Greenhouse Effect Of Water Vapor

The graph below shows how the greenhouse effect varies with humidity during the mid-latitude summer. As you can see, there is a huge increase in downwelling longwave radiation going from no water vapor to 1% water vapor (10,000 PPM.) A little bit of humidity goes a long way towards keeping the surface warm.

One of the reasons why Antarctica is so cold is that there is very little water vapor in the interior of the continent, so little heat reaches the surface from the atmosphere. Likewise, Mars is very cold despite a large amount of CO2 in its atmosphere, due to the low atmospheric pressure and lack of water vapor.

ScreenHunter_4716 Nov. 18 07.17

Also note that you can’t actually go above 40,000 PPM because the air becomes saturated.


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26 Responses to Greenhouse Effect Of Water Vapor

  1. Baa Humbug says:

    At 0 ppm there is still 120 Wm2 DLR. Is that the ‘other’ ghgs?

  2. jmrSudbury says:

    On your thread, Anything is possible on November 17, 2014 at 8:06 pm gave the following link:

    At 8:49 EST today, my local weather station had -11.7C, 99.6kPa, and RH 86%. The humidity calculator showed it was 1939 ppmv

    From where did you get your graph? I would like to see it only up to 40,000.

    Hei hei

    John M Reynolds

  3. Steve Case says:

    I’d like to see a good discussion of greenhouse gas that covers Carbon dioxide, Water Vapor, Methane and Ozone. How do they compare to each other and why? How much is in the atmosphere? What’s their climate sensitivity? What’s their share of the Greenhouse Effect? And what’s the man-made component?

    • BobW in NC says:

      I’m with you, Steve Case. Anthropogenic CO2 comprises only about 4% of all CO2 emitted each year. But as to the rest, been looking for such a comparative study for a long time. CO2 is not evenly distributed in the atmosphere, nor is water vapor as Steve Goddard and others have shown. But water vapor is at least 25X the concentration of CO2, and I have heard, molecule for molecule, is energized by a wider spectrum of LIR. But I would love to see some definitive, quantitative data on all of this.

      Thanks to you, Steve Goddard, for keeping us in the loop.

      • Steve Case says:

        Wikipedia tells us that the percentage contribution to the greenhouse effect on Earth by the four major gases are:
        water vapor, 36–70%
        carbon dioxide, 9–26%
        methane, 4–9%
        ozone, 3–7%

        I’d like to know how they came up with all that.

  4. Steve Case says:

    Here’s a Dewpoint calculator

  5. richard says:

    there is talk of a blanket around a body causes the body to warm. Are we allowed to change the moisture content of the blanket or do we only take a dry one into consideration.

    • Gail Combs says:


      If it is a wool blanket, it will retain 80% of its insulating capacity when wet. If it is a down comforter it is a cold soggy mess. There is also polar fleece but it melts if sparks from a fire hit it. It doesn’t hold much water compared to wool, and will still insulate when wet.

  6. mkelly says:

    The above link is to FLIR video of ice bergs (small ones) in water from a ship. If ice can give off enough heat to be seen in water at night why can’t we see the down welling IR?

    • Gail Combs says:

      I think you are seeing it. If you look at the horizon in the video, the sky is white.

    • terrahertz says:

      I wonder what proportion of the IR from the ice, is emission from the ice, vs reflected IR from the sky?
      It’s also possible the ice (being pure water) is warmer than the surrounding salty water, which freezes around minus 2 deg C.

      • mkelly says:

        I operated FLIR in the S3A Viking aircraft and it had to modes of representation on the screen white hot or black hot. Since the people walking around are black the ice is seen as cold against black hot representation.

  7. terrahertz says:

    The *other* elephant in the room, is that atmospheric water vapor and cloud cover can and does vary synchronously with the day-night cycle. Since solar input occurs 100% during daylight (duh!) the system is a parametric amplifier. Think of a heat shutter, opening and closing in the sky. If it acts entirely randomly, there will be net zero heat flow between the surface and space. If it opens always during the day, closes always at night, things would heat up very very fast. In the reverse, Earth would freeze over very fast. It’s the *difference* from day/night randomness in cloud cover that exerts a major control over surface temperature.
    And of course, since cloud cover has a very complex interrelationship with surface temp, the whole feedback system is complicated.

    I’ve always been amazed that Warmist atmosphere models don’t seem to include the cloud cover day-night correlation, despite that it’s obviously the major factor. Obvious to anyone who’s noticed the radical effect night time cloud cover has on rate of temperature drop overnight.

    • Gail Combs says:

      You also have the condensation of dew over night. At times that is the only water my pastures see but it is pretty reliable unless we geed really dry air.

  8. The problem with such a plot of down-welling radiation is that it is made with a meter which has a cold reference point. That reference point may be at -50C or -70C. Consequently, the meter perturbs the electromagnetic field and it appears that energy is flowing from a colder spot at higher altitude to a warmer spot commonly near the surface. This is not the case. The energy flow is actually from a layer in the atmosphere which is still warmer than the meter’s reference at -50C or -70C. One needs such a reference point for the accurate measurement of temperatures, but it is a detriment to measuring energy flow.

