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

How Do You Like Your Humidity?

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  |  11:00 AM ET, 07/29/2008

Categories:  Extreme Heat

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