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Old 08-10-07, 11:40 PM   #1
cranky old dude
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Humidity is relative

All the recent references to hot weather reminds me of a funny incident
we experienced in Hannibal Mo. while on a family Vacation in 2005. The temp. was
100 or 101 or something like that. As we visited with the nice old lady in the
Visitors Center, the conversation turned to the weather (how novel). She
commented that the heat wouldn't be so bad except for the extremely high
humidity due to the close proximity of the Mississippi River (like accross the street).
Well, we had just climbed that hill at the north end of town (you well read BFers
will know from your Mark Twain studies which hill it is) and there is no way we
could have done that in 100+ degree weather with "high" humidity !!!! We thanked her
for her insights and climbed back into the oven--er I mean the Van--and proceeded
to head south to explore "The Cave". As we were leaving the Visitor Center parking lot the
area weather report came on the radio. Yep, the humidity was a whopping 22%.
Well the wife and I laughed so hard we had tears running down our cheeks. You see, living
on the South shore of Lake Ontario we are accustomed to August humidity readings
upwards of 70%. This week alone I've had my windshield wipers on for the past four
days due to condensation on my windshield...not rain. Right now its clear outside at
01:45 A.M. , 67 degrees with 87% humidity!!!!

So depending on what one is accustomed to, humidity is truely relative
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Old 08-10-07, 11:50 PM   #2
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So depending on what one is accustomed to, humidity is truely relative
Absolutely.
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Old 08-10-07, 11:53 PM   #3
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Good one. Yeah 22% would be paradise. The humidity here has been very high for the last two weeks. I think Saturday will find us some relief, finally.

Someone said, "it's so humid you could take a fish for a walk".
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Old 08-10-07, 11:55 PM   #4
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Good one. Yeah 22% would be paradise. The humidity here has been very high for the last two weeks. I think Saturday will find us some relief, finally.

Someone said, "it's so humid you could take a fish for a walk".
Yep. If we're thirsty we just go outside with a tin cup and spin around for a bit.

We can still smell last Sundays rain !!!!!!!!
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Old 08-10-07, 11:59 PM   #5
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That's odd, because Hannibal is in a region that gets high humidity. 22% in the summer is extremely low for that area. Right now it is 74 there with 74% humidity.

One other thing, when the temp goes up, the humidity falls. There has to be an extremely high amount of water in the air to have a high humidity reading when the temp is over 100.

Ironically, you mention that it is 67 there with 87% humidity. That is exactly the same amount of water in the air as when it is 102 degrees with 22% humidity. Both of those calculate out to a dewpoint of 64 degrees.
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Old 08-10-07, 11:59 PM   #6
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When it rains in N. CA (winter months, mostly) it's still not 100% humidity.

We're lucky to get 22%. You can tell the difference between 10% and 20%. We whine that it's humid, and those with naturally curly hair fret.

Humidity is good for my skin, and my plants, so you won't hear me complaining about it.

You know it's humid when your newspaper is limp.

I'll take sunny, hot and dry any day. Just call me acclimated.
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Old 08-11-07, 12:07 AM   #7
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Another example ... 100 degrees with 26% humidity has far more water in the air than 90 degrees with 16% humidity. It seems like there would be a bit of a difference on first glance, but the real difference is more pronounced than it might appear. 90 w/16% would be a "dry" heat. 100 w/26% would be hot & sticky.
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Old 08-11-07, 12:11 AM   #8
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That's odd, because Hannibal is in a region that gets high humidity. 22% in the summer is extremely low for that area. Right now it is 74 there with 74% humidity.

One other thing, when the temp goes up, the humidity falls. There has to be an extremely high amount of water in the air to have a high humidity reading when the temp is over 100.

Ironically, you mention that it is 67 there with 87% humidity. That is exactly the same amount of water in the air as when it is 102 degrees with 22% humidity. Both of those calculate out to a dewpoint of 64 degrees.
Yup, you're right on the money, DP reported as 62 !!!
I don't understand how it works...I just wipe it off my windows.
Once it gets wet around here in late summer, it often stays wet for quite a spell.
Kinda like a jungle, I think.

So now you've got me researching and I'm confused. I do know we were
more comfortable in Hannibal at 101 degrees than I am now at 67.

Last edited by cranky old dude; 08-11-07 at 12:25 AM.
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Old 08-11-07, 12:29 AM   #9
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As having the temperature and relative humidity readings is the same as having the dew point (since you can calculate the dew point given those two numbers), mugginess is related to both.

In a simplistic view, you can consider the dew point to be the temperature at which given how much water there is in the air, the humidity would be 100%. That is, if the temp is 80 degrees and the relative humidity is 50% (which tells you that the air now contains 50% as much moisture as it could possibly hold), then the dew point would be 64 degrees. Because given the same amount of water in the air, the humidity would be 100% at 64 degrees and thus would form into dew.

Air pressure is also a factor, as that also affects how much water the air can hold.

Thus the simplest way to monitor how sticky/muggy it is, is to watch the dew point. When it is 68 or over, that's very muggy. 60 is much more comfortable, although still a bit on the humid side. 55 is nice and 50 is low. So you can see that relatively small changes in the dew point can have considerable impact on one's comfort level.

