Dew point verses humidity

Judy Good

I always look at the dew point for how uncomfortable the humidity will be. I wonder how many people do? I don't often hear people refer to weather with dew point verse humidity. Here is a wonderful short video on the difference. We are on lake Michigan, so maybe the term is used more here due to being near the Lake.

https://www.youtube.com/watch?v=6mYWhkFm0io


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Jasdip

Our heat is always mentioned as 'temp 28, feels like 35 with the humidex' as an example. I have no idea what a dewpoint is.

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nickel_kg

I finally got it in my head based on this graphic, where Dew Point = 50 Degrees F:



Dew Point is the temperature at which an air mass would be fully saturated, and dew (or frost) occurs. (Air Mass changes as temperature changes --- cold air is more dense than hot air).

Relative Humidity is a percentage of water in the air mass.

(In another thread, Kenswoods and Elmer provided more discussion -- thanks!) (edited to correct names)

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ci_lantro

Central Wisconsin. I pay attention to the dewpoint. Another way to look at dewpoint. Dewpoint is the temperature at which the relative humidity is 100%.


We had a long week where the temperature was in mid to upper 80's. Not a breath of a breeze. The dewpoint hovered between 61 & 71. That, folks, is miserable, lay under the airconditioner & don't move weather.


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Elmer J Fudd

The recent thread referred to by nickel that has more info on this is entitled Thermostat Settings.

Dew point and relative humidity are two different ways to describe and measure the same thing, how much water vapor is in the air at the given temperature.

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Judy Good

I look only at dew point here in Michigan for how "humid" it is. I never use the humidity level to estimate how humid it is. To me humidity means nothing. Different strokes for different folks. I thought the video was pretty good at describing the difference. like I said, maybe it is where you live as to how you read it.

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Elmer J Fudd

Judy, that video is nonsensical because you could make the same argument in reverse, describing the fact that even though the nominal relative humidity number is lower for the higher temperature, the TOTAL water vapor in the air is greater and so it produces a higher heat index.

Heat index can be calculated using relative humidity and temperature only, with no reference to the dew point. The dew point is calculated from relative humidity at a given temperature. Normally instruments measure relative humidity and temperature, not dew point (unless that equation is built into the instrument as it can be with monitoring equipment). So dew point is a derived number. It's a circle of related factors.

It's two different ways to express EXACTLY the same thing. People can pick which they like better but they're the same thing expressed in a different way.

A related example - in the US, gas mileage for vehicles is express with how many miles per gallon are used. In Europe, it's the reverse, how much fuel to go 100 kilometers. You can turn one into the other with simple math. Same thing.

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ci_lantro

Dewpoint & relative humidity didn't mean squat to me when I lived in an arid region of the US. What I wanted to know was the forecast high, how much wind/ breeze and degree of cloud cover.


Now living where we can have muggy summers, I find dewpoint a more accurate gauge of comfort than relative humidity.


Relative humidity is a much more variable, much less reliable gauge of comfort because it is, well, relative to temperature. When the air's temperature matches its dew point temperature, relative humidity is 100%. But, the higher the air's temperature rises above its dew point, the lower the relative humidity gets.


Why is the Dew Point Temperature is the Preferred Gauge of Comfort?

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Lars

Right now in Palm Springs, it is 109° and the dew point is 44° with 11% humidity, and the feels like temperature is 105°. The dew point is meaningless here, as ci_lantro pointed out.

Meanwhile at our house in Los Angeles, the outdoor temperature is 72°, dew point is 66°, relative humidity is 72%, and the feels like temperature is 73°.

Guess which place feels more comfortable. According to this web site, a dew point below 60° is supposed to feel better than one that is above 60°, but I do not find that to be at all accurate. For me, it is difficult to beat the beach areas of Southern California for comfortable temperatures year round. However, I can handle heat if I have a pool.

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Elmer J Fudd

On the issue of dew point VERSUS humidity (verses are what you find in poems and songs) , everyone can prefer what they prefer and there's no one right answer. A simple example to question your view. I've made up the numbers so they may not be exactly within the mathematical function but they're close and illustrate a point.

You get up one muggy morning. It's 68 degrees, the air is hazy, and the ground and cars/houses are wet. It's stiflingly uncomfortable with heavy air. Dew point is 60 degrees. (Mental note, relative humidity is in the 90s)

As the morning progresses, the air clears and objects dry out. By 11:00 it's 82 degrees and the relative humidity is 55 percent. The dew point is still 60 degrees.

