Weather Blog

There's miserable weather, then there's Tuesday in Tennessee

There's miserable weather, then there's Tuesday in Tennessee
Madison Hawkins, 8, enjoys the cool water and hot sun on Monday, July 11, 2011 as she lays on the steps of The Passage in Chattanooga, Tenn.

As many of you already know, much of the nation remains gripped in a massive heat wave where triple digits have been the norm for well over a week -- some spots approaching two weeks.

But unlike Arizona, when even when it's over 110 for ages it's still a "dry heat" (monsoon season notwithstanding) in the Midwest and Southeast, it is most assuredly *not* a dry heat.

One spot that appears to be among the top sufferers is Millington, Tennessee, where the temperature Tuesday afternoon was a sweltering 102 degrees.

But that's not all -- the dew point was a mind-boggling 81 degrees.  Put the two together and it meant that to anyone standing outside, it *felt* like it was 127.

The dew point is a measure of moisture in the atmosphere and tells you what the temperature would have to fall to for the air to be 100% saturated (i.e., foggy). The closer the dew point and temperature are together, the higher the relative humidity.

Humidity affects the body's ability to cool -- we sweat when we're hot and then when that water evaporates off our skin, that process takes heat energy and chills the surrounding area -- making us feel cooler.

When the humidity is low, the air doesn't have much moisture so evaporation process works efficiently.  But when it's really humid, the air is so full of moisture already that it's hard for your sweat to evaporate. So not only do you not get the cooling effects from evaporation, but you're soaked in sticky sweat that's not going anywhere.

That's how a 102 degree can feel like 127 -- your body can't cool itself so it feels even hotter. That 127 degree temperature is called the "heat index" and it's used in a similar way to how wind chill gives you an apparent temperature.  (And as a comparison, in Las Vegas earlier this week, it was also 102 degrees but a dew point of just 28, meaning a heat index of 95.)

But you can't really use relative humidity as a guide for comfort -- a 48% humidity at 102 degrees is light years worse than a 48% humidity at 70 degrees. Instead, you can get an idea of how muggy it is outside by checking the dew point.

Dew points in the 40s and 50s are considered quite comfortable because if temperatures get low enough to where they reach those levels, it's cool enough outside that your body doesn't need to cool off and you don't really notice it's humid.  Seattle spends nearly the entire summer with dew points in the 40s to low 50s -- perfect levels. We start to notice it getting a little muggy here when we creep into the upper 50s.

Once you start getting into the 60s, it's noticeably muggy, becoming oppressive once you get into the upper 60s.  Dew points in the 70s are unbearable and, on the rare occasions when you get into the 80s, it's just not worth going outside.  But 80s are quite rare in the U.S.

(If you're curious, the world record is 95 degrees set in Saudi Arabia.)

Seattle is quite lucky in that most of our weather comes off the chilly Pacific Ocean, where water temperatures are in the 50s year 'round. That keeps the dew points nice and low and when we do get the moist flow off the ocean, it's also a much cooler flow so we don't notice the humidity. 

When we do get hot, it's due to an east wind that holds back the ocean breezes. But those east winds are coming from arid Eastern Washington and dry further when they sink down the western slopes of the Cascades. The relative humidity is usually around 20 percent on our hot days. About the only way we get humid here is to get a south flow that brings up some tropical or monsoonal moisture and doesn't get the cooling effects of the ocean nor the drying effects of the mountains, but it's a pretty narrow window.

Meanwhile the East Coast and Midwest will get that warm, moist air from the 75-83 degree Gulf of Mexico and mixed in with the heat already around -- as is the case this week in Tennessee.