Why Does Humidity Keep Daytime Temperatures Down In The Summer?

Humid climates often have have lower summer maximum temperatures than dry climates.

This is because of evaporative cooling from plants, bodies of water and soils, vegetation shading the ground, rainfall, and haze in the atmosphere which blocks incoming sunlight. Phoenix normally has its hottest afternoons in June, before the monsoon moisture comes in July.

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33 Responses to Why Does Humidity Keep Daytime Temperatures Down In The Summer?

  1. Alan Poirier says:

    Fortunately, H2O exists in all three states on earth, moderating our climate.

  2. Vegetation does more than shade the ground and transpiration. Photosynthesis absorbs tremendous amounts of sunlight and turns it into sugar, which is an endothermic reaction in the same way that burning sugar is exothermic.

    • Gail Combs says:

      In the summer on weekends I routinely stay on the edge of a pine forest at a flea market and visit the guys across the driveway with a EZ-up tent. Both areas are shaded but the forest is noticeably cooler.

      • We should learn how to tap the solar energy that plants absorb and get them to use photosynthesis to produce an energy source that humans can use. Oh wait….firewood!

      • Tel says:

        We have a large frangipani growing on the North side of the house (side facing toward the equator). Highly significant local cooling effect.

        I admit I don’t know how much of that is energy going into photosynthesis and how much is energy going into transpiration, but you need water available for both processes. Very interesting molecule is water, does a lot of things all at once.

        Grass also has a cooling effect, as compared with dirt, rock, car park, concrete, etc. This makes me wonder, does the amount of water available at the grass roots change the cooling effect (dry brown grass vs lush green grass)? Should we go around watering the lawns under all those Stevens Screen measuring stations and freak out the climatologists?

    • Anthony S says:

      Photosynthesis is only about 5% efficient at turning sunlight into sugars. Like CO2, chlorophyll has a very narrow absorption spectra. This is afterall, why leaves are green, because chlorophyll can’t absorb green light.

  3. rah says:

    I know what your saying is correct but that humidity didn’t help much when it was 80 deg. with 80 % even before the sun came up at Ft. Sam Houston and your doing heavy PT. Took some getting used to for an ole winter warrior but by the second summer at 40 y/o I was putting the hurt on those 20 y/o SF wannabes. Amazing how one’s body can adapt if you just get out there and do it.

  4. Chuck says:

    I wish I had known this when I was stationed at Hurlburt Field, FL in July! :/ I do know what you are saying though. I always enjoy the monsoons when they start to break that annoying June heat.

    • rah says:

      Hurlburt Field! SOW?

      • Chuck says:

        Yes Sir.

        • rah says:

          Trained there a couple of times jumping MC-130s and learning to call for fire from AC-130s out on the ranges. And of course running around out in the boonies crossing the Yellow River while the AC-130s hunted us and dealing with the snakes and bears in that swamp land. Last time was summer of 1984. Talked to the Major that I believe was the command pilot of the mission that took the Rangers into the strip at Grenada the year before. His account of the on the spot decision to drop the Rangers instead of air landing them was pretty interesting.

  5. KevinK says:

    Tony wrote; (11/18/2014);
    “A little bit of humidity goes a long way towards keeping the surface warm.”

    Now (11/19/2014) he writes; ”
    “Humid climates often have lower summer maximum temperatures than dry climates.”

    Huh ????

    A warmer surface == higher temperatures == lower maximum temperatures == UNICORNS ???

    Tony, I think you should just go to Mexico for a few months and drink some Coronas, maybe your position will be clear (less cloudy) after that ?

    Maybe you should just revert to designing/testing microprocessors, it’s a good living after all.

    Leave the thermal/optical stuff to folks that have a clue ?

  6. Wholesale cutting of forests around the world is not good for climate stability. Here in BC vast land areas have been cleared of forests. Warms salmon bearing waterways and reduces transpiration. Not to mention the loss of chemical heat storage in new wood growth.

  7. I’ve always thought Earth’s biosphere buffers changing confitions.

  8. Mark Luhman says:

    Humid air hold more heat, it take more energy to heat humid air. I love Arizona in June, that the nicest time of the year down here, yes the day are hot the evenings are lovely. Our highest temperatures are around July 4, right about time the monsoon come in. July and August can be miserable but I have been in the Florida panhandle in July and August, I will take Arizona heat any day of the week over that, is a huge inconvenience having to run you car defroster in July. Some cars will not let you air condition and heat at the same time and that the only way I know to truly dry out the air and keep it comfortable. Here in AZ to the most part we do not have that problem. Yes dew point will creep into the mid seventies but not to often the eighties. Oh by the way in June it the dew point is in the twenties some time the teens or single digits.

  9. higley7 says:

    So, water vapor does NOT keep daytime temperatures down. It’s the evaporative cooling. Good. However, once the sun has set, the water vapor and CO2 serve as radiative gases to convert atmospheric heat to IR radiation which can then be lost to space.

  10. The short sightedness of the large scale burning of forests for electrical generation is astounding! What government agency could possibly think this was reasonable for our planet?

    • richard says:

      not a wool blanket but i wondered what wetting a material type blanket would result in.

      “Wet Bulb Temperature
      A wet bulb temperature reading is obtained by measuring the degree of cooling effect, if any, on a
      thermometer wrapped in a wet cloth. Whenever the relative humidity is below 100% a certain amount of
      evaporative cooling will occur on a wet bulb producing a lower temperature than the dry bulb reading. At
      100% relative humidity, the wet and dry bulb temperatures are equal, because no further evaporation is
      possible. The lower the relative humidity, the greater the temperature drop on the wet bulb. So by knowing
      both the wet and dry bulb temperatures, it is possible to determine the relative humidity of an air-vapour
      mixture

  11. richard says:

    off topic but I never knew this, I though it was just convenience.

    http://www.greenbuildingadvisor.com/stack-effect-when-buildings-act-chimneys

    WHY REVOLVING DOORS WERE INVENTED
    If the stack effect is a big deal in two-story houses, imagine what kind of pressure it causes in high-rise buildings. This pressure is so significant in fact that “when skyscrapers were first developed at the turn of the century, people also had to invent revolving doors because you couldn’t open the front door due to the stack effect pressure,” says Straube. “The cold air was rushing in with so much pressure that it was difficult to push the exit doors open.”

  12. Paul Clark says:

    Increased humidity may appear to bring about increased evaporation, but that’s only because humid climates tend also to be climates with lots of rain & therefore clouds.

    True: sustained rain will grow more plants with cooling effect of shade, evapo-transpiration, lakes etc; basically UHI in reverse; but the increased humidity itself actually retards not assists evaporation.

    Arizona gets cooler in July because with increased humidity comes increased clouds, the latter doing the cooling. Look at Arizona clear days in June Vs July here, there’s a marked difference (scroll down):
    http://www.wrcc.dri.edu/htmlfiles/westcomp.clr.html

    Now if it was true that increased humidity always causes more evaporation, cancelling humidity’s warming effect, then the two are necessarily coupled, therefore increased humidity causes increased cooling, not planet-wide ~33C warming.

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