much better indicator of human comfort.

I hope you all found this informative and educational. Given the start of school in much of the world, maybe this article will make one student’s life a little easier.