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I've always described somewhere as "humid" if the air feels very moist and muggy. So, in the US, I'm thinking of places like Texas (e.g., Houston) and Florida (e.g., Miami). I would consider places like California (e.g., LA) and Colorado (pretty much everywhere) as not humid.

So I thought maybe humidity is the quantifiable metric for this feeling of mugginess. But then I looked up the average monthly humidity for https://weather-and-climate.com/average-monthly-Humidity-perc,Los-Angeles,United-States-of-America vs. https://weather-and-climate.com/average-monthly-Rainfall-Temperature-Sunshine,houston,United-States-of-America, and I was surprised to find that the two were very similar in the summer months, so I'm now confused. I've been to both cities in July, and LA felt very dry, but Houston felt very humid.

San Francisco is somewhere I'd consider to not be humid either, and it seems to have about the same humidity levels as Houston year round: https://weather-and-climate.com/average-monthly-Rainfall-Temperature-Sunshine,San-Francisco,United-States-of-America

So I'm wondering if these humidity measurements is actually representative of the "humid"-ness that I'm feeling when I go to thees places. Should I be looking at other quantifiable metrics?

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  • $\begingroup$ Key word "average" an average may not represent your short term personal experience. $\endgroup$
    – John
    May 7 at 12:18

2 Answers 2

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Relative humidity is just about the most over-used, relatively useless measure of moisture there is. It's just how full the air is, based upon what it can hold, in moisture. Warm air holds more moisture. So 10° F air with a dew point of 10° F is 100% relative humidity... and 80° F air with a dew point of 75° F is lower relative humidity. Suffice to say a cold fog in the deep winter feels a lot different... a lot less muggy... than a soupy summer evening in the south.

Likewise in terms of weather forecasting, relative humidity doesn't tend to say much. It can rain and storm strongly with relative humidity very low at ground level, and yet many times won't rain with relative humidity near 100%. And it's a metric that varies wildly almost daily, basically high at night (when the air is cool and thus nearer full) and low in the daytime (when the air is warm and so can feasibly hold more moisture), despite the level of actual (absolute) humidity being quite slow to change unless airmasses move in from another area.

So use an absolute humidity measure... I prefer the dew point temperature, though other related values like the wet bulb temperature or mixing ratio work as well. They are often useful in forecasting severe weather or heavy rainfall. And to me they're good rough indicators of when it feels muggy. There's still some variation, things like wind and such can still affect how it feels, so perhaps some fancier heat index or apparent temperature may be the most precise. But it's really up to opinion. Yet I still think most experts would agree that relative humidity... doesn't really reflect how moist it feels, and doesn't tell us that much about the weather either. The only thing RH is really helpful with is in fire weather forecasting (plants dry out in low relative humidity and high winds)... but I believe the vast majority of people get nothing useful out of the relative humidity, when instead they could get something better from something like the dew point, if we'd just opt to more actively present it to the public.

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    $\begingroup$ RH seems a useful indicator for avoiding/predicting damp/condensation in property. $\endgroup$
    – Dan
    Apr 23, 2022 at 23:11
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    $\begingroup$ In my experience, RH is usually only mentioned in weather reports during the summer when it's warm. It's a useful point of comparison when other conditions are relatively equal. So if it's 80-90F on two days, it will feel more muggy on the day when RH is higher. $\endgroup$
    – Barmar
    Apr 24, 2022 at 2:23
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    $\begingroup$ @Barmar I think it kind of varies by area. I know I've been in some areas that basically continuously reports the RH. And others where it's basically never mentioned. And I think it is maybe used a little less actively than it was thankfully. Got me thinking a theory... perhaps the areas that use it a bit more are highly agricultural. And I could envision that being roughly useful to such people if they follow it enough to offer hints on when to plant, when to harvest, etc. And yeah, same with particular needs like excessive dampness, or even when dew may happen. $\endgroup$
    – JeopardyTempest
    Apr 24, 2022 at 11:20
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    $\begingroup$ @JeopardyTempest I'm on the Boston area, I think it's common to report it in summer, similar reporting wind chill in winter. $\endgroup$
    – Barmar
    Apr 24, 2022 at 16:55
  • $\begingroup$ RH is very useful it is just not the sole predictor of "mugginess" Its like complaining temperature is useless because temperature alone also can't predict mugginess. mugginess is high temperature combined with high humidity which limits evaporation rate and thus the ability of sweat to cool. $\endgroup$
    – John
    May 7 at 12:23
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There are a number of things going on here.

One is microclimate. The data for Houston most likely comes from Houston Intercontinental Airport (IAH), which is 70 miles from the Gulf Coast. Travel ten or twenty miles toward the coast from IAH (and you're not yet in downtown Houston) and you will notice a difference in climate. The data for Los Angeles most likely comes from Los Angeles International Airport (LAX), which is right in the coast. Travel even a few miles inland from LAX and you will notice a huge difference in climate.

Another factor is that there is a huge difference between 60% relative humidity, where sweating can still help your body remain cool, and 80% humidity, where sweating starts to fail to function.

Yet another factor is temperature. Comfort, or lack thereof, is a combination of temperature and humidity. San Francisco is notoriously humid, but it also has notoriously cool temperatures. "The coldest winter I ever spent was a summer in San Francisco," supposedly uttered by Samuel Clements, aka Mark Twain.

I suggest that you look at dew point rather than humidity regarding comfort level.

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