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. momtopugs

    maybe it's jut me because I'm on the west coast, but don't we get/hear the deadly heat wave stories on the East every summer? And every summer its the "it's never been the hot before" or "record breaking heat". Just curious is this is journalist reaching for ratings and people forgetting how hot summer is?

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

      When they say record-breaking, they generally mean a daily record. The hottest it's ever been on this date since we started keeping track in the 1900s was 89°, but today it was 90°, that kind of thing. They don't generally mean the entire summer was hotter, and when they do they usually mean the average day was hotter than the record average day. There have been some records like that broken lately for averages, though. 1976 was profoundly weird and had days where it was cool in Death Valley in summer and hot in the far north in record-breaking amounts.

      July 22, 2011 at 1:01 pm | Report abuse |
    • Chesapeake Bay Girl

      I'm in Virginia, on the Chesapeake Bay. It's normal to have really hot weather in July and August. But temps in the 90's are usually followed by thunderstorms and a little relief from the heat. This year has been truly bad. Today, we have a heat index of 120. That's unusual. And there have been no storms to cool us off. Rough summer this year.

      July 22, 2011 at 1:35 pm | Report abuse |
    • stacy12

      this kind of heat doesn't happen every summer 120 isn't a common site people just don't want to admit that there is sum thing going on with our climate they either to ignorant or stupid don't really know which is what the weather is getting worse every year and jus like this spring when the tornado's hit and killed over 2000 people were like everybody is scared there is nothing going on idiots wake up cuz this time next year things are gonna be allot worse look at the tropics its starting to pick up now we are seeing 5 systems in the Atlantic cant wait to see what happens next

      July 22, 2011 at 3:29 pm | Report abuse |
  2. Ana W

    What's behind it? Uh...climate change moron! Geez..I still don't understand why the media refuses to acknowledge Global Warming as the reason behind this extraordinary weather we've been having. Scientists for DECADES have been saying that the faster we pump GHG into the air the more we will destabalize the climate leading to much more severe weather then normal, ie: frequent heat waves, heavier precipitation (yes that means snow too for you morons who think snow somehow makes AGW a myth), drought, famine, heavier storms. I guess CNN needs to post headlines like this for a dumbed down public. Yes, climate change is natural- but yes we are also accelerating that natural cycle.

    July 22, 2011 at 12:21 pm | Report abuse |
    • DaveNYUSA

      The media DOESN'T? What friggon rock have you been living under?

      July 22, 2011 at 12:46 pm | Report abuse |
    • Ana W

      Dave, do you see any indication in this article that ackowledges climate change? neither did I.

      July 22, 2011 at 12:56 pm | Report abuse |
    • Al Gore

      It's summer idiot. Those of you kool-aid drinkers still believing in global warming should really be excused from normal society, you bring down the collective IQ. Is there a weather pattern or occurence that you nimrods haven't tried to claim for global warming? Here's a hint, in a few months we're going to go through a little period of global cooling. The temperatures will plummet, and we might even seem some snow. It's called winter, and it's coming. And there's nothing we can do about it, not even with your expensive and ugly new lightbulbs. Is the climate changing? Perhaps. Did mankind do anything to cause it? Absolutely not. Can we do anything to stop it? Nope. Should we try? Nope. What about the Maldives? Don't care. Tell 'em to move, or swim.

      July 22, 2011 at 1:42 pm | Report abuse |
    • Joe

      Anna, YOU are the moron. I have to laugh every time someone starts yelling that environmentalist crap because it's HOT in JULY. No, REALLY? Chicago had one of its deadliest heat waves ever in 1995. I guess since we haven't had such a big heat wave since then, the climate change has gotten better, huh? It's nature doing its thing, Ana. We've had excessive heat and humidity before, as have generations before us. Global warming isn't behind this. Wake up.

      July 22, 2011 at 1:43 pm | Report abuse |
    • AlfonsoGore

      Single meteorological events do not prove nor disprove anything. Many places had record cold and snowfall; does that disprove global warming? No, just as record heat does not prove it.

      Fact is, global warming – as defined by increase in global average temperatures – is real, so that's not even the debate... The cause is what is being debated among scientists and many pseudo-scientists (every donkey and elephant in the room who suddenly understand climatology and paleoclimatology) alike.

