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. NoodleHat

    Sometimes I light my flatus. Does that make me a bad person? Does that contribute to the problems?

    July 23, 2011 at 10:01 pm | Report abuse | Reply
    • tokyo joe

      its the bombing heat from the Libyan bombs that have drifted into our corner

      July 24, 2011 at 3:44 pm | Report abuse |
  2. Realist

    You are a problem alright.

    July 23, 2011 at 11:06 pm | Report abuse | Reply
  3. Cathey

    PLEASE, OH PLEASE, don't say it is global warming. It is THE WEATHER, MORONS. Unusual things happen from time to time. This is one.

    July 24, 2011 at 12:21 am | Report abuse | Reply
    • Sunny Guy

      And the severe drought is another one. And the record snowfalls are another one. And the tornados are another one. And that's just this year.

      Sunny Guy

      July 24, 2011 at 3:11 am | Report abuse |
    • Jennifer

      Its not Global Warming, its Climate Change.... and yes its behind it also! Hence the flooding in the midwest, which is what happens when you plow the entire Mississippi river flood plain.... Oh and the extra run off water...

      July 24, 2011 at 6:56 am | Report abuse |
    • Ken

      Ok I won't say it's Global Warming...I'll say its JULY = it's summer time stupid, get over it, it's suppose to be hot. Or one could do what Obama does and blame it all on Bush .. he did it..oh wait can't say that he did ... How about blaming the SUV's that people used when the dinisores were running around on the planet..oh wait..can say that either...how about it's the microwaves from all the security cameras around the world spyin on us all .. no .. can't say that either...I revert back to ... IT"S JULY .. ain't it suppose to be hot this time of year?

      July 24, 2011 at 9:39 am | Report abuse |
    • jean2009

      When it happens consistently for several years then yes, it is global warming.

      For an understandable read try: Mind of Dan website: mind.ofdan
      Topic: U.S. National Academy of Science re-confirms that humans are responsible for climate change- .posted May 25, 2010.

      July 24, 2011 at 10:56 am | Report abuse |
    • Timm

      As a meteorologist and a staunch environmental supporter, I am saddened by the ignorant and uneducated people on here blaming global warming for weather patterns that have been ongoing for millenia.

      July 24, 2011 at 3:09 pm | Report abuse |
    • Priapus

      And how is it that the Pacific Northwest was covered in ice years before Bush took office? Thousands of years.

      Libs grab any data point and connect A dot. Morons.

      July 24, 2011 at 4:22 pm | Report abuse |
  4. WIL69R

    GLOBAL WARMING

    July 24, 2011 at 12:33 am | Report abuse | Reply
    • Tim

      Yep, Climate Change = Global Warming, no matter how often people stick their fingers in their ears and sing "la-la-la-la-la-la" it is GLOBAL WARMING!

      July 24, 2011 at 1:11 pm | Report abuse |
    • Priapus

      Someone please call China and ask them to stop making all the crap that you people keep buying, PLEASE. And how is it that the Pacific Northwest was covered in ice years before Bush took office? Thousands of years.

      Libs grab any data point and connect A dot. Morons.

      July 24, 2011 at 4:21 pm | Report abuse |
  5. tman

    it's really not that no one should in the us should complain when us troops overseas fighting for you guys excperince 130 degree days every day with 60% + humidity stop with the it's too hot

    July 24, 2011 at 1:41 am | Report abuse | Reply
    • Pastor Nick

      Two words: Volunteer Military

      Welfare queens, there hasn't been a real threat to the US in over 60 years. You just oversea protecting war/oil profiteering!!!!

      July 24, 2011 at 7:01 am | Report abuse |
    • jean2009

      Thank you for your service. Keep safe!

      Might I add...for Pastor Nick....9/11.

      July 24, 2011 at 10:59 am | Report abuse |
    • Timm

      Don't be a moron tman. I have been over there and it is not that much hotter. Also, my family is not in the middle east, they are right here. I think the young healthy men and women who volunteered to work and fight in such conditions will be o.k., drama queen.

