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. frank

    No mention of climate change in this or the previous article? Interesting, despite the record highs that are occurring here and globablly...

    July 21, 2011 at 3:08 pm | Report abuse |
    • Barb

      Climate change should be a huge topic of discussion. We have had so much extreme weather due to climate change– perhaps the corporate interests that sponsor news organizations are keeping them from reporting the truth.

      July 21, 2011 at 7:08 pm | Report abuse |
    • XFitPete

      Probably because every time the weather varies more than one standard deviation people come out of the woodwork and crow Global Warming! Climate Change! No need to beat a dead horse, and frankly (haha) I get tired of hearing about it. Instead they focused on why it feels hot and not what is causing it.

      July 21, 2011 at 7:09 pm | Report abuse |
    • Jeff

      There is always climate change. This is assumed.

      July 21, 2011 at 8:23 pm | Report abuse |
    • zlul

      Or maybe its just standard climate cycle changes. You know Earth has gone through multiple heatings which melt caps and glaciers and subsequent coolings(ice ages) long before humans were around. You know that volcanoes give off just as much co2 as humans do, but they've been doing it for the past 4.5 billion years, instead of the past couple hundred years.

      July 21, 2011 at 8:25 pm | Report abuse |
    • Mike

      You are correct, Barb. (Or should I say, BarbARA. Oh, we know . . . we know!). Except it's not just corporate interests, but a new alliance between the corporations and the vaaaaast right wing conspiracy. We just finalized things last week. In any case, you're clearly on to us, so prepare now for capture and deportation to a CIA prison nearest you. Also, I note that you did not emit your share of carbon last week, so your house will be burned to make up the difference. Have a nice day.

      July 21, 2011 at 8:25 pm | Report abuse |
    • Barb

      To Mike,
      I will run away from you in my Prius 🙂

      July 21, 2011 at 8:41 pm | Report abuse |
    • Ana W

      Of course not, the media refuses to ackowledge any problem and instead poses stupid questions like .."hmm..what IS behind this heat??"...whomever thinks that an extreme heat wave, a record blizzard, record storms and droughts all in one year is normal, is only fooling themselves. Science has said for decades that we were impacting this and would one day reap the consequence. So stop wondering CNN, get your head out of the sand, and report what is actually happening.

      July 22, 2011 at 12:28 pm | Report abuse |
    • Rob

      Not globally just in the U.S do some research fool. It's cool and Rainy in Europe.

      July 22, 2011 at 4:17 pm | Report abuse |
    • Ana W

      Rob why don't YOU do some research....yes, only the US is dealing with AGW, of course not the Somolians and their famine, or Bangladesh and their floods...China and drought..should I go on??

      July 22, 2011 at 5:45 pm | Report abuse |
    • bailoutsos

      All the methane discharge from cows is causing it.

      July 23, 2011 at 4:43 am | Report abuse |
    • mike

      Go to www dot skepticalscience dot com

      You can get *detailed* descriptions of proven science for each of the claim used by denialists there, with links to additional resources. Read for yourself and become *educated* on the issue and do not let simple single-minded denialist responses sway you. If you read up on the issue(s) and still disagree, at least you are better informed

      July 23, 2011 at 2:02 pm | Report abuse |
    • bob

      What about the record cold winter we've had 2 years in a row?

      July 24, 2011 at 7:42 pm | Report abuse |
  2. Thermometer

    It's as if nobody even believes ME any longer. (shrug) It's happening to a friend of mine too, your dictionary.

    July 21, 2011 at 3:40 pm | Report abuse |
  3. Queen

    My daughter just got back from Minneapolis last night...on an un-air- conditioned plane. She was *not* a happy camper. It is 102 degrees here, heat index, 115...I'm not a happy camper either! My pool is 90 degrees, so it's not very refreshing, but it's better than nothing!

    July 21, 2011 at 3:51 pm | Report abuse |
    • L. Abner

      Throw a mess of ice in your pool.

      July 21, 2011 at 6:47 pm | Report abuse |
  4. nancy

    Its coming a storm here. I hope my trailer holds up. At least it might cool thangs off some. We got us a new winder unit air condishoner last month with our welfare check. It helps make thangs tolerable.

    July 21, 2011 at 5:41 pm | Report abuse |
    • L. Abner

      Wayell I recon I'll half to wait till ma next welfare check cause I just spent ma last un on ice cream.