      Anyone who tries to say that GW is without doubt anthropological or non-anthropological in nature is a only fooling themselves. It's called a theory for a reason; neither side has produced enough evidence to support their claim(s).

      July 22, 2011 at 1:45 pm | Report abuse |
    • Guest

      When there are cold snaps and people try to cite that as evidence that the climate is not warming, they're shouted down as idiots for confusing weather and climate. Why is it suddenly evidence of global warming if it's a heat-related weather phenomenon instead of a cold-related one?

      July 22, 2011 at 1:48 pm | Report abuse |
    • usmc1

      There is no solid evidence of global warming, only junk science. Its called summer.

      July 22, 2011 at 2:46 pm | Report abuse |
    • Bubba

      Pretty obviously humans have been burning wood and stuff for fuel since forever, but only in the last few centuries have we gotten so numerous that we've started tearing down forests faster than they could grow. You don't have to be very smart to see what happens if you light a fire inside a closed room, and we're putting a lot of smoke and stuff into a closed system worldwide. It would be a miracle if it wasn't making changes, and in fact even the climate-change deniers acknowledge that things are changing. The argument is still about what's happening, what can we do, and are we headed for an ice age or a meltdown? Here's the thing: if we can head off the melt down by tipping back towards the ice age, and then avert the ice age by tipping the hot way again, we can go on forever. We have to get the facts and work on controlling our emissions.

      July 22, 2011 at 4:28 pm | Report abuse |
    • Ana W

      Keep your heads in the sand..it's much easier to believe this is all natural and you are all smarter then 98% of the scientific community. Nobody is denying climate change, it's not debatable anymore. AGW is also not debatable as nearly all of science has warned us for decades that the increase in carbon pouring into the atmosphere will without a doubt be the cause for more severe weather in the coming years, which we are now seeing come to pass. This is NOT debatable any longer and serves no interest to anyone to pretend it does not exist. The only thing it does is help you sleep a little better at night. Keep moving along, everything is normal, this is a normal cycle..nothing to see here folks..1995 also had a heat wave...it snows in the winter...keep moving along...

      July 22, 2011 at 5:42 pm | Report abuse |
  3. BlameTheLatinos

    The heat waved was caused by old man John McCain's hot air

    July 22, 2011 at 12:32 pm | Report abuse |
  4. DaveNYUSA

    "What's behind deadly summer heat?"
    Uhm, SUMMER, maybe?

    July 22, 2011 at 12:45 pm | Report abuse |
    • Jack Wagon

      You are an idiot

      July 22, 2011 at 1:08 pm | Report abuse |
  5. redeemed

    Read the Global warming deception,by Grant R. Jeffrey. Its really quite an eye opener.

    July 22, 2011 at 12:46 pm | Report abuse |
  6. Bubba

    Come on, we all know this is being caused by Daylight Savings Time. Those scientists should never have added the extra hour of daylight; it ain't natural and it's making the world hotter and hotter.

    July 22, 2011 at 12:54 pm | Report abuse |
    • redeemed

      @ bubba,LMAO

      July 22, 2011 at 12:55 pm | Report abuse |
    • brew987

      That's funny.

      July 22, 2011 at 2:16 pm | Report abuse |
  7. krehator

    The Sun.....duh.

    July 22, 2011 at 1:12 pm | Report abuse |
  8. herpderples


    July 22, 2011 at 1:46 pm | Report abuse |
  9. HistoricalPatterns

    To say extremes in weather isn't natural is a stretch. There are multiple records of when the weather was abnormal for no obvious reason. There was a period in south america that brough the moche (i think) civilization to a halt because it was a freakish flood that was consistant for decades followed up by drought. They found evidence of it in the snow/ice samples taken from the mountains. Wonder how much GHG the pre-columbian society was pouring into the air to cause that.... 😉

    July 22, 2011 at 2:10 pm | Report abuse |
  10. tommy

    well crap here in a small town in texas we have had almost fifty days of 100+. We also have not had any significant rain since September last year. Between the towns going dry and the fires it is getting a little rough here. Also, if any of you think prices for beef are high wait tell next year. I am seeing large herds die or get sold all over texas. Partner up with someone and buy a half of a beef....All and all I guess we can be thankful that we do not have the high dewpoints. Hell we would be happy if we could just get fog, dew, or a small shower. Anything!!!!!

