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. effelbee

    Rush Limbaugh, in his typically idiotic way, says this is a government conspiracy to persuade us that global climate change is real Step outside and stand there for a few hours, Rush.

    July 22, 2011 at 9:49 am | Report abuse |
    • LeftMind

      Brought to you by the same person who recently asked why people that can't feed themselves should be given the right to vote. He's a bigot, and idiot, and a poison to our society.

      July 22, 2011 at 9:54 am | Report abuse |
  2. Bill

    What's causing the heat? My money is on the SUN.

    July 22, 2011 at 10:00 am | Report abuse |
    • Congress are Liars

      well played Bill 🙂

      July 22, 2011 at 10:23 am | Report abuse |
    • Congress are Liars

      well played Bill 🙂

      July 22, 2011 at 10:24 am | Report abuse |
  3. damn texan

    DUH! Has anybody ever heard of global warming? I 've heard it don't exist?like what does this article have to do with a fat wind bag named rush. now i know he's full of hot air but he hasn't caused the heat wave.let's keep it honest

    July 22, 2011 at 10:03 am | Report abuse |
    • Derp

      If it's global warming, why was past winter even colder? Derp.

      July 22, 2011 at 10:22 am | Report abuse |
    • oldostritch

      He may not have caused the heat wave single handedly, but his level of irresponsibility is not a model for remediating the environmental degradation that our offspring will have to deal with eventually to survive.

      July 22, 2011 at 10:31 am | Report abuse |
    • Ana W

      Derp- global warming destabalizes the climate so you will in fact experience higher highs in some areas, and lower lows in others. also, one cold winter or summer does not indicate GW does not exist. Its the longer term trend that is concerning. GW may also bring about harsher winters and record snowfall...many people believe that a bad winter somehow displells GW, but in fact it does more to confirm the fact that extraordinary weather across the board is not only tied to warmer temps.

      July 22, 2011 at 12:52 pm | Report abuse |
    • Ana W

      sorry, I meant to say that GW does not just mean warmer temps or "hot" summers, winters, etc.

      July 22, 2011 at 12:53 pm | Report abuse |
    • Samdromeda

      Rush might have something to do with Global Dimming. He did say that he is an entertainer. Put mind in neutral and drink in Rush. It is not good to let entertainers shape you. It ultimately causes global dimming and characterological weakness. Our nation is weak enough as it is.

      July 23, 2011 at 1:45 am | Report abuse |
  4. Fupped Duck

    Rush beats the meat with his left hand...Oh, I mean heat.....

    July 22, 2011 at 10:03 am | Report abuse |
  5. Paul Ronco

    The heat is being caused by man-made global warming.

    July 22, 2011 at 10:03 am | Report abuse |
    • JdJr

      Riight...and that symposium of international scientists was soo successful proving global warming is man made...

      Face it; Greenland was GREEN a few centuries ago. Maybe it was warmer then?

      July 22, 2011 at 10:28 am | Report abuse |
    • kap912

      @JDJr Greenland was never green. It was named such to misleadingly attract settlers. For all the idiots out there global warming is the term for a simple global increase in temperature of about couple of degrees. This seemingly small change causes more extreme weather including colder winters and hotter summers. This is occurring right now and this is indisputable fact. The only question up for debate is whether this is man-made or not although most scientists seem to think it is man-made. And for those you who actually think thousands of scientists can be bought off to perpetrate this massive "hoax of global warming," I recommend that you check yourself into the nearest loony bin. Scientists as whole community are impossible to buy off and there is no massive conspiracy to make Al Gore rich.

      July 22, 2011 at 10:56 am | Report abuse |
  6. guy

    It's all the freakin' pollution we're producing.....humans are the dirtiest creatures on this planet. Go green and the earth won't be some mean!

    July 22, 2011 at 10:04 am | Report abuse |
  7. NRMD

    Folks, We survive temperatures much higher than 100 using green technology. Very few families have AC units and they can not use either since we have 70% power cut, leave alone it is 3 times more expensive using it. The green technology we use is very simple, wear 1 extra cotton wear, then you feel like you are taking a dip but not in water but with your sweat that provides 10-15 degrees cooler than it is outside of the 2nd layer of your own clothes. Enjoy the heat.