From a muggy-ness point of view, the 82 degrees is going to feel drier and more comfortable than the 68 degrees of dampness. Dew point hasn't changed so it tells you nothing. The relative humidity has changed and its story is useful. Unless you don't think so. Fine with me.




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Richard (Vero Beach, Florida)

At my local...

95°F Actual

117°F Real Feel (Per wunderground. Includes UV and wind?)

102°F Heat Index

71°F Dew Point

45% Humidity

0 mph wind

11 UV index


At a hypothetical location... at a mountain campsite?

60°F Actual

100% Humidity

60°F Dew Point

ignoring wind and UV


Higher humidity in the mountains.

Higher dew point in the mountains. (Nope, I looked at that wrong, it's lower)

Which would I rather sit around a campfire roasting hot dogs? (the mountains, assuming of course it's only foggy, not raining or storming)


You need actual temperature and either humidity or dew point to calculate "heat index".

And you need to add in UV, wind, sun angle, cloud cover, etc. to calculate "real feel". (What is RealFeel?)


It seems to me that neither humidity nor dew point means very much comfort-wise when taken alone.


(It looks like Elmer posted a similar scenario while I was slowly composing but... I'll hit submit anyway.)

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Lars

Meanwhile, in Laredo, Texas it is now 107°, dew point is 59°, relative humidity is 21%, and the feels like temperature is 110°. I've been to Laredo quite a few times, and it was never pleasant, that I can remember.

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bragu_DSM 5

once the temp reaches the 'dew point' you have precipitation.

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Lars

It is also clear from the graph of Laredo that the dew point goes up when the temperature goes down and goes down when the temperature goes up, at least in Laredo.

The last time I was in Texas, I saw a lot of billboard on Interstate 35 advertising "Laredo Is Safe!" If you have to advertise this, there might be a problem IMO.

My advice is that you might want to avoid Laredo.

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Elmer J Fudd

richard, "Higher humidity in the mountains" is far from true in the arid West. where summertime air is usually quite dry. The Rockies, the Sierra Nevada, and other similar mountain ranges normally have dry air (humidity wise) when it's hot.

as for

"once the temp reaches the 'dew point' you have precipitation."

I think yes and no. Precipitation used colloquially is used to describe rain and snow. While theoretically condensation of water vapor from heavily laden air is a type of precipitation, people (at least in my area) don't typically call it such. Stretches of the California coast have foggy mornings at times in the summertime and people tend to call it dampness or condensation or dew from the fog rather than precipitation. Sometimes, light drizzle from the fog, but not the word precipitation itself, people understand it's from the ground-hugging fog and not overhead clouds. When it warms up (and relative humidity drops to clear the fog) , there are usually blue skies overhead. Usage may vary.


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ci_lantro

You can't ignore the breeze/ wind factor that helps cool you when the mercury & dew point start edging up. With a nice breeze, it might be tolerable out. Same temp & dew point & zero breeze will feel much hotter.

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matthias_lang

In warmer months I prefer to have the heat index or at least relative humidity reported. However, in colder months, particularly in the weeks close to the transition toward the next season, I prefer to have dew points reported, as dew points help me predict what might happen in my garden so that I can take action of need be. For example, I might turn on misters if the dewpoint is at a temperature where ripening squash might get frosted. (The mist would add warmth, warding off damaging ice crystals on surfaces.) Or in late winter, expecting the dew point to be reached might make me leave certain seed trays out in the overnight weather to take advantage of that amazingly gentle surface watering. My local news outlets don't usually report dew points anymore.

(Remember Defense Early Warning Line, the DEW Line?)

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Lars

Frost is unknown where I live in Los Angeles, which might be why dew points are not paid that much attention to there. I do bring certain orchids indoors when the temperature drops below 50°, however, generally late November through early February. Since those days are few, I generally leave most of them outside most of the time. I mostly just bring phalaenopsis inside, and I can give them plenty of light indoors in the winter.

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nickel_kg

If you want, go to 'weather underground' website and type in your location. Go to the "hourly" view and you'll see dew point and humidity (and temp, etc). Dew point varies by a couple degrees while humidity goes way up and way down. This makes sense when you think about weather 'happening' because of air masses that are moving over us. The total amount of water vapor in a given mass of air will be more even than the temperature of the same mass of air.

edited to add: this makes perfect sense to me now. But will I remember it in a couple months? lol maybe, maybe not.

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joyfulguy

Not to discourage you, nickel, but ... for most of us, as we get older, that "hard to remember" stuff, gets more frequent, oftener, more common/intense - take your pick.

ole joyfuelled

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