    There are two reasons at least for the increase in measured energy flow to the reference point as the water vapor concentration increases. First, the greater water content causes an increase in the absorption of incoming solar radiation in the atmosphere, so the atmosphere above the surface is warmer relative to the surface than it usually is. Second, because of the short mean free path for absorption of radiated IR from a water vapor molecule, as the humidity rises, the IR reaching the meter is from a lower and lower altitude. The lower altitude is warmer.

    It should be remembered that whenever a water vapor molecule absorbs energy from incoming solar radiation, half or slightly more of the energy is transported toward space. As such, it is a cooling mechanism with respect to the surface temperature. It kept half of the solar radiation that might have warmed the surface from ever arriving at the surface. Much of that energy winds up quickly in the form of rising convection currents. Then of the portion emitted toward the surface as IR, some is absorbed by another lower altitude water molecule and half of that is emitted away from the surface. Some gets re-routed downward from a higher altitude, but there is no way to gain energy at the surface due to water molecules intercepting solar radiation. This is always a loss mechanism, which is consistently underrated.

    That part of the longwave IR emitted by the surface into the atmospheric window escapes the atmosphere and is a powerful cooling mechanism. That part which is absorbed by IR-active gases, such as water vapor or CO2, has such a short mean free path that most of the energy (about 80%) is thermalized with the surrounding atmosphere a mere 1 meter to 50 meters above the surface. Since the usual temperature differential between the surface and 1 meter or 50 meters is small, the absorbed energy is therefor small. Of the remaining 20% of the IR-active gas molecule absorbed from the surface, half is emitted to space and half to toward the surface. Since that re-emission is prior to energy losses to a collision with a surrounding gas molecule, the emitting molecule is actually at the temperature of the surface. That re-emitted energy can be absorbed by the surface. But, this back-radiated energy is very small compared to the hugely exaggerated energies of Kiehl-Trenberth Earth Energy Budget diagrams.

    On the other hand, the absorption of IR emitted from the surface closer and closer to the surface as the humidity is increased does warm the air near the surface due to the 80% of the radiated energy being thermalized or equilibrated with nearby air molecules. This decreases the temperature difference between the surface and that layer of air, which decreases the IR energy flow between from the surface to that layer of now warmer air. This effect is strong enough that it serves to slow down night time cooling noticeably when the humidity is high. But during the day, it loses out to the increased absorption of incoming solar radiation, so the daytime net effect of higher humidity is a cooling effect.

  9. higley7 says:

    One HUGE reason the Arctic is cold is that it receives little in the way of energy input, seeing only sunlight at a low angle for a couple of months of the year. No amount of water vapor in the atmosphere can warm a system with no energy input. Duh!

  10. Timothy Dunlap says:

    Water vapor has mass and latent heat- which it got when it evaporated from the surface. Thermal mass doesn’t mean downwelling radiation.

  11. Timothy Dunlap says:

    P.S. It’s cold outside. I’ll close one of my windows to make it warmer in here. They say it works in the atmosphere.

  12. nickreality65 says:

    Cold air can not/does not absorb/hold as much water vapor as warm air.

  13. nickreality65 says:

    Part One: Heating the earth

    A popular global heat balance shows 340 W/m2 incoming radiative flux at the top of atmosphere. A watt is a power unit, energy over time, equaling 3.41 Btu of energy/heat/work per hour. Over a 24 hour period the earth’s ToA semi-spherical surface would collect 7.13E18 Btu of energy.

    Dry air is mostly nitrogen and oxygen with a heat capacity of about 0.24 Btu/lb-F. For dry air to absorb 7.13E18 Btu would require a temperature increase of about 2.63 F. Over 24 hours.

    Water vapor evaporates/absorbs, condenses/releases, energy/heat at about 1,000 Btu/lb. For atmospheric water vapor to absorb 7.13E18 Btu through evaporation would require an amount equal to 25.5% of the current atmospheric water vapor content, i.e. more clouds, more albedo, more reflection, a self-correcting thermostat. That’s the entire ToA!

    Part Two: IPCC RCPs

    IPCC AR5 states that between the years 1750 and 2011 man generated GHGs increased the RF by less than 3 W/m2. (Is that the downwelling?) Contrast that figure with the ToA.

    IPCC bases its various computer model predictions on four cases:
    Case………….…CO2 ………….……Radiative……Dry air, ΔF………..Increase in atmospheric
    ………………….Concentration……..Forcing………………………………water vapor content
    RCP 2.6…………421 ppm CO2……..3.0 W/m2………0.02……………..……….0.2%
    RCP 4.5…………538 ppm CO2……..4.5 W/m2………0.03………………………0.3%
    RCP 6.0…………670 ppm CO2……..6.0 W/m2………0.05………………………0.4%
    RCP 8.5…………936 ppm CO2……..8.5 W/m2………0.07………………………0.6%

    It’s the water vapor thermostat that controls the greenhouse, not CO2. It’s the water vapor thermostat that controls the simplistic blanket analogy as well. The hiatus heat went into a few more clouds, not the ocean.

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