      July 24, 2011 at 3:25 pm | Report abuse |
  6. dave76

    it's called solar maximm, it happens every 11 years, this time it's happening in our summer. more radiation more heat, geez read a science book

    July 24, 2011 at 3:45 am | Report abuse | Reply
    • Jennifer

      You should try it, the solar cycle has no effect on the current temperature. It most is about the magnetic fields from the sun, not heat..

      July 24, 2011 at 6:58 am | Report abuse |
    • jean2009

      From the Environmental Protection Agency's report for August 14, 2002.

      Climate Change
      "According to the National Academy of Sciences, the Earth's surface temperature has risen by about 1 degree Fahrenheit in the past century, with accelerated warming during the past two decades. There is new and stronger evidence that most of the warming over the last 50 years is attributable to human activities. Human activities have altered the chemical composition of the atmosphere through the buildup of greenhouse gases – primarily carbon dioxide, methane, and nitrous oxide. The heat-trapping property of these gases is undisputed although uncertainties exist about exactly how earth’s climate responds to them."

      Changing Climate

      Global mean surface temperatures have increased 0.5-1.0°F since the late 19th century. The 20th century's 10 warmest years all occurred in the last 15 years of the century. Of these, 1998 was the warmest year on record. The snow cover in the Northern Hemisphere and floating ice in the Arctic Ocean have decreased. Globally, sea level has risen 4-8 inches over the past century.

      Worldwide precipitation over land has increased by about one percent. The frequency of extreme rainfall events has increased throughout much of the United States."

      July 24, 2011 at 11:08 am | Report abuse |
    • sciguy73

      Did you read the book? Solar irradiance varies by only a few tenths of a percent with the solar cycle.

      July 24, 2011 at 11:13 am | Report abuse |
  7. mmi16

    Normal is derived from the extremes

    July 24, 2011 at 6:10 am | Report abuse | Reply
    • blah,blah, blah.

      Well said. Also, we only have records dating back a hundred years or so.

      July 24, 2011 at 11:20 am | Report abuse |
  8. SamQuentin

    Heat...humidity...dew point?
    Why not keep it simple? It's called the Humidex!
    See: http://en.wikipedia.org/wiki/Humidex

    July 24, 2011 at 7:20 am | Report abuse | Reply
  9. SamQuentin

    The Solar Maximum for this Solar Cycle 24 will not be reached until approximately 2013.
    See this: http://science.nasa.gov/science-news/science-at-nasa/2009/29may_noaaprediction/
    In any case, it is still only a theory that solar activity affects long term climate on earth. It may be only partially true.

    July 24, 2011 at 7:24 am | Report abuse | Reply
    • sciguy73

      I love how you say "it is still only a theory" as if that somehow indicates how true/likely it is. Quantum mechanics and relativity are "still only theories" too, but are most decidedly correct. The value of a theory is determined by how often its predictions are shown to be correct. There are good theories and bad ones. The evolution theory is supported by mountains of evidence, and is a good theory. The 9/11 consipracy theory is supported by very little evidence, and is a bad theory. This is how science works.

      July 24, 2011 at 11:22 am | Report abuse |
  10. Josh

    Regardless of the article I read, the comments section is vastly more entertaining. Reading this stuff cracks me up no end. Keep up the good work

    July 24, 2011 at 9:50 am | Report abuse | Reply
  11. Damawn

    Gulf of Mexico oil spill anyone?

    July 24, 2011 at 11:38 am | Report abuse | Reply
  12. Mary Jane Burns

    Ah derp...think maybe the sun might have something to do with it herp?

    July 24, 2011 at 11:52 am | Report abuse | Reply
  13. ELH

    Being from Minnesota, I can now legally blame Iowa–STOP growing all that corn!!

    July 24, 2011 at 12:46 pm | Report abuse | Reply
  14. julianpenrod

    New World Order "explanations" and machinations are obvious throughout the denuncuation of climate change here.