    July 22, 2011 at 2:23 pm | Report abuse |
    • guest

      That stuff is occuring because you live in Texas. Its a giant arid state. Maybe not the far eastern sections but your basically close to the only area ever to be dubbed "The Dust Bowl". Heck your a stones throw away from North Americas only deserts. No offense to you or any of the cattle hearding sorts of Texas, but your state isn't known for growing lush pastures. I'd say it'd be best to do cattle growing somewhere a little more dependable on rain. The south and southwestern portions of our country go through droughts quiete often.
      I hope our Texan brothers get some rain soon though... either way it sucks and would be nice to get you guys a little water down that way.

      July 22, 2011 at 3:39 pm | Report abuse |
  11. same

    Person 1 says something
    – Reply: You ARE AN IDIOT
    Person 2 says something
    – Reply: You ARE AN IDIOT
    Person 3 says something
    – Reply: You ARE AN IDIOT
    Person 4 says something
    – Reply: You ARE AN IDIOT
    Person 5 says something
    – Reply: You ARE AN IDIOT
    Person 6 says something
    – Reply: You ARE AN IDIOT

    That's how most CNN comments go... the thing is that we are not talking to each other in person, just online, everyone has the gut to talk down at each other... when we DO see each other, we put on a mask and act all civil and nice and understand, and not rude. Just an observation, Happy Summer!

    (Waiting for someone to call me Idiot and enjoy it at the same time)
    – Reply: You ARE AN IDIOT

    July 22, 2011 at 2:51 pm | Report abuse |
    • Alistair Cooke

      You are a cultured, discerning individual of exquisite taste and intelligence.

      It is my pleasure to make your aquaintence. We must get together some time to discuss the theatre.

      Please have your manservant contact mine. In the interim, please accept this amusing Chablis as a token of my esteem.

      July 22, 2011 at 3:10 pm | Report abuse |
    • Alistair Cooke

      Oh, blast; I've misspelled "acquaintance".

      I am ever so sorry, dear chap.

      I am mortified, and shall slit my wrists and move to Borneo forthwith.

      July 22, 2011 at 3:17 pm | Report abuse |

      You are an idoit

      July 22, 2011 at 4:29 pm | Report abuse |
  12. Alfred E, Neuman, c. 1955

    "It's not the heat, it's the humility."

    July 22, 2011 at 3:04 pm | Report abuse |
  13. The all potent one

    The impact of emissions can't be determined because we only have one earth so no control sample. Earth goes through temperature changes but comparing where we are now to where we would be if no fossile fuels is not possible as there is no way to know. This will allow the global warming argument to go on forever.

    July 22, 2011 at 3:05 pm | Report abuse |
  14. Lance

    It's summer. It's hot. It's humid. This isn't typical?

    July 22, 2011 at 3:18 pm | Report abuse |
  15. Proof

    Since the industrial revolution began over 200 years ago, water has been evaporating more slowly as pollution levels rise, changing weather patterns in ways never before seen by man. Pollution isn't the cause of global warming. The earth is naturally getting warmer. Our pollution only serves to make what would have been a nice, natural transition into one filled with violent extremes in weather. Now you know why governments are in no big hurry to reduce allowable levels of pollution. It won't stop "global warming" but it will cut-into corporate profits. Hence the lack of urgency.

    July 22, 2011 at 3:20 pm | Report abuse |
    • Ana W

      Lance, nobody is saying that heat and humidity in the summer is not typical...nor is anyone saying one heatwave is atypical- what is being said is that it has been predicted for decades that pumping excessive amounts of carbon into the air will speed up the natural cycle of climate change at an unprecedented level, which is what we are seeing now- that is not debatable. Something that typically takes hundreds or thousands of years to do is now taking place in the span of decades. a single weather event cannot be tied to this change, but the overall trend we are seeing now is just about where things were expected to go with the continuation of GHG into the atmosphere. and yes, this is an issue being ignored mainly bc. solving the crisis will cut into corp profits.

      July 22, 2011 at 6:16 pm | Report abuse |
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