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. bipar08

    just the earth going thru its normal cycle, warming and cooling, next will be frozen earth.
    or maybe the sun is starting to become a red giant

    July 23, 2011 at 1:30 pm | Report abuse |
    • My2Sense

      Can "unprecedented rate of change" simply be dismissed or ignored?

      July 23, 2011 at 1:34 pm | Report abuse |
    • andrew

      Word. Glad to see someone else has a rational thought in their brain.

      July 23, 2011 at 1:41 pm | Report abuse |
    • Callista

      The sun won't become a red giant for quite a while yet. But I think maybe you weren't quite serious about that one. Curse this lack of ability to express sarcasm on the Internet!

      July 23, 2011 at 3:01 pm | Report abuse |
  2. The voice of Reason

    It's easy to cry Global Warming or more recently Global Climate change, and blame them on things like greenhouse gas emissions, and over consumption. The fact of the matter is, it all comes down to money. The planet is going to go through cycles much like it has done over millions of years. It's going to get hotter, then it's going to get cooler, then it's going to get hotter again. This is the way it has been long before humans started driving to work or waiting in line at McDonalds.

    Now that said. Why would someone be so gungho about global warming and using all of these different products that are "green" and "Safe for the environment?" Simple. It makes them money. Chances are you've noticed that if something is environmentally friendly (recycled perhaps) It generally costs more than something made without it. Here's the kicker, It costs LESS to make, but you're paying MORE for it. Why? Because you feel like you're doing something to help the environment. This planet has been around far longer than we have, and will be around long after we're gone. It's really pretty arrogant and vain to think that we make a single lick of difference on this planet.

    July 23, 2011 at 1:41 pm | Report abuse |
    • My2Sense

      I believe "we" have developed and stockpiled weaponry to devastate all life on this planet.

      That alone could make a "single lick of difference"...

      July 23, 2011 at 2:12 pm | Report abuse |
    • pn

      1) Global warming is about CLIMATE not WEATHER. A heatwave is a weather event.
      2) You seriously believe that the mainstream scientific community earns commission on the sale of solar panels?
      3) If you want to replace your "reason" with FACTS I suggest you go to NASAs homepage. They have raw data posted in an easy to read format.

      July 23, 2011 at 2:31 pm | Report abuse |
    • g$


      learn yourself...

      July 23, 2011 at 3:01 pm | Report abuse |
    • Andrew

      Your "reason" clearly does not translate to facts.

      You are correct that the earth has gone through cycles over millions of years. That is exactly what is startling about the current rate of climate change, since it is supposed to occur over millions of years and not over a matter of centuries.

      But by all means put your head in the sand and continue to poison the environment we have to live in.

      July 23, 2011 at 4:25 pm | Report abuse |
    • mjack

      In the future, you may study economics. You will learn the generally, prices do include all costs. But there are exceptions. An example of an exception is pollution. If a utility company uses inferior prodcuts and causes pollution, this saves money for that company, but overall costs are increased, because taxpayers have to pay for cleanup.
      Thus, we see minimum standards applied to all energy producers, and enforced by regulatory agencies.

      July 23, 2011 at 6:41 pm | Report abuse |
    • Greg

      g$ – go here: https://docs.google.com/Doc?id=dnc49xz_0fb228shr&hl=en

      to see that over the last 3000 years or so, there have been multiple times the temperature has increased dramatically in a short period of time. The current trend might, or might not, be just another of these cycles.

      July 23, 2011 at 8:21 pm | Report abuse |
  3. laquer

    can't convince conservative that is hotter!!!!!

    July 23, 2011 at 1:42 pm | Report abuse |
    • kman02

      You can't convince a conservative that the sky is blue if it does not go along with his religious/political ideology at the time.

      July 23, 2011 at 5:07 pm | Report abuse |
    • kman02

      Really? Not one right-wing nut-job would disagree? Really? REALLY? Puxxys.