    "The earth has always gone through climate changes. This is only one." But, natural cilmate changes have been slow, taking centuries. More has occurred in just the past ten years than in millenia of what are considered normal climate variations! And, past climate changes were always found to have a cause, but, for all that they denounce the man made climate change idea, the nay-sayers offer no alternative cause for what is happening. If pro-man made climate change individuals offered no alternative explaation, the nay-sayers would be o them in an instant. That's a characteristic of the "economic theory" crooks, too, they make grand declarations, but never once actually explain how they are supposed to happen.

    "It's just weather, it's not climate." Climate is a sum total of weather. That's by definition. ANd a consistent patten of weather defines climate. And weather for the last decade or so has been consistently setting records in high temeprature, as well as producing tornadoes where they were never know before, such as Brooklyn; melting glaciers at an unprecedented rate; and even [producing the first new cloud species in a half century, the undulatus aspiratus.

    "The sun is causing the heat increase". As with all other natural type changes, this would take far longer than just a decade. ANd notice the willingness to blame it all on the sun and completely ignore that, if the sun is heating, that doesn't mean there can't still be man made changes, as well!

    "It's cold in winter, so that means the atmosphere is not getting warmer." Even :"gobal warming" never said it would all get uniformly hot at the same time! Not uniformly hot! They said everywhere would increase in temperature, not that they would all go instantaneously to the same temperature! The poles are at an average of about -15 degrees Fahrenheit year round, meaning. at times, they will got to -45 or so. Even if the earth fose 10 degree Fahrenheit, the Arctic would still be below freezing all year! That and the lessening air pressure at the poles and the greater moisture in the air could produce massive snow storms, even in a warming earth! Eventually, the earth could become intolerably hot, but the poles could still be cooler than the rest of the globe!

    "We've had warm years before." There is a distinct and fundamental difference between a single year when temperature spikes and a span of unnaturally warm years! If a year has a temperature 10 degrees above normal, but a decade on either side had normal temperatures, it wouldn't necessarily mean as much as twenty years in a row having temperatures 5 degrees above normal!

    And notice the craven, catty use of legitimate criticisms of the New World Order against those who are saying climate change is occurring. "I will not listen to someone so arrogant as to think man can change the envrionment", in direct contrast the malignant greed beyond arrogance that motivates those who deny climate change. "Don't be sheep! Don't follow the drumbeat of the climate change forces! Think for yourselves!" But the "think for yourselves" these envision is really following their drumbeat! "Don't be led astray by the climate change forces who are only trying to steal your money for their projects!" The money funneled into any climate change analyzing project is less than one tenth of one pecent of what New World Order oil corporation thugs get every year!

    July 24, 2011 at 12:56 pm | Report abuse | Reply
    • ok

      Its called HAARP for the climate change, not saying i agree with everything said but thats the weather part of it not NWO!!

      July 24, 2011 at 4:14 pm | Report abuse |
    • julianpenrod

      ok claims to support part of what I said, but then takes it back by dismissing the major part of the rest. That's a common tactic of the New World ORder, to pretend they're your friend, gain your confidence, appear to be acting honorably, then stab you in the back. People on the outside think the stabbing wasn't a craven act and you can be conned into thinking it was deserved! "It's called HAARP for the climate change", ok says, "thats [sic] the weather part of it [sic] not NWO!!" Who does ok think is manning HAARP, and why does ok think they're doing it? And wouldn't a defender of the New World Order make sure specifically to end an attempted exoneration of them with as insistent multiple exclamation point? Why is ok so determined to make sure no one suspects the New World Order?

      July 24, 2011 at 4:26 pm | Report abuse |
  15. cybrsage

    OK...for all you who are claiming global warming is the cause, you need to read part of the article again...I will quote it for you:

    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.

    If the record set in 1966 was tied, what does that mean for all the years in between 1966 and 2011? IT MEANS THE TEMP WAS LOWER! That is right...global warming over the last several decades created lower temps.

    Figured many of you are told what to think so you do not bother to actually see that obvious when it is placed directly before you...but you no longer have that excuse. Free your minds, think for yourselves...do not let others control your thoughts with drivel. Learn to be yourself and think for yourself.

    July 24, 2011 at 4:07 pm | Report abuse | Reply
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