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. Mark L.

    I'll take 100°+ temp.'s anyday over minus 20° below zero wind chills in the winter...No complaints here. Just so long as I got my sports-bottles filled with ice-cold water – One for me and one for my pet dog...{:o)

    July 22, 2011 at 7:50 am | Report abuse |
    • Kelly

      That's the thing that gets me now. I remember, on the mid eastern seaboard 20 years ago, we had 4 seasons. now we usually just have oppressively hot summers and bitterly cold winters, nothing like 20 below, but pretty darn chiily nonetheless. I want 4 seasons back!

      July 22, 2011 at 9:28 am | Report abuse |
    • greg

      I am the exact opposite. I would take cold (maybe not 20 below) over hot anyday. You can always put stuff on to keep warm. Can't always take things off to get cool.

      July 22, 2011 at 9:36 am | Report abuse |
    • Sean

      @ greg, you are absolutely right, I've been saying that for years. Always better to be cold than hot. Can always bundle up, can only go so far to unbundle...

      July 22, 2011 at 10:43 am | Report abuse |
    • 108inNYC

      I don't know, there's something physically painful about cold. Hot is just uncomfortable. I'd take discomfort over pain any day...

      July 22, 2011 at 5:30 pm | Report abuse |
    • Mr. Wizard

      Absolute cold is -460 F and absolute hot is 100 million million million million million degrees. Humans feel comfortable in the 60 – 80 F degree range. The tiniest shift from absolute cold to absolute hot would cause every person on earth to be instantly dead.

      July 24, 2011 at 5:22 pm | Report abuse |
  2. FL Gulf Coast

    The record humidity comes from all the condensation. All the condensation comes from the melted polar ice caps releasing massive amounts of water. The melted ice caps come from global warming. Global warming comes from burning so much oil. Like Al Gore said...

    July 22, 2011 at 8:09 am | Report abuse |
    • Doug


      July 22, 2011 at 8:19 am | Report abuse |
    • Doug

      Glorrrrrrry Glorrrry Al Gore luuuuuuuu yah, Gorrrry glorrrry Al gore luuuuuuu yah. Gorgeous als truth keeps marching on. Knock me over with a featha. Its al gore with the weatha. Doug.

      July 22, 2011 at 9:50 am | Report abuse |
  3. Heat Miser

    It aint the meat, its the motion.

    July 22, 2011 at 8:20 am | Report abuse |
  4. truthsayer2

    Wow, not one single mention of global warming in an article about the heat from CNN! Have I stepped through a rip in time and space?

    July 22, 2011 at 8:25 am | Report abuse |
    • MJ

      Well, ever since global warming alarmists were proven to falsify their research, and neglect the fact that the earth has cooled since 1998 dramatically (using their own data!), the theory of global warming has been outrightly repudiated.

      Science should be based on hard facts and not have conclusions predetermined. Everything alarmists have done was in violation of the Scientific Method.

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

      Wow MJ..I feel sorry for you...falsifying research?? Have you looked into how many billions of dollars from Big Oil are being poured in to help deny AGW? You would much rather believe Big Oil and Congress who are getting rich over this denial then 98% of the scientific community whom have nothing to gain. Hows that sand feel on your head in this heat?

      July 22, 2011 at 12:40 pm | Report abuse |
  5. CTYank

    Where's Rush Limpbutt? To him this "heat index" stuff is all a gummint conspiracy. (Like to see him do a day's work outdoors in conditions like 116 heat-index.)
    Deniers like him would never understand that CO2, water vapor and methane are all greenhouse gases, and that one "feeds" off the other. Ain't ignorance blissful?

    July 22, 2011 at 8:29 am | Report abuse |
  6. drednox

    I love it. Hopefully this will increase each year on a steady curve. The heat affects the areas where most people and political circles which insist there is no global warming are located. This way they are actually the ones that will die off 1st from what they claim does not exist. That is kinda good for the rest of us 🙂

    July 22, 2011 at 8:58 am | Report abuse |
  7. Mr. Obvious

    It's obvious whats causing all the heat....THE SUN!...Just like someone saying "the cause of the fire is unknown"....no it's not, it was a spark that caused the fire.