      July 23, 2011 at 6:21 pm | Report abuse |
  4. My2Sense

    Additionally, I'm not sure this "follow the money" argument makes any sense.

    I can't imagine any company dedicated to "green" or "Safe for the environment" products amounting to more than a pimple on ExxonMobil's backside.

    Your position defends the most profitable companies in the history of the industrialized world – and yet you're suggesting that "green" companies are in it for the money, and therefore must be dishonest...

    July 23, 2011 at 2:48 pm | Report abuse |
  5. Jones in CO

    The highest temperature ever recorded for the month of July occurred on July 10, at Greenland Ranch, Calif. (elevation –178 ft), where the temperature reached 134°F.

    In 1913.

    The highest temperature ever recorded for the month of June occurred on June 23, at Volcano Springs, Calif. (elevation –220 ft), where the temperature reached 129°F.

    In 1902.

    July 23, 2011 at 3:49 pm | Report abuse |
    • kman02

      So? Average IQ of an idiot who does not believe the planet is warming at an alarming rate: 65. Get my drift, moron?

      July 23, 2011 at 5:11 pm | Report abuse |
    • Brian Dodge

      It also reached 129 F in Pakistan last June. High temperature records in June this year in the US beat low temperatures by 11 to 1. According to scientists at UCAR "...for the period from January 1, 2000, to September 30, 2009, the continental United States set 291,237 record highs and 142,420 record lows, as the country experienced unusually mild winter weather and intense summer heat waves." "If temperatures were not warming, the number of record daily highs and lows being set each year would be approximately even."

      Asingle weather evetn in 1913 or 1902 says little about the climate.

      July 23, 2011 at 6:07 pm | Report abuse |
    • Alfred W

      "A single weather event says little about the climate." Yeah, and such a small sample of reliable weather readings over the past 120 years says even less. "Climate Change" is a term coined by Madison Avenue (true story) after environmental activists were becoming concerned that "Global Warming" had too many positive connotations and thus was not properly motivating people through fear of annihilation. Want a true scientific test on whether Climate Change is a scientific or political doctrine? Ask yourself: why aren't non-political solutions to this imaginary problem being pushed? For example, if every homeowner had a white roof, and every urban area took away some of the blacktop and replaced it with other materials that didn't absorb heat, terrific gains could be made toward reducing some of the "scientific" concerns. But Climate Change is not about science, its about shifting money and political power from one segment of society to another. And any "solution" to this imaginary problem that does not involve one group cashing in or another group being empowered to tell others what to do is a non-starter.

      July 23, 2011 at 8:07 pm | Report abuse |
    • Steve


      July 23, 2011 at 9:46 pm | Report abuse |
  6. Jones in CO

    This planet was at one time covered mostly in ice and snow, yet it warmed somehow, long before Man came along. How did that happen?

    July 23, 2011 at 3:51 pm | Report abuse |
    • ron281969

      by oil cause if the oil was there then it would be still cold but it's like an car as earth is and it needs oil to stay cool but the more we use for like car to stay cool then earth will be warmer.

      July 23, 2011 at 4:26 pm | Report abuse |
    • Brian Dodge

      Milankovic cycles in orbital parameters increased the insolation slightly in the Northern hemisphere, starting the melting of glaciers. The exposure of dark earth that had previously been covered by white, highly reflective ice cause more solar energy to be absorbed. This caused positive feedbacks from more ice melting, and warming of the ocean surface. The warming of the oceans caused the emission of dissolved CO2 and increases in water vapor(humidity), warming the planet even more by decreasing the outward emission of infrared radiation. The maximum of the last glaciation was about 19000 years ago, and the warming terminated about 10000 years ago, in other words, it took 9000 years. during that time CO2 increased from 190 to 280 ppmv and global average temperatures rose about 8 degrees C. It wasn't a continuous smooth transition- Heinrich events and the Younger Dryas cause bumps along the way. By comparison, fossil fuel use has increased the atmospheric CO2 from 280 to 390 ppmv in less than 150 years. This is the highest they have been in 15 million years. For a more complete discussion see "The Last Glacial Termination" by Denton et al in Science, June 2010.

