High dew points breed summer misery
When the dew point is high, the sweat on your body will not evaporate, and the body overheats.
July 21st, 2011
02:15 PM ET

High dew points breed summer misery

What's all the talk about a heat dome causing a heat wave that is blistering the eastern third of the nation? You may have heard your local TV meteorologist talk about a "dome of high pressure" being responsible for this heat wave.

Essentially, a heat dome is just another word for a dome of high pressure that forms south of the polar jet stream, usually during the summer months, in the Northern Hemisphere, said CNN meteorologist Chad Myers.

But why has this heat wave been so severe and deadly?

How many times have you also heard, "It's not the heat, it's the humidity," or, "It's a dry heat"? Believe it or not, the amount of water vapor present in the atmosphere can actually make a huge difference in the severity of a heat wave. The amount of moisture the atmosphere holds affects how severe the heat is to the human body.

Meteorologists use the dew point and the current temperature to calculate the heat index. When a parcel of humid air is cooled at a constant atmospheric pressure, the temperature at which water condenses is called the dew point and the condensed water is called dew. The higher the dew point, the higher the heat index, and the more severe the heat is to the body.

Dew points are downright oppressive when they are over 75 degrees Fahrenheit. By contrast, dew points of 40 to 50 degrees are very comfortable. With dry heat (lower dew point) the body can withstand much higher temperatures because when your body sweats, the sweat evaporates and cools the body. However, if the dew point is high, then the sweat on your body will not evaporate and the body overheats, Myers explained.

A very uncomfortable record dew point of 82 degrees was recorded Tuesday in Minneapolis. This also caused the heat index to reach 118 degrees, tying the record set in 1966 for the highest heat index ever recorded at the Twin Cities. Dew points soared into the 80s in the Midwest on Wednesday, causing heat indices to reach an oppressive 123 degrees Fahrenheit in Council Bluffs, Iowa.

Heat indices this high usually occur near the Gulf of Mexico or the coast of Saudi Arabia near a relatively shallow, tropical body of water where dew points are high. In fact, Dhahran, Saudi Arabia, located near the Persian Gulf, had the highest dew point ever recorded, 93 degrees, in 2003.

The unusually high dew points (in some cases, record all-time high dew points) combined with very warm temperatures is making this heat wave unusually deadly. At least 22 heat-related fatalities have been reported with this heat wave, and that figure will probably increase.

But why are the dew points currently at such high levels? Part of the answer is the record floods experienced this spring in the Midwest. The ground is still saturated, rivers and lakes are out of their banks and all of this water is turning to vapor and being released into the atmosphere. High dew points also are common in the Corn Belt of the upper Mississippi River Valley.

Surprisingly, the high dew points are enhanced by the mature corn itself, drawing water from the saturated soil and releasing it into the atmosphere. This process is what meteorologists call transpiration.

So the next time your neighbor jokingly says "it's not the heat, it is the humidity," that's a fact.

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Filed under: Uncategorized • Weather
soundoff (381 Responses)
  1. Beastman13

    I'm in Virginia and the weather has been blistering hot. My 10 minutes car ride to work was like sitting in an oven. My friend walked outdoors to get his dogs and came back sweating. The weather is supposed to get even hotter tomorrow but I saw the forecast is calling for iso-storms later in the evening. Maybe it'll help cool the area and get some water to the dying vegetables in my backyard, this heat is roasting them! I can't imaging how the athletes have it each day/night for MLB. I wouldn't be surprised if attendance was lower than normal due to extreme heat.

    July 22, 2011 at 3:24 pm | Report abuse |
    • omegarising

      It's a good thing you don't live in Arizona, you would be arrested for growing those evil vegetables in your own yard.

      July 22, 2011 at 4:10 pm | Report abuse |
    • Anonymous

      Storms = rain = more moisture in the air = more humidity = higher dew points. Have fun.

      July 22, 2011 at 4:45 pm | Report abuse |
    • Beastman13

      @Anonymous – It's a temporary fix for starters. The plants can most definitely use the hydration. Also, it doesn't always get bad after a storm.

      July 22, 2011 at 5:42 pm | Report abuse |
  2. BBoy

    It's a balmy 55f (13c) here in Calgary and overcast... send us the heat!

    July 22, 2011 at 3:52 pm | Report abuse |
  3. USMC79

    It's called SUMMER deal with it. In the winter they'll be complaining about heavy snow.

    July 22, 2011 at 4:08 pm | Report abuse |
    • Frenchy

      probably.....so what's yer point?

      July 22, 2011 at 4:23 pm | Report abuse |
    • Sean

      The media is just over hyping the weather.

      USMC’s point was very clear. Yours however seems to lacking.

      July 22, 2011 at 4:30 pm | Report abuse |
    • Ross

      It "is" summer – moron – but record breaking heat is obviously NOT normal summer temps. Summers continue to get warmer each year. I wonder will you be so cavalier in a few years when the average high is 110?

      July 22, 2011 at 4:37 pm | Report abuse |
  4. rtbrno65

    I tend to find that Summer is hotter than Winter and that Winter is colder than Summer.

    July 22, 2011 at 4:14 pm | Report abuse |
    • Blatent Sarcasm

      Thats just the Liberals playing tricks on your mind with HAARP!

