It's Not the Heat, It's the Humidity!
November 19, 2018
In most of the Northern Hemisphere, July is the hottest month. But when folks grumble about feeling uncomfortable, they often say, “It’s not the heat, it’s the humidity.” That’s so true.
How Can You Tell That It’s Humid?
You can quickly judge moisture by glancing up.
- A deep blue sky means dry air.
- A light blue sky with a nearly white horizon means average humidity.
- A sky that’s milky overhead is very humid. That’s the summertime norm in the Carolinas and the Gulf States.
Warm air can hold far more water than cold air. And the best measurement of the air’s dampness is dewpoint. That’s the temperature at which the current air mass, if cooled down, would not hold its moisture anymore, so its water changes from invisible gas to countless liquid droplets. It’s when fog forms and dew appears. When you breathe on a mirror, it fogs up because the cool glass has lowered your breath to its dewpoint.
What Is Humidity?
Let’s make sense of humidity. Let’s say it’s early morning, the air is 68°F, and it’s holding all the water it can. This means that there’s fog outside or dew on the ground. Since this air is saturated at 68º, this air has a dewpoint of 68. Its relative humidity is 100%. The temperature and dewpoint are the same.
But six hours later at midday, the air is 95°F. This hot air is now capable of holding twice as much water, so the relative humidity is now 50%. Thanks to the increased temperature, the relative humidity has changed radically. Yet it’s the same air as before, moisture-wise. Its dewpoint is still 68°.
So dewpoint is a much better gauge of air moisture than relative humidity. It’s the language spoken by meteorologists and weather nerds.
When Is Air Humid?
What’s important to know is that a dewpoint of 65ºF or higher means very humid air. A dewpoint in the low 60s is somewhat humid. A dewpoint in the 50s is pleasant. A dewpoint in the 40s feels wonderfully dry, like the air in Montana.
Here’s one more very cool fact: Air never cools below its dewpoint. So by looking up the current dewpoint, you instantly know the lowest the temperature can get to tonight. That’s assuming some new air mass isn’t marching in.
How does humidity affect the sky watcher? Let’s have more fun with humidity!
If it’s extremely hot and humid where you live, practice summer weather safety. See these sun safety trips and heat index chart.
About This Blog
Welcome to “This Week’s Amazing Sky,” the Almanac’s hub for everything stargazing and astronomy. Bob Berman, longtime and famous astronomer for The Old Farmer’s Almanac, will help bring alive the wonders of our universe. From the beautiful stars and planets to magical auroras and eclipses, he covers everything under the Sun (and Moon)! Bob, the world’s mostly widely read astronomer, also has a new weekly podcast, Astounding Universe!