    July 22, 2011 at 10:04 am | Report abuse |
  8. Moderate

    Why do so many people insist that every article regarding the weather has to parrot back to global warming? There are still other factors that affect the earth's weather patterns, and this article is specifically pointing to the humidity causing above-average temperatures this summer. What's wrong with that? We hear enough about global warming as it is. It's perfectly OK for CNN to write an artlicle about the heat without reminding us about global warming, and it does not mean that they are ignoring climate change. Sheesh!

    July 22, 2011 at 10:07 am | Report abuse |
    • oldostritch

      Actually the article focuses on how humidity makes higher temperatures feel even hotter. Since the earth is more than two thirds covered by water, and less and less of it is ice, smart people can see where that's going to lead.

      July 22, 2011 at 10:19 am | Report abuse |
  9. Donaldk

    Wel-come to my World.

    July 22, 2011 at 10:14 am | Report abuse |
  10. ThatGuyBill

    its been this hot before. im sure it will be this hot again. global warming is a bunch of b.s.

    July 22, 2011 at 10:14 am | Report abuse |
    • Rick

      Ill bet you believe in God too!

      July 22, 2011 at 10:20 am | Report abuse |
    • oldostritch

      Soon it will be cooler. It's called winter. Then next summer it may get hotter. We hope not, but trend analyzers think so.

      July 22, 2011 at 10:24 am | Report abuse |
    • sick of republican phonies

      Global warming is a FACT. Even the most cynical oil company-employed "scientists" agree. Their skepticism (or, grasping at straws) regards how MUCH or LITTLE human activity has to do with the undeniable warming that is occurring. But when you have nitwits that make stupid statements, like "global warming is B.S.", without any regard for the facts- how do you fight rampant stupidity?

      July 22, 2011 at 10:37 am | Report abuse |
    • Bubba

      Sigh. Why must you be so dumb? Look it up yourself, I ain't takin' you to raise. If doctors told you you had cancer, would you disbelieve that too?

      July 22, 2011 at 10:59 am | Report abuse |
  11. Though I agree it's offensive

    I still fart in elevators.

    July 22, 2011 at 10:15 am | Report abuse |
  12. Donaldk

    It's Called Summer

    July 22, 2011 at 10:15 am | Report abuse |
  13. Christophorm

    All of these up-ticks in valcanos and earthquakes and strange weather.. all happening at once.. is due to this Non-comet nasa is calling Elenin...Comets don't Effect the sun and planets like this.. see Nasa /Jpl and type in C/2010 x1 and see at every major earthquake date, this Elenin is aligned with earth...not conspiracy this time!

    July 22, 2011 at 10:24 am | Report abuse |
    • Bubba

      Please, can you HAARP nuts and Planet X crazies and October Comet loons all go somewhere else?

      July 22, 2011 at 11:00 am | Report abuse |
  14. T

    So are you saying 1966 had just as much "global warming" problems as today...if so then I would say we are on the right track of controlling global warming since it hasn't changed much since 1966. The earth is a billion years old, of course it's going to evolve and wear down!


    July 22, 2011 at 10:27 am | Report abuse |
    • CodeMan

      Exactly my thinking. When something tied or breaks a record that was set in 1966, etc....then that means it was the same temperature (or very close to it) back in 1966. And that means it hadn't been that temperature again, between the years 1966-present. But again, many years back, in this case 1966, it WAS that same temperature.

      July 22, 2011 at 10:35 am | Report abuse |
    • sick of republican phonies

      The problem is when people who know nothing of the science of global warming look at individual days and say, "Gee, it was 110 in 1966, hasn't been that hot again until today, so I guess everything is about the same." The mean (average) temperature is the story. It's figured out over a LOT of days, not by looking at one or two unusual days and saying, "It's real hot- must be global warming" or " It's real cold- I guess there's no such thing as global warming". And the mean temperature is rising, year after year.

      July 22, 2011 at 10:45 am | Report abuse |
  15. Vic of New York

    Hey, no such thing as global climate change. So let's let Arizona burn to the ground, Virgina can go into the toilet along with the debt ceiling default, and would some one PLEASE put Rush Limbaugh out of his misery? Remember, Democrats have Second Amendment rights too!

    July 22, 2011 at 10:29 am | Report abuse |
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