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. 2bits

    The vacuum in DC is sucking up the heat from the equator.

    That giant sucking sound is all those empty heads in Congress sucking the life out of America.

    July 21, 2011 at 9:00 pm | Report abuse |
    • DonBeal

      We, the People, are responsible. We want government programs but we don't want to pay taxes. We elect some representatives for one reason and we elect some for the other. Until we, the people, can decide, we will continue to have gridlock.

      July 22, 2011 at 4:30 am | Report abuse |
  2. BigJoe

    Humans are destroying this planet.

    July 21, 2011 at 9:20 pm | Report abuse |
    • Dawn

      Humans are destroying the planet, the planet is fighting back and destroying us... so you're both right. The problem is that people don't WANT to take any responsibility for this because ignoring it, or blaming it on some outside source, is so much easier than changing their own behaviors. Government is too greedy about oil to invest more in solar and wind energy, or other clean energy sources. Alternative fuel/electric cars are too expensive for many people to buy. Prices are kept high on these alternatives because oil is more profitable and to hell with what we are doing to the planet in the process...

      July 22, 2011 at 3:20 am | Report abuse |
  3. Ancient Texan

    I was bracing for a lot of global warming comments and only see one so far. In West Texas, we've had 31 days so far this summer. Setting a new record for this period in time, beating the total in 1934. But the strange fact is that we are not in most cases exceeding the record high tempertures of 1934-1935. Did we also have SUV induced global warming 77 years ago?

    July 21, 2011 at 9:37 pm | Report abuse |
    • Voig Nederlander

      The 1934-35 heatwave was a two year phenomenon. It's true, the earth cycles. However, this one has been going on over a decade. Big diff.

      July 21, 2011 at 10:44 pm | Report abuse |
    • dofacc

      But gee, Ancient, the mid-30's were peak Dust Bowl years. You may actually recall some of those years, if you are as old as you imply. The Dust Bowl had a large man made component to them also. If you are using the Dust Bowl years as some sort of yard stick to measure with, you are measuring how awful things can really get.

      July 22, 2011 at 12:41 am | Report abuse |
    • Elizabeth

      The dustbowl was caused by scraping all the vegetation off the land, including wind-breaks; also a human-caused disaster. But warming is as much caused by black asphalt (paint roofs white) as it is by cars. Time to try a different color for roads and parking lots.

      July 22, 2011 at 2:44 am | Report abuse |
  4. gfg

    the sun. case closed

    July 21, 2011 at 9:38 pm | Report abuse |
    • Can11

      You are an idiot, the suns output has only increased by 30% over the last 4 billion years and is not responsible for this heat wave.

      July 21, 2011 at 10:56 pm | Report abuse |
    • Mr. Wizard

      Ha! Your right.

      I like how Can11 calls you and idiot and says that the sun is not responsible for the heat wave. Then he says that the heat from the sun has 'only' increased by 30%. Some people do not know how stupid they sound.

      July 24, 2011 at 5:09 pm | Report abuse |
  5. Ancient Texan

    Meant to say 31 days above 100 degrees actual temperture, not discomfort index.

    July 21, 2011 at 9:39 pm | Report abuse |
  6. rdg18

    The heat is from all the propaganda the demorats are spewing out 24/7 lately.

    July 21, 2011 at 9:40 pm | Report abuse |
    • NobodySpecial

      . . . . . . Said the Right Wing Nut Job Tea Baggy.

      July 22, 2011 at 12:02 am | Report abuse |
    • Wayne Schmand

      Why is someone this paranoid on a CNN website. Get thee to Fox News!

      July 22, 2011 at 3:40 am | Report abuse |
    • Ana W

      Faux News called...it wants it's audience back

      July 22, 2011 at 12:34 pm | Report abuse |
  7. Coach Tru

    "This process is what meteorologists call transpiration." Actually, that's a biological term; why am I not surprised that the reporter didn't research that?

    July 21, 2011 at 10:12 pm | Report abuse |
    • noly972

      Transpiration takes place in both biology and meteorology. Maybe you should do some checking, yourself.

      July 22, 2011 at 12:53 am | Report abuse |
    • Alex

      I'm a Geologist and we use the term "transpiration" because of its effects on the atmosphere.. Not really for sure how it effects the plant...

      July 23, 2011 at 3:25 am | Report abuse |
  8. glu

    Deadly?? You bunch of sissies.

