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. cybrsage

    Also, just wanted to point out that we all agree the world is getting slowly warmer. Why, just a mere 10,000 years ago there was ice covering half of the northern hemisphere. Yes, a good 1/4 of the planet was under ice. The natural cycle, which has made the planet hotter than it is today several times in the past all by itself, did not vanish...mankind is not the sudden cause of something which has existed for hundreds of millenia before today.

    However, for all those who think mankind is now the problem, I beg you to stop eating or using any product derived from cows (steaks, milk, leather, etc.). Cows are far more damaging to the Earth (global warming gasses) than CO2. If you actually believe the Earth is in danger and want to control what everyone else does because of it, you need to lead by example and stop supporting the cow industry. Anything less means we should ignore you.

    July 24, 2011 at 4:12 pm | Report abuse |
    • CTYank

      For a wannabe "sage" that's pretty simplistic, and simply wrong.
      To in any way equate however many millions of cattle with coal, oil and gas burners is plain silly by orders of magnitude.
      To say that because there are natural climate cycles MEANS that human behavior cannot affect climate is logically stupid.
      Study up and get some facts straight, before next mounting yer soapbox. (Clue: ignore Rush, Hannity, etc.)

      July 24, 2011 at 7:13 pm | Report abuse |
    • RajS

      So these prior climate fluctuations you're talking about raised the temperature within a 100 years did they?
      And there were no serious consequences for ecosystems?

      Yeah, I didn't think you'd taken the time to really have a think about it.

      July 24, 2011 at 8:58 pm | Report abuse |
  2. bob

    all that heat and iam freezing my butt off in western Oregon.

    July 24, 2011 at 6:19 pm | Report abuse |
  3. CharlieSeattle

    Hot Air in Washington DC ia causing it...NOT global Warming!

    July 24, 2011 at 6:22 pm | Report abuse |
  4. bob

    El Nino/Nina is responsible for the heat. Clearly the heat has shifted further north this year causing a heat wave in large parts of the country and record rain further north.

    July 24, 2011 at 7:42 pm | Report abuse |
  5. FU

    HAARP...google it...get angry!

    July 24, 2011 at 8:57 pm | Report abuse |
  6. StephensB

    Responsible for the heat ?????? Gotta be Al Gore !!!!!!!!

    July 24, 2011 at 9:22 pm | Report abuse |
  7. Smitty77

    I've lived a long time and it is sure as hell hotter now, with less severe winters than it was 75 years ago! Each year is warmer than the previous ones. Stop listening to the Gore bashers who most likely have not spend most of their lives studying the environmnet!

    July 24, 2011 at 9:44 pm | Report abuse |
  8. Mike

    It's summer – there are always hot days in summer !

    I live in chicago area – the only thing making the weather an issue is the news & weatherman trying to get the big ratings – if you look at the record highs for chicago area, the past couple weeks don't come close to breaking the record highs – most of which were set from 1910 thru 1930's –

    July 25, 2011 at 10:39 am | Report abuse |
  9. Cattle barrier

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    April 10, 2012 at 3:43 am | Report abuse |
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