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. Barry

    Are you sure it's not the hot air from all of our politicians that's causing the heat?

    July 22, 2011 at 10:30 am | Report abuse |
    • Victor

      Why does everything has to be so political? This is a very good information piece on the weather and folks like you just have to mix politics in it.....

      July 22, 2011 at 10:38 am | Report abuse |
    • No

      @Mr. Sunshine, dude it's sarcasm, relax

      July 22, 2011 at 10:43 am | Report abuse |
    • No

      whoops, that should have been @Victor

      July 22, 2011 at 10:44 am | Report abuse |
  2. nimirra

    People dont seem to understand climatic change is what global warming should be called, Global warming is misleading for common people trying to understand whats going on. Look at the amount of snow Chili got which is another side affect of the climatic change.

    July 22, 2011 at 10:32 am | Report abuse |
    • Mr. Sunshine

      Well climate change is natural and normal and then the politicians could not scam us.

      July 22, 2011 at 10:39 am | Report abuse |
    • Carla Hurst-Chandler

      @ Mr. Sunshine...it doesn't really matter if Climate Change is natural and normal if it causes the Earth to become uninhabitable- (by humans) does it.

      Think People.

      We have survived as a species (thus far) because of our ability to adapt/evolve. Their are species who had a far longer run than mankind that are now extinct because they could not adapt. The Planet will be just fine...the people...not so sure about that.

      July 22, 2011 at 11:24 am | Report abuse |
    • Carla Hurst-Chandler

      "there" not their.

      July 22, 2011 at 11:25 am | Report abuse |
    • Paul

      The hot weather is called SUMMER, it happened every year since GOD made the universe. You need to get away from the AC and go outside once in a while. The temp is no different than when my grandfather lived but he was use-to-it. Today it seems hotter because people are always sitting in front of an AC vent.

      July 22, 2011 at 11:48 am | Report abuse |
  3. Monrob

    I feel like I've taken a deep breath and I am waiting for summer to be over to exhale. I hate summer, it is not healthy.

    July 22, 2011 at 10:33 am | Report abuse |
    • Howard

      Without summer, there would be very few food crops, for people or for livestock.

      July 22, 2011 at 11:12 am | Report abuse |
  4. Mr. Obvious

    @Public – Thank you for the science lesson! I'm pretty sure that nobody here knew that "the sun OBVIOUSLY hits the whole earth at some point in a given day"...I will have to look into that phenomenon!...All I said was that the sun causes the heat...true? Without that big, bright, yellow ball full of hydrogen and helium up there, it would be just a little bit cooler and darker here don't ya think? So yeah, the sun is the one causing the heat...unless it is everyone that is heated up because of the debt ceiling...hmmm.....

    July 22, 2011 at 10:35 am | Report abuse |
  5. Nonya

    It's because the poles are beginning to flux, at the same time as the largest solor flair activities are starting. Will next year will be hotter than this year? 3000+ years, now how old is that calendar? How old are the pyramids, older pyramids, locations, shadow difference, pyramids in China age? Mongolia, age? Nile climate change age? Core drill at pole results? There are more than a few coincidences here. Who knows?

    July 22, 2011 at 10:37 am | Report abuse |
  6. usaidwhat2me

    Can some please start a fashion trend with the umbrella hats !!! Strength in numbers..... who cares, we may look stupid but we cool........ not sayin jus sayin. Who the F cares, it hot, SO WHAT!!! My Gosh, Bummed the casey anthony trial is over, media want to beat our brains in with this now.

    July 22, 2011 at 10:45 am | Report abuse |
  7. maine liberal

    Some one should inform Rushbo he thinks the heat index is a government hoax.
    The heat bwave has shut up the global climate change deniers that were screaming back in february that it was cold in tx and snowing therefore global warming was a myth.

    It has been 92+ in maine for last 4 days... warmest summer on record . 2010 was warmest year on record.
    while it was snowing in DC it was spring rain and flowers in maine.

    July 22, 2011 at 10:47 am | Report abuse |
  8. the Wash

    Sounds like a great time to visit Seattle: 75 degrees and sunny this weekend (subject to change)!

    July 22, 2011 at 10:53 am | Report abuse |
  9. richp

    I attributed it to all the lies threats and rhetoric coming out of DC and all three houses and being amplified by media scare tactics.

    July 22, 2011 at 10:55 am | Report abuse |
  10. RightOn

    I'm guessing the heat wave of 1896 in NY City was cause by all the horse and buggys...and the Ice of the Ice Age was melted by flying saucer exhaust. This heat sure causes some confused thinking,but it sure makes the global warming worshippers happy to forget all about the record cold we had this winter.

    July 22, 2011 at 10:55 am | Report abuse |
  11. nwo

    Comet ELENIN (Extinction Level Event Nibiru In November) is not a comet but our suns brown dwarf companion locked in a 3600 yr eliptical orbit, dubbed Nemesis by NASA in 1984 in an effort to explain Uranus' perturbed orbit . Every alignment of the earth between our sun and comet (wink wink) Elenin has produced a major 6.0 quake or above since its approach through our solar system. 11/21/11 will be a destructive day.

    July 22, 2011 at 11:10 am | Report abuse |
  12. nwo

    Comet ELENIN (Extinction Level Event Nibiru In November) is not a comet but our suns brown dwarf companion locked in a 3600 yr eliptical orbit, dubbed Nemesis by NASA in 1984 in an effort to explain Uranus' perturbed orbit . Every alignment of the earth between our sun and comet (wink wink) Elenin has produced a major 6.0 quake or above since its approach through our solar system. 11/21/11 will be a destructive day.

    July 22, 2011 at 11:13 am | Report abuse |
  13. lou

    This article is nothing new. Heat and humidity have always combined to make for miserable conditions. It does NOT answer why the TEMPERATURES are so high this year. That's the point that is different. I am soooo thankful I live in the west!

    July 22, 2011 at 11:39 am | Report abuse |
  14. Paul

    The hot weather is called SUMMER, it happened every year since GOD made the universe; get away from the AC once in a while. The temp is no different than when my grandfather lived but he was use- to-it. Today it seems hotter because we are always sitting in front of an AC vent.

    July 22, 2011 at 11:49 am | Report abuse |
    • noteabags

      Not true. It is hotter than usual. You can't deny facts.

      July 22, 2011 at 11:52 am | Report abuse |
    • Salop

      Paul is right. Even with this latest 'heat wave', where I live there have been no records set. Those records occured in the 1900's & 1930's, well before A/C was around. Plus the record string of 95 deg + days was set in that era, too (the current string of days was half the record). So why haven't the years been getting progressively hotter, year by year if 'global warming' is reality? It's because, although man DOES have impact on the climate, it is not as significant as nature's own impact. One good volcanic blast has multiple times more impact than man; temps can be lowered almost immediately and last for up to a year by such a blast. My opinion???global warming is a contrived way to help explain (read justify) alot of expensive science projects. Remember, these same 'scientists' can't tell you whether or not it will rain in the morning one week from now. But they know for sure to what degree man is changing the climate? Give us all a break.

      July 22, 2011 at 12:58 pm | Report abuse |
  15. noteabags

    We all know there is no such thing as global warming. At least that's what the conservatives were saying in January when we had snow.

    July 22, 2011 at 11:50 am | Report abuse |
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