      July 21, 2011 at 6:38 pm | Report abuse |
    • joenunya

      Wow. Nancy – your a real piece of work. Enjoy spinsterhood.

      July 21, 2011 at 8:06 pm | Report abuse |
    • kman02

      Good try, TROLL. Not really. That was a WEAK shot into the lower class from somebody sitting at a computer in an airconditioned office. STFU!

      July 23, 2011 at 5:16 pm | Report abuse |
  5. gn7465

    People who think "It's not the heat, it's the humidity" is a joke are morons. It's a basic scientific fact that everyone should be learning in grade school.

    And seriously, the midwest floods are causing all the humidity? Really? Gee. I thought it would something more like the Gulf of Mexico. Rivers, even flooded ones, are insignificant compared to the the Gulf, oceans, or even the Great lakes.

    July 21, 2011 at 5:58 pm | Report abuse |
    • Mike

      Actually, the rivers themselves are not so much to blame, as the residual flood waters caught behind levees and the such. But, as the article mentions, and anyone in a farming area can tell you, agricultural crops put of a tremendous amount of humidity, as the article mentions. Also, remember that the great leaks are smack dab in the middle of the upper half of this mess. Lot of humidity to be had there too. They actually shut the beaches down in Chicago on Tuesday afternoon because of fog rolling in off of the lake. The Life Guards could not see the swimmers in the water.

      July 21, 2011 at 7:21 pm | Report abuse |
    • kman02

      Are you serious?. Really. Look at the actual flood maps and compare them to the heatwave maps. You are really stupid if you blame floods.

      July 23, 2011 at 5:20 pm | Report abuse |
  6. Jason

    All I hear is "waaahhh its hot!". Please...I live in Arizona...yes it is a dry heat however for three straight months we deal with heat well in excess of 110; yet the news never makes a huge story out of it. The key is to stay indoors in an air conditioned building, in your air conditioned car, or in a nice cool body of water such as a pool, lake, water park, etc.

    July 21, 2011 at 6:01 pm | Report abuse |
    • Burbank

      I have been in Tuscon in August during monsoons in 115 degree heat and the humidity was nothing compared to stepping off a plane in Hawaii when it was only 80 degrees. Until you actually experience something like that you can't understand. Yes, Arizona gets extremely hot, I have spent a lot of time all over that state in all seasons, but it is not the same as these high humidity places, not even close.

      July 21, 2011 at 6:24 pm | Report abuse |
    • NoDoubt

      Jason, I live in Arizona too, but before I moved here I lived in Louisiana for 14 years..and let me tell you, our 110 degree heat is NOTHING compared to 95 degrees in the South or anywhere else with humidity. Flat-out no comparison. Sure, it gets blistering in AZ, but its bearable if you can find shade and have water. In humidity, there is nothing that you can do but stay inside.

      July 21, 2011 at 7:36 pm | Report abuse |
  7. Jeremy Fritz

    Whats behind this summer heat? Answer is simple. its part of God's judgment!

    July 21, 2011 at 6:10 pm | Report abuse |
    • L. Abner

      What's he a judgin us about?

      July 21, 2011 at 6:42 pm | Report abuse |
    • Barb

      it is man's stupidity. We should have been off of fossil fuels by now.

      July 21, 2011 at 7:10 pm | Report abuse |
    • OceanLight

      Blasphemy. Killing defenseless babies. Promoting perverted lifestyles. Dwelling in luxury when others are starving. The Bible says the rich will get punished most severely in the Endtime. Read the Book of James.

      July 21, 2011 at 7:15 pm | Report abuse |
    • Jeff

      Jeremy, I disagree. God placed the judgment of the world on Jesus.
      Will there be a final judgment? Yes. These are what the Bible calls "birth pangs". Or a "turning up the heat" on people so they would come to Jesus and live. Not in judgment, but out of compassion.

      July 21, 2011 at 8:21 pm | Report abuse |
    • Ana W

      humans will do anything to put the blame on God..sorry pal, not getting out THAT easy.

      July 22, 2011 at 12:31 pm | Report abuse |
  8. Burbank

    Duh.... Maybe, just maybe it's global warming?

    July 21, 2011 at 6:21 pm | Report abuse |
    • Rod C. Venger

      It's Summer, stupid. Off the meds!