If you want to get an idea of how comfortable it will be over the next week, just look at the high temps and the dew points. If the highs are close to 90 and the dew points are in the upper 60s, then it's going to be bad. But if the highs are in the lower 90s with dew points in the lower 50s, then it should be relatively nice.

And if you are in a place like Baton Rouge this weekend, where the temps are supposed to hit 100 with dew points around 71-72, then you have my sympathies.
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Old 08-11-07, 12:50 AM   #10
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Thank You!

I shall start paying more attention to Dew Points in regards to comfort.
Thanks for taking the time to explain, you got me researching.
Hopefuly with a few days worth of review of both your posts on this subject
and research on the internet, maybe I'll start to understand better.

Dew Point.....Temp at wich condensation occurs & good indicator of comfort zone
Relative Humidity...amount of moisture in the air vs amount it can hold (kinda sorta)

If it gets too confusing, I'll just stick my head out the window.

Do I hafta add this to the things I learned this year thread ?
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Old 08-11-07, 01:02 AM   #11
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The most common misperception is for people to think relative humidity is some kind of constant way to think of mugginess. They will think that 40% humidity at 70 degrees produces the same stickyness in the air as 40% at 80 and and 90.

The key word that gets lost in the shuffle is "relative."

Admittedly, it is asking a lot for someone to know how to compare 50% humidity at 70 degrees to 35% at 90 degrees. That's where the dew point measurement makes it simpler.
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Old 08-11-07, 01:06 AM   #12
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That's odd, because Hannibal is in a region that gets high humidity. 22% in the summer is extremely low for that area. Right now it is 74 there with 74% humidity.

One other thing, when the temp goes up, the humidity falls. There has to be an extremely high amount of water in the air to have a high humidity reading when the temp is over 100.

Ironically, you mention that it is 67 there with 87% humidity. That is exactly the same amount of water in the air as when it is 102 degrees with 22% humidity. Both of those calculate out to a dewpoint of 64 degrees.
My math sucks...but there are some dew point calculators on the internet!
Double check me, but I came up with a dew point of approx 54 at 101 degree F with
22% humidity, which would be in the more comfortable zone?!?! If I used these calculators
correctly, and that's a big if, then that would explain why we weren't overly uncomfortable
on our vacation. You have opened my eyes Tom Bombadil, and I thank You.

I was, up till now, a humidity watcher...high humidity equalls discomfort. I'm cured. From now on I'll hope for less than 55 degree dew point.

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Old 08-11-07, 05:07 AM   #13
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I'm on my way out to ride now. The temps are fairly mild, mid 80s. But the dew point is 71. I guess I'd better take a towel!
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Old 08-11-07, 07:27 AM   #14
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I don't know what the relative humidity was yesterday morning, but when I was riding and would go down into even a slight depression and the temp would be a degree or two cooler, when I would ride back up the other side and it warmed back up again, my glasses, mirror and bike computer fogged over. For several minutes at a time. It was in the upper 70's at that time of the morning, and that must have been very close to the dew point. It has been rather muggy here for the last ten days or so, with temps at or near 100. We will go over 100 the next few days they say with maybe some relief towards the end of next week.
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Old 08-11-07, 05:43 PM   #15
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My math sucks...but there are some dew point calculators on the internet!
Double check me, but I came up with a dew point of approx 54 at 101 degree F with
22% humidity, which would be in the more comfortable zone?!?!
101 at 22% humidity has a dew point of 63.2 degrees. Which is within the uncomfortable zone but not terrible.

There are a number of dew point calculators on the net, here's one:

http://www.decatur.de/javascript/dew/index.html
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Old 08-11-07, 05:50 PM   #16
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101 at 22% humidity has a dew point of 63.2 degrees. Which is within the uncomfortable zone but not terrible.

There are a number of dew point calculators on the net, here's one:

http://www.decatur.de/javascript/dew/index.html
Sorry !!!!
My mistake...I didn't specify...101 degrees Fahrenheit.
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Old 08-11-07, 05:53 PM   #17
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Hot is hot. over 90, I am house bound. Excess humidity just means the A/C has to work harder. what I do hate about excess humidity. That means the nights are equally miserable, so not even the possibility of a night ride.
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Old 08-11-07, 05:55 PM   #18
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Oh, I didn't notice the temp selector. Most of them default to Fahrenheit and I just assumed this one did.

A dew point of 55 is very low for the Hannibal area. Right now it is 97 there with 35% humidity, for a dew point of 65. Tomorrow it will be climbing above 70 with temps around 100 - nasty. The Mississippi River Valley rarely has dry air in the summer.
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Old 08-11-07, 06:00 PM   #19
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No worries...now at least I understand the comfort index thing a little
better. Always good to have helpful forumites around to educate and inform.
Thanks again.
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Old 08-11-07, 06:11 PM   #20
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Does not most of the nation from Houston to Newark have pretty high humidity index's most of the summer.
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Old 08-11-07, 06:26 PM   #21
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If you have an interest in monitoring dew points, check out this map

http://www.intellicast.com/National/.../DewPoint.aspx
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Old 08-11-07, 06:53 PM   #22
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Don't you want to come MTB riding with me tomorrow afternoon? Temps in the high 90's, humidity around 50 and dew point around 70. Ah, summer in middle Georgia.
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