      July 22, 2011 at 6:00 pm | Report abuse |
  5. Guest

    heading for a 6 hour drive to pittsburg. wish me luck

    July 22, 2011 at 4:18 pm | Report abuse |
    • Mike in NYC

      Good luck. 6 hours to Pittsburgh ..... lets see .... that should put you @ Scranton or Indianapolis (depending on your direction), I hope. 'Cause if its gonna take you 6 hrs to get to Pittsburgh from Steubenville then there's a probelm. Of course you'll also need luck if you're trying to get to Pittsburgh in 6 hrs from Albuquerque!!! lol

      July 22, 2011 at 4:48 pm | Report abuse |
  6. Ishmael

    Why do Americans whine so much? I am from Iraq and we have much more many problems . Also whether is hot . Quit whining like childrens. Americans are spoiled.

    July 22, 2011 at 4:51 pm | Report abuse |
    • Ozymandias71

      Freedom of Speech = Freedom to whine... and gripe... and rant... and discuss. We do like our freedoms. 🙂

      July 22, 2011 at 5:04 pm | Report abuse |

      Why do Iraqis whine about other people whining?

      July 22, 2011 at 5:47 pm | Report abuse |
    • Blatent Sarcasm

      In Iraq when unhappy, we blow things up in protest, like men!

      July 22, 2011 at 5:59 pm | Report abuse |
    • little one

      I agree with you whole heartedly, we are spoiled and often times forget we are NOT the only people inhabiting this planet. thanks for being honest.

      July 22, 2011 at 6:22 pm | Report abuse |
  7. Kyle Overboard

    CNN posting an article from Yahoo! that was likely heisted from AP. Sad.

    July 22, 2011 at 4:51 pm | Report abuse |
  8. Dan

    But according to the republicans, there is no global warming...

    July 22, 2011 at 4:52 pm | Report abuse |
    • Ana W

      hmm...GOP whom are tied to Big Oil and their corporate interests or 98% of the scientific community? who should i believe?? hmmm...

      July 22, 2011 at 5:50 pm | Report abuse |
  9. Chris

    I thought political rhetoric was behind the current heat. We never talk about global warming during some of the coldest days that we had this past winter, however....

    July 22, 2011 at 5:00 pm | Report abuse |
    • Ana W

      We don't talk about AGW period...and when it was cold the idiots used that as some sort of proof that AGW doesn't exist..but in educated circles this discussion was very much going on. Climate change will cause a destabalization in the general climate leading to higher highs, lower lows, increased precipitation and increased drought. yes, you CAN have it all...just bc it's called warming does not mean cold and snowy winters dissprove AGW- any extreme weather only serves to help support the argument as these were predictions made decades ago. You don't see science changing its story when snow hits.

      July 22, 2011 at 5:54 pm | Report abuse |
    • Blatent Sarcasm

      Well if it gets cold in the Winter, then obviously global warming is a Socialist Nazi Communist plot to steal our Bibles.

      July 22, 2011 at 5:56 pm | Report abuse |
  10. BOB

    Texas NEEDS RAIN!

    July 22, 2011 at 5:15 pm | Report abuse |
  11. Ana W

    So ridiculous. I just listened to this Chicago program where the interviewer was talking to two meteorologists about the extreme weather this year across the country, in this case Chicago, and asked a lot of simple questions that anyone would wonder. He pointed out the Blizzard in March, the tornadoes, extreme thunderstorms in spring, and now the extreme heat in summer. As SOON as the show was wrapping up the host says to them both "So some people are attributing this to climate change as scientists predicted, what do you think of that?"..It was shocking how both meteorologists just froze like he said the "n" word (but it was the "c" word) and said it was "dangerous" to attrbute these events to CC. Sorry, but it would seem to be exponentially MORE dangerous NOT to even entertain the idea as erring on the side of caution. When did this become such a bad word and WHY do ppl act like this is something we should not even discuss?? Isn't it always better to play it safe ESPECIALLY when scientific prediction has come to pass and weather is doing EXACTLY as they said it would the more we pumped carbon into the air? Its like a bunch of robots there refusing logic. Just astounds me!

    July 22, 2011 at 5:28 pm | Report abuse |
    • ER

      Because if they had answered the question, they would have revealed their political stance, which would lower their credibility in the eyes of the opposite party. That's why it's "dangerous" to discuss it. Unfortuneatly, politics takes precedent over science and popularity is more important than reason or logic.

      July 22, 2011 at 6:00 pm | Report abuse |
  12. Jeremy Fritz

    JESUS IS COMING!!! thats whats causing the Heat to intense.

    July 22, 2011 at 5:29 pm | Report abuse |
  13. Dale

    Look at La Niña information you might understand what's going on it is low pressure and high pressures causing this weird weather.

    July 22, 2011 at 5:39 pm | Report abuse |
    • Ana W

      Yes, but while drought or excessive wetness usually accompanies an El Niño or La Niña pattern, these dry or wet spells have been more intense in recent years. So the nina/nino argument does not explain the unprecendented extremes.

      July 22, 2011 at 5:58 pm | Report abuse |
  14. Jeremy Fritz

    JESUS IS COMING!!!!!!!!

    July 22, 2011 at 5:53 pm | Report abuse |
    • Mr. Wizard

      Where is he right now?

      July 24, 2011 at 5:31 pm | Report abuse |
  15. Blatent Sarcasm

    We all know why its so hot, its so obvious you just have to look out the window. Aliens are moving the Earth closer to the Sun!

    July 22, 2011 at 5:57 pm | Report abuse |
    • sick of republican phonies

      As an idiot spouting off, you ought to learn how to spell "blatant"- especially if it's part of your name. Your posts still would be stupid, but it wouldn't be immediately obvious.

      July 23, 2011 at 6:48 am | Report abuse |
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