Wednesday, July 25, 2012

Weather Whys Wednesday: Humidity Can Be Relatively Useless

For today's Weather Whys, we'll tackle Humidity. When I worked in television, I made it my goal to teach viewers about how dewpoint is the single most vital piece of information there is to help gauge how comfortable you will feel if you step outside in the summer. We always hear what the relative humidity is, but does it really add anything to our knowledge of what the weather's actually like? Let's explain a bit.

Dewpoint is an actual temperature. According to the American Meteorological Society's Glossary of Meteorology, dewpoint is "the temperature to which a given air parcel must be cooled at constant pressure and constant water vapor content in order for saturation to occur." Let's elaborate on this a bit. Air always has at least some moisture in it. It may not always be a ton, but gaseous water vapor is always there in some fashion.

Dewpoint Map From Mid-Afternoon Wednesday
Let's take a hypothetical late September example. The air temperature is 65 degrees at 7 PM. At that same time, the dewpoint is 50 degrees. If everything were to stay unchanged and you cooled the air around you from 65 degrees to 50 degrees, the air would no longer be able to hold any of that moisture (water vapor), so it would have to let it out would see condensation on glass, dew on the grass (frost if it were colder), fog, etc.

While temperatures can change rather quickly, dewpoints generally react more slowly. Temperatures rise and fall from day to night. Dewpoints will often stay steady or even come up at night. Generally though, when it's a mainly clear night, the temperature will fall toward the dewpoint. Usually, if there's a large difference between the air temp and the dewpoint temp, the air temperature will fall quickly. If there's a small difference, it will fall slowly, because like I just said, dewpoints don't like to change much. You know this because drier air cools off and heats up quicker than more humid air in most cases.

So let's now focus on comfort.

Map of Relative Humidity values from mid-afternoon
Wednesday. Credit:
When the air is saturated, the relative humidity is 100%. In other words, you could have a temperature of 55 and a dewpoint of 55 as well and your RH is 100%. Alternatively, you could have a temperature of 75 with a dewpoint of 75 and have a relative humidity of 100%. So the humidity in both examples here is 100%...the air is saturated.

But think about it... if you step outside and it's 75 degrees with a 75 degree feels flat out disgusting! But if you step outside at 55 and may feel a bit damp (the word clammy comes to mind), but it's actually not too bad. Yet the relative humidities are exactly the same!! So if you're watching the news, RH tells you next to nothing here, except that the air is saturated.

Let's go one step further. If the temperature is 95 degrees and the dewpoint is 75 degrees, the relative humidity is about 50%.

Interestingly enough, if the temperature is 75 and the dewpoint is 55, the relative humidity is also around 50%.

Which example feels more pleasant?

This is why dewpoint is one of the most important items to talk about when it comes to weather. In the winter, this isn't so important (it's not so useful when discussing comfort in winter, but has large implications on rate of temperature drop, precipitation type, start time of snow, etc.), but in the summer it has huge implications on how it really feels. It's not the heat, it's the humidi-- dewpoint.

I developed this guide to dewpoint a few years ago to make it easy to describe to others:

If the dewpoint is 70 or higher, it's very uncomfortable outside.

If the dewpoint is 65-70, it's slightly uncomfortable outside.

If the dewpoint is 60-65, it depends on your own personal viewpoint...some days this is pleasant...other days it feels a bit humid.

If the dewpoint is 50-60, it's relatively comfortable outside.

If the dewpoint is less than 50, it's extremely comfortable outside.

While I don't advocate eliminating RH as a weather variable completely, it's important to understand the context here. Most of the time in summer, RH won't help you much. Dewpoint, however, will.