Joe Lundberg

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It's Not The Heat, It's The Humidity

June 25, 2014; 11:25 AM ET

Wednesday, 11:55 a.m.

I lead our local cycling group's Tuesday evening rides each week, and this year it seems we have been plagued by unsettled weather more often than not. I did a pre-ride ride with a friend who remarked how high the relative humidity seemed. Then, during the regular ride, once a couple of thundershowers rolled by, we rolled out, and one of the guys in my group was complaining about how humid it was. Of course, I was enjoying the hard workout and the sweat just dripping off me, as did the other two riders in our particular group.

In both rides, though, different people reacted differently to the high humidity - or, more specifically, the high dew points. By the time we shoved off on the first ride, the dew points had risen to 70 degrees locally, and the showers that came through just before the regular ride at 6 p.m. only added to the steambath conditions. On the one-hand, the one individual commented on the relative humidity, not really understanding that it is the dew point that really matters, while the other in my regular ride fully knew how high the dew points were, as I work with him!

It is all relative. The 70 degree dew point is high most any place, more so the farther north you go. If the temperature is 90 with that kind of dew point, the 'relative' humidity is 51% - which doesn't seem high, but it is merely relative. By the same token, you can have an air temperature of 60 with a dew point of 50 - hardly a humid air mass - and come up with a relative humidity of nearly 70%. The relative humidity is deceptive, and it's why I much prefer looking at the actual dew point temperature.

And it really is all relative. A dew point of 55 in Baltimore at this time of the year would be considered dry, and you'd probably forecast a beautiful day with at least partly sunny skies and no chance of rain! Take that exact same dew point and put it in Denver, as was the case yesterday, and has been so far this morning, and it would hardly be considered dry! Indeed, it helped fuel some nasty thunderstorms over eastern Colorado yesterday afternoon and last evening and will probably do so again this afternoon and evening. Were the dew points to get any higher, I would think flooding downpours and nearly spontaneous combustion would be in order there!

Going forward, the humidity - more specifically, the dew points - are likely to remain pretty high over a large portion of the country. Here's what they look like now:

Some of the higher dew points are in the Ohio and Tennessee Valleys to the mid-Atlantic states, at least when compared to normal, with many locations up near 70. With the upper-level trough of low pressure currently swinging through the Great Lakes and Ohio Valley into the Northeast, all of this moisture is contributing to a lot of showers and heavy thunderstorms, not so much severe weather. As pointed out earlier, even back into the eastern Rockies the dew points are high with respect to normal, and that can lead to a few thunderstorms with flooding downpours again this afternoon.

Look at this is projected to change by late Friday, according to the 12z June 25 NAM forecast:

We would expect the dew points to remain high throughout the South at this time of the year, but note they will come down some in the Northeast as this upper-level trough moves away, and high pressure sends drier air into the region. In contrast, look at the surge in very humid air up into the northern Plains and Midwest, this in advance of the next storm gelling in the Dakotas. The combination of that developing storm and these high dew points will lead to more showers and heavy thunderstorms Friday into Saturday from the eastern Rockies and particularly the central Plains up into the Dakotas and across the Midwest, so more flooding can be expected.

Aside from your standard heat in the South, and the typical triple-digit heat in the Southwest deserts, there really won't be a lot of heat across the country, but there will be plenty of humidity - more specifically, high dew points, making it uncomfortable for many, and fueling some heavy thunderstorms in the process.

The views expressed are those of the author and not necessarily those of AccuWeather, Inc. or AccuWeather.com

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Joe Lundberg
Joe Lundberg, a veteran AccuWeather.com forecaster and meteorologist, covers both short and long-term U.S. weather on this blog.