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View Full Version : Why does my local news tell me the "dew point" as a humidity indicator?


JohnT
08-15-2014, 10:52 AM
In Atlanta and Knoxville, when one watched the local weather you used to (haven't lived in those cities in years) be told that the humidity was "40%" or "20%" or "100%" (essentially meaning that it's raining).

This way might not have been the most accurate - I have no idea how it was calculated - but you could easily gather that a humidity index of 20% means it's going to be a much drier day than those days where the humidity index is 80%.

However, in San Antonio the local news uses something called the "Dew Point" to indicate humidity... and as far as I can tell, the statistic is utterly worthless. For example, last week the temperature range on a particular day was 76F for the low, 101F for the high, with a dew point of 65F.

Notwithstanding the obvious question of "WTF sense does it make to tell me that the 'dew point' is a temperature that we're not even going to reach?" (though if you could clue me in on that too, that would be nice), what does dew point have to do with telling me how sticky the air is going to be when I step outside? And how do I translate a nonsensical fact like "dew point" to relatively understandable terms such as "Damn, it's going to be a miserable day" like I could in the ATL when the temps get to 95F and the humidity is 90%?

Thanks!

kunilou
08-15-2014, 11:02 AM
The weatherpersons around here have lately begun explaining why they've changed from RH to dewpoints. It comes down to two basic things.

Relative humidity readings are relative, depending on the temperature, so a 40% RH at 20 degrees isn't the same as 40% at 40 degrees, and your perception of it is different.

The difference between dewpoint and the actual outside temperature is easy to understand. The closer the dewpoint to the actual temperature, the closer the air is to saturation (hence, forming dew) and the more you're going to feel it.

chrisk
08-15-2014, 11:03 AM
WAG, aided by a little wiki-ing, it's one way of representing the moisture in the air that doesn't depend on the air temperature. Normalizing to an effective % at a constant temperature would be another way. And one reason why it's relevant is that the air you breathe gets close to body temperature when it's in your lungs, so the % given doesn't apply any more.

The wikipedia article for 'Dew point' (http://en.wikipedia.org/wiki/Dew_point) even has a handy translation chart for what the dew point means for human comfort.

scr4
08-15-2014, 11:08 AM
And how do I translate a nonsensical fact like "dew point" to relatively understandable terms such as "Damn, it's going to be a miserable day" like I could in the ATL when the temps get to 95F and the humidity is 90%?

Dew point: relationship to human comfort (http://en.wikipedia.org/wiki/Dew_point#Relationship_to_human_comfort). Basically, high dew point = humid. If dew point = actual temperature, you've got 100% relative humidity.

Dew point is literally the temperature at which dew forms. So if dew point is 70F, it means "it's so humid that even a lukewarm 70-degree can of coke has condensation on it."

kinoons
08-15-2014, 11:48 AM
It is also worth adding for homes in locations that evaporative cooling may be popular the difference between the air temp and dew point is very helpful.

jtur88
08-15-2014, 11:51 AM
This has bothered me greatly, ever since I discovered that Dewpoint is crucial and Relative Humidity is meaningless.

My benchmark is 70-75. Here in South Texas, when the dewpoint is below 70, it is a tolerably comfortable day, even if the temperature is 105. But if the dewpoint is over 75, it is uncomfortable, even if the temperature is 80.

Furthermore, the dewpoint remains fairly stable through the day, typically, in summer, fairly close to the overnight low. I can tell by the dewpoint at 9 am, whether it is going to be a muggy afternoon or not.

Kimballkid
08-15-2014, 01:59 PM
Every once in awhile they'll explain it on the local news here. He says dew points in the 60's and 70's mean it's going to be humid and dew points lower than that are comfortable to dry.

dracoi
08-15-2014, 02:08 PM
This has bothered me greatly, ever since I discovered that Dewpoint is crucial and Relative Humidity is meaningless.