      July 23, 2011 at 5:42 pm | Report abuse |
    • Moooo

      Some prehistoric cows survived the ice age by going into a cave where moss and grass grew, and they ate the grass and farted. The methane in their farts escaped from the cave and began the warming cycle. Sadly, the cows died, but life returned thanks to their farting life back into global climate.

      July 23, 2011 at 11:03 pm | Report abuse |
  7. Joe San Antbnio

    What contributed more to the summer heat in the east coast is the Republican and Democratic party's hot air spewing out from the Capitol bldg in DC.

    July 23, 2011 at 5:08 pm | Report abuse |
  8. m.r.s.f.

    What's causing this heat? Alright everyone, lean in close, I have the secret.



    It's summer.

    July 23, 2011 at 5:21 pm | Report abuse |
  9. cjs

    You don't have to worry about climate change because a huge asteroid will do the earth in. Saw it on PBS.

    July 23, 2011 at 5:29 pm | Report abuse |
    • kman02

      We hear from the moron club enough! I understand it's easy to get into the 'moron' club, but give the rest of us a break for crying out loud!

      July 23, 2011 at 6:18 pm | Report abuse |
  10. Typinator

    Short term weather changes are not worth predicting. Trends over geological time is the best determiner of trends and those changes outlive us as a species. As to some other comments I see... The sun has been increasing in luminosity since it entered the "main sequence". Life will no longer be sustainable in approximately 1 billion years, so basically you could say that there is global warming, but the big picture shows that the tipping point won't be for quite a while.

    July 23, 2011 at 5:55 pm | Report abuse |
  11. kman02

    Officials accepting climate change as fact is the same as officials accepting debt issues as fact. It won't happen until the problem has grabbed their collective scrotums and is pulling in the opposite direction that they are leaning.+

    July 23, 2011 at 6:16 pm | Report abuse |
  12. Steve

    I don't give a crap about the heat wave going on here, how do we get the rain to come? Lets do some cloud seeding or some damn thing

    July 23, 2011 at 7:07 pm | Report abuse |
  13. Steve

    Record temperatures in the early 1900's for Death Valley aren't indicative of much. Record temperatures (both high and low) occur regularly.

    The argument that nothing's happening, move along, nothing to see is that a) CO2 levels have no measurable effect on climate, and b) modern human activity has no measurable effect on CO2 levels.

    Even IF the world climate is changing at unprecedented rates due to natural cycles, if we're affecting them in any way, and the direction it's heading is counter to our interests, we'd be foolish to not try to reduce our impact in pushing it in an undesirable direction.

    In addition, "going green" is good from a resource point of view. Landfill is expensive, digging new raw materials out of the ground (whether it's oil, coal, ores) is expensive and can damages the environment in many ways, and getting rid of toxic by-products from mining, smelting, power production is expensive and can be dangerous.

    July 23, 2011 at 9:40 pm | Report abuse |
  14. Brenda

    I cannot absolutely deny the theory of "global warming," but I do know that even in my lifetime, we have gone through some pretty dramatic weather phases. And don't forget the Plains states' horrible Dust Bowl days. I will admit that - with the drought, the heat, the dying vegetation - here in Texas, I am certainly recalling those poignant photos of Dust Bowl families trying to survive. So far as I know, we just have to hang on.

    July 23, 2011 at 9:43 pm | Report abuse |
  15. Professional Biscuit Taster

    OK, so now I fully understand the problem. It's not that CO2 causes warming, it's that warming causes CO2. When you burn stuff you get CO2 (along with lots of heat), and when you want to put out a fire, you apply CO2. Simple!

    July 23, 2011 at 9:57 pm | Report abuse |
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