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Posted at 11:00 AM ET, 07/29/2008

How Do You Like Your Humidity?

By Jason Samenow

Discomfort Discussion

Capital Weather Gang reader Dave sent us an email asking:

Why isn't humidity included as part of regular forecasts? The "detailed" forecasts from [the National Weather Service, NWS] make no mention of humidity, and this seems like an important aspect.

I don't have a good answer to this question but my guess it that it's because NWS has "always done it that way." To be fair, NWS does provide predicted humidity levels and the dew point (a measure of how much moisture is in the air) in its Tabular Forecast -- although the audience for that product is outside the mainstream.

In our forecasts here at the Capital Weather Gang, we try to communicate how humid it's going to be. But are we effective?

Keep reading to explore with us new ways of communicating information about humidity. To find out how humid it will be over the next several days, see our full forecast.

The simplest way we describe humidity is simply low, moderate, high, or very high. We'll also use a series of adjectives (that may vary somewhat from forecaster to forecaster) to describe the spectrum of summer humidity:

Low humidity = comfortable
Moderate humidity = tolerable
High humidity = sticky, muggy
Very high humidity = oppressive, sultry

These levels and adjectives are simple and descriptive, but can become repetitive and are somewhat subjective. What's moderate and tolerable to me might be high and muggy to you.

We'll also, at times, refer to the heat index, also known as apparent temperature. This metric indicates how hot it feels (in degrees) when humidity is added to the actual air temperature. We usually only mention the heat index when it's going to be really hot, say 95 or higher. The index doesn't stop working when it cools down, but, under the circumstances, it no longer makes sense to call it a heat index. So we can then either use the geeky sounding term "apparent temperature" which few understand (even though it's the same concept as the heat index) or none of the above. We have chosen the latter.

Once in a while, we'll talk about dew points. The problem with dew points is that many people don't know what they are. The USA Today Weather Guys recently blogged "What the *$#@ is the dew point?" and quipped:

If you took a survey of 100 Americans, how many would know (or care) what the dew point is? Maybe 10? 20, tops?

While dew point is a harder concept to comprehend than temperature, it's not that complex. It's simply the temperature at which the air becomes saturated. Because warm air holds more water than cold air (at saturation), the higher the dew point, the more water in the air. In other words, the dew point is an absolute measure of the air's water content, aka humidity. Here's how dew points fit into our summer humidity spectrum:

Low humidity = Dew points in the mid 50s or below (comfortable)
Moderate humidity = Dew points from the upper 50s to low 60s (tolerable)
High humidity = Dew points from the mid-to-upper 60s (sticky/muggy)
Very high humidity = Dew points of 70 or higher (oppressive)

Of course, this assessment of what dew points are low and comfortable is calibrated to typical D.C. summer conditions. A dew point of 56 on a late July afternoon may feel like heaven to Washingtonians, but a visitor from Flagstaff, Az., used to dew points in the 40s, might feel quite differently.

A metric you won't see discussed within a Capital Weather Gang forecast is relative humidity. Relative humidity tells you how much water there is in the air as a percentage of the maximum possible amount. It varies according to temperature and might be 97% in the morning but drop to 40-something percent in the afternoon even when the amount of moisture in the air doesn't change. The dew point, under these conditions, would not change providing a more useful absolute sense of how moist the air is.

So there are a lot of metrics and ways to talk about humidity. Each have their advantages and disadvantages. I've thought about devising a 1-10 scale for rating humidity levels during the warm months -- where 1 represents very low humidity and 10 is as humid as it gets (sauna-like). Would you find that useful?

Vote in the poll that follows...

By Jason Samenow  | July 29, 2008; 11:00 AM ET
Categories:  Extreme Heat  
Save & Share:  Send E-mail   Facebook   Twitter   Digg   Yahoo Buzz   StumbleUpon   Technorati   Google Buzz   Previous: Forecast: Will the Delayed 90s Finally Arrive?
Next: CommuteCast: Humidity's Unwelcome Return


"Bombocast" currently uses "muggy" especially overnight; may start using "sultry" and "oppressive" as well!

Breaking weather news: Topper Shutt just marked us in for near-century 98F next Monday, could mark a real heat wave. In addition, the mesoscale convective outlook is predicting some of those intense "Chippewa County thunderboomers" in Northern Wisconsin today. They could extend as far east as Rhinelander.

Posted by: El Bombo | July 29, 2008 12:26 PM | Report abuse

I learned what a dew point is, relative humidity and how to calculate it (wet bulb,dry bulb) in 8th grade science class.

Dew point, heat index, the levels, and the adjectives, all seem interchangeable to me.

Relative humidity and a 1 to 10 scale somewhat less so.

Posted by: wiredog | July 29, 2008 1:33 PM | Report abuse

You know how you do the snowflakes, apples for snow? Maybe you could do a humidity the frizzometer....sweatdrop-o-meter...4 baseball caps means give it up...the humidity will win...2 shirt changes means partial humidity...I could go on...

Posted by: Sara in Oakton | July 29, 2008 1:48 PM | Report abuse

For that matter(I said I could go on) you could have little metro trains for heat ratings. 2 cars means it's going to be hot enough that chances are good AC will break down in the cars actually making it hotter in the train than it is outside. 4 cars means chances are something will break down, the "bus bridge" will be chaos, plan on walking from one station to another. OK, now I'm done.

Posted by: Sara in Oakton | July 29, 2008 1:52 PM | Report abuse

I just spent two months in Vegas and 110 degrees with no humidity was more uncomfortable than our 90 degree days with high humidity. I never thought I'd feel this way but I actually missed the humidity. The absolute desert dryness was extremely irritating and drying to my skin and eyes, my lungs felt like they were drying out and made it difficult to breath, also my nasal passage became very dry and caused frequent nose bleeds.

So bring on the mugginess and sticky, sweaty skin, at least I won't dry up into nothing-ness!

Posted by: rjmdog in dc | July 29, 2008 4:41 PM | Report abuse

I'm okay with just the dew point, having learned it junior high science class like wiredog. But, I also love Sara in Oakton's suggestion of using a Metro train icon! If not humidity, there must be something CWG can indicate with the Metro train icons.

Posted by: ~sg | July 29, 2008 9:40 PM | Report abuse

Dewpoint, please! As you indicated, it's the only objective, easily understandable way of understanding how it's going to feel outside. The more you and others use dewpoints, the more common they will become.

Posted by: dal20402 | July 29, 2008 11:21 PM | Report abuse

I think if you are going to use subjective terms (which I prefer) to describe the humidity, you should include the term "soup" in your vocabulary.

For example, on the way home from the Nats game tonight, I was not breathing air, I was breathing soup.

This has been a bad week for allergies so far. I think when the air is this humid the mold spores go nuts or something.

Mold soup.

Posted by: Laura in NWDC | July 29, 2008 11:41 PM | Report abuse

The comments to this entry are closed.

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