A hot mess - and why dewpoint is more important than humidity

An Excessive Heat Warning has been issued for the majority of the metro (Fayette County is technically under a Heat Advisory, but I'm not quibbling when the heat index has reached the danger level). The Excessive Heat Warning is in effect until Sunday at 8pm, but I have high confidence that we'll see it extended, possibly through Wednesday, before a front finally starts to move in our direction by week's end.

Excessive Heat Warning through Sunday in pink, Heat Advisory in orange. Graphic courtesy NWS-Memphis.

After a short break from the mid and upper 90s (but not the high humidity) earlier this week, plus a few rounds of strong thunderstorms, upper level ridging builds anew and can be thanked (or cursed) for pushing temps back into the mid 90s to near 100 for the next several days.  Rain chances are minimal to nil through Wednesday as well. These are the dog days... woof.

If it's any consolation, we are nearing the end of the hottest part of the year, climatologically. When eliminating rounding of temperatures to the nearest degree, the period from July 11-27 has the highest average daily temperature at 82.8°. Starting next week we slip a tenth of a degree or so! OK, that didn't help you any.  Maybe this will...

Perhaps this will make you feel a little better Memphis!
Posted by MemphisWeather.net on Saturday, July 25, 2015

So, besides hot temperatures, why are we seeing heat index values near or above 110°? Maybe you've heard this before: "It's not the heat, it's the humidity." (Well, in this case, it's the heat too. Let's not kid ourselves!) But there is truth to that statement. Let's look a little closer at humidity.

Relative humidity vs. Dewpoint

When humidity values are reported, it's actually "relative humidity" that we're talking about. It's relative to the temperature. For instance, if you track humidity during a normal day, you'll see that it peaks around sunrise and bottoms out in the late afternoon. However, even though the air is closer to saturation when relative humidity is highest, relative humidity doesn't actually tell you how much water content is in the air. Besides temperature, water content is the most important factor in determining how "uncomfortable" it feels.

Consider a calm, clear spring morning with temperatures in the mid 50s. The relative humidity is 90%. Now consider early this morning, with temperatures near 80 and relative humidity of 80%. In our example, the humidity was 10% lower this morning, so it wasn't as sticky as our spring example morning right? Wrong! Behold, I give you the more appropriate measure of water content (and uncomfortability - yes I made that up): DEWPOINT.  Dewpoint is the temperature to which air must be cooled to be saturated (or achieve 100% relative humidity).

Let's look at yesterday's observations from Memphis International Airport (below). Notice the humidity (yellow highlight column) falls from 88% in the morning when the temperature was 76° to 49% in the afternoon when the temperature was 95° (orange highlighted rows). That humidity value is "relative" to the temperature. Did the amount of water content in the air change from one time to another? Barely. We know this by looking at dewpoint (green highlight column). It was 72° when the temperature was 76° and 73° when the temperature was 95°. It inched up 1°, thus the amount of water content of the air changed very little. You'll also notice that the dewpoint is generally is more consistent (in the 72-76° range all day) than relative humidity, which varied from about 50-90%.

So, you say "50% humidity doesn't sound all that high!" Did you go outside yesterday afternoon? The air felt very humid! 50% in the morning is low humidity, but 50% in the afternoon is high! Dewpoint helps straighten all this out, since it measures the amount of water vapor in the air no matter the temperature!

Dewpoints above 65° usually cause people to start to notice the "humidity" in the air. Dewpoints above about 72° are very uncomfortable and when a dewpoint reaches the upper 70s to near 80° it's best to just avoid the outdoors altogether, especially if you have health issues! The amount of water vapor in the air at a dewpoint of 80° is so high that it can cause heat illness fairly quickly for those who aren't taking every possible precaution.

It's rare to see dewpoints this high at properly sited professional equipment that is well-maintained and in open areas, such as at large airports. However, in the MWN backyard, the grass is green and thick, moisture is constantly evaporating due to the heat, and there is less mixing of the air due to being sited in a neighborhood with obstructions. Even with well-maintained semi-professional equipment, the dewpoint has been reading 80° recently. I can assure you, having to mow that green thick grass that I can feel every degree of that ridiculous dewpoint! The sweat does little evaporating with that much moisture already in the air.

MWN's Bartlett reporting station, surrounded by grass and slightly sheltered by fences. These factors contribute to a higher dewpoint in this locale than at the airport.

So when someone comments on the humidity from now on, you can take a mental note that it's actually the dewpoint that determines how uncomfortable it feels!

Erik Proseus
MWN Meteorologist

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