I wouldn't say RH is meaningless. It does convey a specific and potentially useful piece of information. The problem is that most people misinterpret it. They see 80% humidity on the morning news and believe that this is like the high and low temp - a piece of information that will be true all day long. Of course, RH goes down quickly as temperature goes up and if you don't follow it every hour, then you may have meaningless information. That's how you wind up with idiots saying "It's 100 degrees and 100% humidity today!" Uh, no... not unless there is water condensing onto your skin. By the time it gets to 100 degrees, RH may only be 40%.

Dew point is so useful because its more or less true throughout the day, and you can sort of guesstimate RH based on the difference between actual temperature and dew point temperature.

OldGuy
08-15-2014, 02:19 PM
The thing to remember for comfort is the temperature will seldom drop more than a degree or two below the dew point. The reason for this is the dew point is the temperature at which dew forms -- that is the temperature at which the air becomes saturated with water vapor. As the temperature drops below this, the air becomes supersaturated and water vapor begins to condense out of the air. This condensation releases the heat of vaporization back into the air.

To get a notion of how much heat this is: The heat of vaporization of water is 540 calories per gram. The specific heat of water is 1 calorie per gram. So condensing water releases as much heat as 54 times as much water would release when cooling 10 degrees. All this released heat keeps the air warm.

If the dew point is 75 F, then the overnight temperature isn't going to fall below 73 and probably not that low. So it's going to be unpleasant sleeping unless you have A/C.

zweisamkeit
08-15-2014, 02:47 PM
Dennis Mersereau wrote an excellent post explaining why the dew point is more important/relevant (http://thevane.gawker.com/your-weather-report-is-duping-you-1577269171). I've sent the link to several people and they loved it, too. It also has a quick rule of thumb guide which I've applied all summer:

If the dew point is 61°F, it means that the air temperature has to cool to 61°F to become fully saturated and reach a relative humidity of 100%. The warmer the air temperature gets — or the farther away from 61°F it climbs — the relative humidity will start to drop. As I noted above, this is why relative humidity often reaches the 90-100% range at night and quickly drops to 50% or below during the heat of the day.



Glancing at the dew point can give you a good idea of how comfortable or gross it will feel once you go outside. Here's a general scale to help:



·< 50°F = comfortable

·50-60°F = noticeable moisture but still not bad

·60-65°F = what most people would call "humid"

·65-70°F = muggy, approaching uncomfortable

·> 70°F = oppressive; can be dangerous above 75°F

TroutMan
08-15-2014, 05:30 PM
my benchmark is 70-75. Here in south texas, when the dewpoint is below 70, it is a tolerably comfortable day, even if the temperature is 105. But if the dewpoint is over 75, it is uncomfortable, even if the temperature is 80.

it also has a quick rule of thumb guide which i've applied all summer:

< 50f = comfortable

50-60f = noticeable moisture but still not bad

60-65f = what most people would call "humid"

65-70f = muggy, approaching uncomfortable

> 70f = oppressive; can be dangerous above 75f


Evidently South Texans are a tough bunch.

jtur88
08-15-2014, 05:35 PM
I wouldn't say RH is meaningless. It does convey a specific and potentially useful piece of information. The problem is that most people misinterpret it. They see 80% humidity on the morning news and believe that this is like the high and low temp - a piece of information that will be true all day long. Of course, RH goes down quickly as temperature goes up and if you don't follow it every hour, then you may have meaningless information. That's how you wind up with idiots saying "It's 100 degrees and 100% humidity today!" Uh, no... not unless there is water condensing onto your skin. By the time it gets to 100 degrees, RH may only be 40%.

Dew point is so useful because its more or less true throughout the day, and you can sort of guesstimate RH based on the difference between actual temperature and dew point temperature.
Around these parts, the Relative Humidity is nearly always about 100% when you get up on a summer morning. If there is dew on the grass or your parked car, it must have been 100% when the dew formed. Which is completely meaningless later on in the day, when the temperature rises through the 90s, and the RH steadily drops (as it must), as you say, to 40%, as the temperature rises.

Outside my door right now, the temperature is 98, the dewpoint 71, the RH 43. What useful information does that RH impart?