    July 22, 2011 at 9:10 am | Report abuse |
    • Public

      Wow. I think that's about all that needs to be said. Mr obvious is obviously not bright enough to know the real reason for the heat (if it was just the sun, captain obvious, then everywhere would be the same temperature cuz the sun OBVIOUSLY hits the whole earth at some point in a given day)

      July 22, 2011 at 9:18 am | Report abuse |
    • virgoptrex

      I love to read and ROFL at comments from fool's paradise!

      July 22, 2011 at 9:27 am | Report abuse |
  8. Drowlord

    Wow. I almost never see mentions of my hometown (Dhahran, Saudi Arabia is an expatriate community for oil workers where my family - we're Americans - lived for a decade). They're totally right about the dew point thing, too. In the evening when it starts to cool off, everything gets sopping wet because the humidity is so close to 100% that the moisture in the air condenses even when it's still hot outside.

    July 22, 2011 at 9:13 am | Report abuse |
    • AF Retired

      Ahh...Good ol' Dhahran! I deployed there 4 times during my AF career! Talk about one hot humid place, I remember working on the jets (midnight shift), and just being totally drenched with sweat. It would get up to 140 degress on the black top during the day, where the jets were, and we could only work 15 minutes at a time before having to take mandatory breaks. Thanks for the memories!

      July 22, 2011 at 9:42 am | Report abuse |
  9. foreverwar

    I'm suprised they have not come up with a stupid name for this yet like "hotpocolypse" or "humidageddon"

    July 22, 2011 at 9:16 am | Report abuse |
  10. boogietime

    This is obviously Al Gore's fault. He flies around on his private jet doing those global warmfullness talks and pollutes the atmosphere.

    July 22, 2011 at 9:24 am | Report abuse |
  11. Thinking7

    CNN wants us to believe global warming is not occuring. The heat melts ice in the poles. The ice turns into moisture, which will then make things colder in the winter. I guess the germ "global warming" would make more sense to be called "global climate change." The facts are the facts. It is here, and if you can't feel it, you need to step outside.

    July 22, 2011 at 9:27 am | Report abuse |
    • Ana W

      CNN just appealing to the dumbed down public

      July 22, 2011 at 12:41 pm | Report abuse |
  12. GFUNK


    July 22, 2011 at 9:30 am | Report abuse |
    • greg

      It's actually climate change but why let that little fact change your view

      July 22, 2011 at 9:38 am | Report abuse |
    • Matthew

      You do know that the melting of the arctic ice caps creates a vastly different effect across varied regions. Global Warming, or more correctly: Climate Change, creates higher highs and lower lows. The moisture in the atmosphere builds up causing much more extremes. With Climate change we can expect crazier extremes. More snow in the winter, hotter summers, stronger hurricanes.

      July 22, 2011 at 9:42 am | Report abuse |
    • rob2tall

      rather silly remark...
      global warming does not mean its warmer everywhere! try some real research to discover the true meaning and the affect of it...

      July 22, 2011 at 9:43 am | Report abuse |
    • rob2tall

      This data is a bit outdated-but part of global warming is shifts in weather patterns.Rather than NY getting warmer-the reverse is true-far heavier snowfall /rainfall/or drought is possible...global warming is the affect warmer oceans and our Ionosphere being heated up both by natural elements , man made pollution- and stupid projects like HAARP which superheats a huge portion of the upper atmosphere above Alaska-which created a giant hole in the Ionosphere...messing with that region that normally protects us from UV radiation is a big mistake..
      heres some raw data below about possible side affects to real global temp rises:

      The Consequences of Global Warming
      On Weather Patterns
      Higher temperatures could lead to increased droughts and wildfires, heavier rainfall and a greater number of Category 4 and 5 hurricanes.

      More Powerful and Dangerous Hurricanes
      Warmer water in the oceans pumps more energy into tropical storms, making them stronger and potentially more destructive.

      Warning signs today:

      The number of category 4 and 5 storms has greatly increased over the past 35 years, along with ocean temperature.

      The 2005 Atlantic hurricane season was the most active Atlantic hurricane season in recorded history, with a record 27 named storms, of which 15 became hurricanes. Seven of the hurricanes strengthened into major storms, five became Category 4 hurricanes and a record four reached Category 5 strength.

      Hurricane Katrina of August 2005 was the costliest and one of the deadliest hurricanes in U.S. history.