    July 21, 2011 at 11:38 pm | Report abuse |
    • Elizabeth

      My husband has cancer. I have rheumatoid arthritis. I'm not afraid to die. Obviously you are not afraid to call other people names.

      July 22, 2011 at 2:46 am | Report abuse |
  9. Mark L. Luce

    The heat wave can hardly be blamed on the Sun, as the Sun has been unusually quiescent for the past decade. Very few sunspots, which means the Sun is a bit cooler than normal.

    That we are having such a monster heat wave even while the Sun is being quiescent ought to worry any thinking person.........

    July 22, 2011 at 12:07 am | Report abuse |
    • Caz

      The sun was more active between 1960 and 1990 than it had been for hundreds of years. That heat was stored in the oceans, which then warmed the atmosphere between 1980-2000. The lag time between peak sun and peak ocean temperature is similar to the daily lag between noon-time peak sun and 1-2pm peak temps.

      July 23, 2011 at 4:57 pm | Report abuse |
  10. s~

    Someone go to God and demand he install a central AC control.

    July 22, 2011 at 12:25 am | Report abuse |
  11. Aerin

    American Physical Society national policy statement:

    "National Policy
    (Adopted by Council on November 18, 2007)
    Emissions of greenhouse gases from human activities are changing the atmosphere in ways that affect the Earth's climate. Greenhouse gases include carbon dioxide as well as methane, nitrous oxide and other gases. They are emitted from fossil fuel combustion and a range of industrial and agricultural processes.
    The evidence is incontrovertible: Global warming is occurring.
    If no mitigating actions are taken, significant disruptions in the Earth’s physical and ecological systems, social systems, security and human health are likely to occur. We must reduce emissions of greenhouse gases beginning now.
    Because the complexity of the climate makes accurate prediction difficult, the APS urges an enhanced effort to understand the effects of human activity on the Earth’s climate, and to provide the technological options for meeting the climate challenge in the near and longer terms. The APS also urges governments, universities, national laboratories and its membership to support policies and actions that will reduce the emission of greenhouse gases. ..."


    July 22, 2011 at 12:45 am | Report abuse |
    • myisland2@

      nice try. to get moving in a positive direction will take the human race to help each other; transportation, agriculture is a starting or turning point for climate. energy, the planet is water and gravity could we use it.

      July 22, 2011 at 10:57 pm | Report abuse |
  12. M.C. Guegan

    And here, in Nothern Europe at least, we have no summer. Low temperatures, rain, cloudy weather. We can hardly see the sun.

    July 22, 2011 at 2:53 am | Report abuse |
  13. mightyrock

    Is it not obvious that this world is getting crazy? Record breaking heat, droughts, thousands of birds dropping out of the sky, millions of fish mysteriously dieing, earthquakes, severe storms, rumors of wars, floods, wild fires, and ungodliness. What a coincidence that the prophecy in the MOST POPULAR BOOK of ALL TIME is being fulfilled today. These things are announcing the coming of God loud and clear. www theeternalwisdom com

    July 22, 2011 at 4:04 am | Report abuse |
    • Shaif Yirboosch

      NOooooooo....they're announcing the destruction of the eco-system by mankind.

      Gawd-DUH has nothing to do with it.

      And the most popular book ever printed is the 1st issue of Superman magazine comics. It's also about as real as that other book.

      July 22, 2011 at 6:25 am | Report abuse |
    • Ana W

      Tis' much easier to blame on God and the world rather then on ourselves and change our own habits.

      July 22, 2011 at 12:36 pm | Report abuse |
    • Anonymous

      It wasnt less than a month ago that people like you were preparing for Doomsday. Dont worry you still have December 2012 left.

      July 22, 2011 at 5:07 pm | Report abuse |
  14. Tim

    Well, all I can add is ... it's darn good thing that Global Warming is a myth . . .

    July 22, 2011 at 5:18 am | Report abuse |
  15. Shaif Yirboosch

    It's called Global Warming; you have heard about it haven't you?

    July 22, 2011 at 6:23 am | Report abuse |
    • C Being

      Actually, Global Warming (while not a myth) is a misnomer. Taking into consideration the high pressure dome covering a majority of the USA is HOT, the correct term, also considering England and Brazil's colder temperatures, would be:


      This is indicative of the problems inherent in picking a sound bite that catches on before the complexities are evident. Remember the first winter after the GW term was applied gave birth to numerous jokes about how could the globe be warming when we're freezing? What mankind should be concerned with is:


      July 22, 2011 at 7:09 am | Report abuse |
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