The uncomfortable weather of July is often due to humidity, not heat. Find out more about humidity and what it means to be humid!
How Can You Tell That It's Humid?
In most of the Northern Hemisphere, July is the hottest month. But when folks grumble about feeling uncomfortable, they often say, 鈥淚t鈥檚 not the heat; it鈥檚 the humidity.鈥 Want to know if it鈥檚 going to be humid? Look up to the skies! Here鈥檚 how.
How Can You Tell That It鈥檚 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鈥檚 milky overhead is very humid. That鈥檚 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鈥檚 dampness is dewpoint. That鈥檚 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鈥檚 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.
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What Is Humidity?
Let鈥檚 make sense of humidity. Bear with me.
- Let鈥檚 say it鈥檚 early morning, the air is 68掳F, and it鈥檚 holding all the water it can. This means that there鈥檚 fog outside or dew on the ground. Since this air is saturated at 68潞, 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掳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鈥檚 the same air as before, moisture-wise. Its dewpoint is still 68掳.
So, dewpoint is a much better gauge of air moisture than relative humidity. It鈥檚 the language spoken by meteorologists and weather nerds.
When Is Air Humid?
What鈥檚 important to know is that a dewpoint of 65潞F 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鈥檚 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鈥檚 assuming some new air mass isn鈥檛 marching in.
How does humidity affect the skywatcher? Let鈥檚 have more fun with humidity!
If it鈥檚 extremely hot and humid where you live, practice summer weather safety. See these sun safety trips and heat index charts.
Bob Berman
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Being in the south, Alabama, 20 miles from the Gulf, as the article mentions, it's very humid in the summer. As the saying goes, you can cut the air with a knife. Knowing the humidity makes the air very uncomfortable. Curiosity, I bought gages with both the temp and relative humidity readings. Having three of them, they all were within 1-2 degrees, so were fairly accurate it seemed. I have charts that notes the Heat Index for various readings as OFA also has. But many times the Heat Index is off the chart(s). Searching the NET found a web site that inserting the temp and
humidity it instantly gives the Heat Index. From NOAA https://www.wpc.ncep.noaa.gov/html/heatindex.shtml It's a fun site. One table is for Dew Point and another is for Relative Humidity.
To make matters more confusing, I found a site showing the formula to determine the Heat Index, quite complicated. From the National 蜜桃恋人 Service: https://www.weather.gov/ama/heatindex
A sampling of my record keeping My Codes:
FS = Full Sun, gage placement
PS = Partial Sun
OC = Over Cast
HI = Heat Index Temp (from chart) degrees
HM = Humidity
HD = Heat of Day @ 2:00 pm
OT = Outside Temp
IT = Inside Temp
7/07 OT FS HD 115 F HM 90% HI = off the chart HI = 151 degs (per the input table)
Ready for winter. The south may be hot, but in Vermont 2 or 3 years ago it was 104 in the shade.
Hate the summers heat and humidity. I'm already counting down the days for Fall and my favorite season Winter. I'm much more comfortable in colder weather than summer. Now I'm very irritable, sluggish, can't breathe, loose weight from lack of eating. For those who love summertime heat,you can have this horrible heat I'm praying for the quick end of summer.
I feel the same. I'd rather have much cooler weather all year long. This heat and humidity is awful.
Good morning Bob! Thanks for another good article! The dew points in Houston have been in the high 70s almost all summer, each morning it is 79掳 and I have seen several handfuls of 80掳and 81掳 dew point readings. I鈥檝e lived here all my life and it seems like this summer has been the worst. Even five years ago our dew points rarely got above 76. I鈥檓 not sure what the change is, but I don鈥檛 like it! So many people don鈥檛 even know what I鈥檓 talking about when I mention this reading, so I鈥檓 going to send them your article! Happy stargazing and I hope you have clear skies for Mars viewing!