      July 21, 2011 at 7:34 pm | Report abuse |
  9. David

    I live in Las Vegas where it is supposed to be 110 plus right now. Wierdly enough, we had our fourth latest 100+ degree temperature this year, and it just barely hit 110 for a few days and then dropped back down into the uper 90's. This week is supposed to be 101 to 104 then we get hit by another monsoon at the end of the week which usually pushes us back under 100. We only have a month and a half of time for our normal 110's to 115's and then its over, This is turning into one of our coolest summers on record. But I do sympathise with all You easterner's. Stay cool!

    July 21, 2011 at 7:21 pm | Report abuse |
    • rick springfield

      I've not had trouble in Las Vegas heat. Its actually kind of fun as long as you stay hydrated. I really like the sugar free icees they have at the 7-11 north of the MGM and also at the Circle K south of the Stratosphere. I sometimes will buy a bicycle and ride it around Vegas and take in a lot of the off-strip attractions. I also find many of the small bar type casinos that are so much fun to play. My first trip to Vegas in 2000 was fun because everything I did was new to me. We were there in July and it was like 110 degrees. It rained very heavy and the place flooded bad. I see where they have installed huge tunnels under many of the streets now. One of the stupid things we did on that trip was to walk from the MGM to the Rio and back. Getting to the Rio was not so bad but we came back from behind Cesar's Palace. We entered through an employee entrance and wound up walking by the soft count room. Finally someone noticed us and directed us to the Appian Way Shops back staff entrance. We were actually looking for the Forum Shops. So we notice these shops are like extremely exclusive and most even had armed guards. We finally found the Forum Shops and just added this memory to our collection. We then walked back to the MGM and tried to recover.

      July 21, 2011 at 7:36 pm | Report abuse |
  10. Juan Banuelos

    @Ocean, did you know there are 11 commandments? If the need shall arise, You may let people fuc your mother for money. Remember to give 10 % to charity.

    July 21, 2011 at 7:21 pm | Report abuse |
    • Jeff

      Actually there are 613. The 10 were the moral law, the 603 were the governmental law.

      July 21, 2011 at 8:23 pm | Report abuse |
  11. Rod C. Venger

    It's currently 100 degrees in Bryan, Texas but the humidity today is pretty low, only 29%. Yesterday the temp was like 94 and the humidity was way up around 70%. You walk outside to check the mail and come back in soaking wet, all in 60 seconds. Today is "pleasant" even if it is too hot. Yesterday was just miserable.

    July 21, 2011 at 7:33 pm | Report abuse |
  12. Kevin Roberts

    We caused this! We depend on electricity for everything and are ruining the planet! We didn't listen and THIS HAS GOT TO STOP!

    July 21, 2011 at 8:15 pm | Report abuse |
    • Jeff

      I don't believe, for a minute, that we caused any change in climate. We are far too insignificant for that. Not saying that we shouldn't look into smarter energy sources, I think we should...it is good management of our environment.

      July 21, 2011 at 8:25 pm | Report abuse |
    • Precursor

      Then perhaps you should stop using electricity? I mean if you think that's the cause of all these problems then why are you clearly still using it?

      July 22, 2011 at 10:29 am | Report abuse |
  13. Gus

    Earth cannot sustain sin. it groans when this happens. Gay people play a big part on severe weather. If i was the earth i wouls swallow them all alive!!!!

    July 21, 2011 at 8:26 pm | Report abuse |
    • Storm

      @ Gus. Gus, I can't believe that people with your single digit IQ and knuckle dragging mentality still exist, but sadly you do. If I were the earth I'd strike your mouth off with a bolt of lighting so the rest of the planet doesn't have to live with the stupidity you spew forth.

      July 21, 2011 at 8:55 pm | Report abuse |
    • SDCyclist

      Sounds like someone likes the idea of "swallowing" some gay people a little too much... Swallowing? Really? You chose to use that word? What's really going on in that head of yours, Gus? Hmmmm... me thinks thou doth protest too much!

      July 22, 2011 at 7:24 pm | Report abuse |
  14. Gus

    It is because of sin. Period.

    July 21, 2011 at 8:28 pm | Report abuse |
  15. terry wrist

    her in Texas we get used too it after a month or so, and we really dont see a change until late September early October,besides not much we can do about it except stay indoors.

    July 21, 2011 at 8:34 pm | Report abuse |
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