      Drought and Wildfire
      Warmer temperatures could increase the probability of drought. Greater evaporation, particularly during summer and fall, could exacerbate drought conditions and increase the risk of wildfires.

      Warning signs today:

      The 1999-2002 national drought was one of the three most extensive droughts in the last 40 years.

      Warming may have lead to the increased drought frequency that the West has experienced over the last 30 years.

      The 2006 wildland fire season set new records in both the number of reported fires as well as acres burned. Close to 100,000 fires were reported and nearly 10 million acres burned, 125 percent above the 10-year average.

      Firefighting expenditures have consistently totaled upwards of $1 billion per year.

      Want ideas on how to fight global warming?
      Sign up for our monthly newsletter and get the latest advice and tips.
      See the latest issue

      Intense Rainstorms
      Warmer temperatures increase the energy of the climatic system and can lead to heavier rainfall in some areas.

      Warning signs today:

      National annual precipitation has increased between 5 and 10 percent since the early 20th century, largely the result of heavy downpours.

      The Intergovernmental Panel on Climate Change reports that intense rain events have increased in frequency during the last 50 years and human-induced global warming most likely contributed to the trend.

      According to the National Oceanic and Atmospheric Administration, the Northeast region had its wettest summer on record in 2006, exceeding the previous record by more than 1 inch.

      July 22, 2011 at 9:53 am | Report abuse |
    • oneSTARman

      STOP SCREAMING CLIMATE NAZIS – Even with ALL the MONEY the Oil and Power companies are throwing against SCIENCE – we can ALL SEE that Floods and Famine and Wildfire and Tornadoes and Drought and crop failure – are becoming the NEW NORMAL – CLIMATE has ALREADY CHANGED.

      July 22, 2011 at 9:55 am | Report abuse |
    • rob2tall

      yet another example of what I am talking about is below-from todays news..

      Chilean officials declared a "catastrophe" in eight municipalities Wednesday after heavy snow blanketed communities and blocked roads in what the nation's interior minister called a "white earthquake."

      "It has snowed more than ever," Curacautin Mayor Jorge Saquel told CNN Chile Tuesday. "This is an anomaly. ... This worries us because the meteorologists assure us that new snowstorms are coming."

      In the city of Lonquimay, officials said snow had piled more than 2.3 meters (7.5 feet) high.

      in one day Chile got 7.5 feet of snow! Since this has never happened in recorded history-it too can be part of the shift in global weather patterns-but we will have to wait several years to find out if thats the true cause..

      July 22, 2011 at 10:02 am | Report abuse |
    • Ana W

      GFUNK a typical example of the fools that think snow=cold=opposite of warm= no global warming. what do you think snow is? MOISTURE. Rising oceans and heat trap moisture into the air and in the winter, guess what happens? more snow. Sorry, go read a science book.

      July 22, 2011 at 12:44 pm | Report abuse |
  13. mixedapproach

    I think alot of this heat is a direct result of that bright star in the sky. Maybe I'm wrong.

    July 22, 2011 at 9:31 am | Report abuse |
  14. xtremeflyr

    If you cannot take the heat, get out of the kitchen. Now, there is an old, old saying. Whatever did the saying actually mean? Someone please explain...

    July 22, 2011 at 9:37 am | Report abuse |
    • oneSTARman

      I take it you have never WORKED in a Kitchen with Pots Boiling and the oven on – on a Summer Day. Well..that is what it will feel like in North Dakota in for Thanksgiving 2061 – The Turkeys who CAUSED the Climate change will be long DEAD and our GOOSE will be about COOKED.

      July 22, 2011 at 9:47 am | Report abuse |
  15. Bubba

    Rush says Obama is causing the heat wave with Socialism and Democrats. No, really, that's what he says. Loco del calor.

    July 22, 2011 at 9:47 am | Report abuse |
    • oneSTARman

      CRAZY from the Heat? Or more like Crazy like a FOX. The largest Corporations on EARTH are spending BILLIONS to buy Congressmen and Talk Show Bovicators to HYPNOTIZE you poor morons into believing Cigarettes are GOOD for You and that Fossil Fuels are not destroying our Grandchildren's Future.

      July 22, 2011 at 9:51 am | Report abuse |
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