Downwelling Infra-Red Radiation and Temperature: Part 1

Way back in July last year I posted about the long term decrease in downwelling IR at Cape Grim and Alice Springs, despite rising CO2.

From the Bureau’s solar radiation glossary,
“Downward infrared irradiance is a measurement of the irradiance arriving on a horizontal plane at the Earth’s surface, for wavelengths in the range 4 – 100 μm (the wavelength emitted by atmospheric gases and aerosols). It is related to a `representative (or effective radiative) temperature’ of the Earth’s atmosphere by the Stefan-Boltzmann Law:
E = σ T4
Where: E = irradiance measured [W/m2]
σ = Stefan-Boltzmann constant [5.67 x 10-8 W/m2/K4
T = representative atmospheric temperature [K]
Consequently, this quantity will continue to have a positive value, even at night time. It can be measured using an Eppley PIR pyrgeometer.”

As atmospheric temperature increases, DWIR must also increase. This would be a symptom of warming.
A reader commented: ”What we need is DWIR nighttime measurements only (preferably without clouds) in a location where there is little or no water vapour. Atacama Chile would be perfect. Alice Springs maybe but less so. i am willing to bet that one couldn’t measure the DWIR at night without clouds in Atacama because it would be so low.”
I am unable to get data for Atacama, but here is DWIR data for Alice Springs for July 2018. July is mid-winter and usually dry and cloud free. No rain fell in July 2018 at the Alice.
Figure 1 shows maxima and minima for the month:
While July had no rain, there were several large weather changes shown by the spikes and dips in temperature. Coldest temperatures were on 12-13-14 July.
Fig.1: Surface temperatures Alice Springs July 2018

Next, downwelling IR. The weather changes show up in IR as well.
Fig.2: Downwelling IR Alice Springs July 2018

Now for IR in the hours of darkness:
Fig.3: Downwelling IR Alice Springs July 2018 at night (6pm to 6am)

Clearly, DWIR is real and measurable at night, in all conditions. It usually (but not always) decreases in a smooth curve. Putting it together, we see a clear daily cycle: DWIR usually increases rapidly in daytime, and decreases at night.
Fig.4: Downwelling IR Alice Springs July 2018 by day and night

Now we look at typical IR behaviour in cool, dry conditions on 12 and 13 July 2018. The x-axis is in 3 hourly divisions and I have marked in midnight of 12-13.
Fig.5: Downwelling IR Alice Springs 12-13 July 2018

Note the curve is not completely smooth: there are little variations due to pockets of different temperatures in the air. The lowest DWIR values (227.36 Watts/sq.metre averaged over one minute) are reached around 8.00 a.m. shortly after sunrise, then values rise rapidly before tapering off to peak in the late afternoon. During the night they decrease until the sun heats the ground again in the morning.
Now for the period 5 to 8 July:
Fig.6: Downwelling IR Alice Springs 5-8 July 2018

On the 6th and 8th strange things happen after midnight, almost certainly clouds.
Strange things also happen from 23 to 25 July. On the 24th a heavy bank of cloud comes over and clears with a sudden dry change after sundown, with more separated clouds arriving later at night before finally clearing about 9 a.m. next morning.
Fig.7: Downwelling IR Alice Springs 23 – 25 July 2018

How do I know those spikes were caused by clouds? Here’s direct radiation and IR for 23-25 July.
Fig.8: Downwelling IR and Direct Irradiance Alice Springs 23 – 25 July 2018

Direct irradiance is the radiation from the sun’s direct beam. It is zero at night but rises rapidly to peak at local solar noon, then rapidly falls to zero at dusk. Not all solar radiation reaches the surface. Some is reflected, some is scattered by dust, smoke, or rain drops, but on a clear day the pattern is like 23 July. On 24 July clouds block the sun’s direct rays for most of the day, and downwelling IR increases markedly. This is from warm moist air in the cloud which has come from somewhere else.
My conclusion:
Downwelling infra-red radiation (so called “back radiation”) is real and measurable including at night.
It is greatly increased by cloud and humidity, and there is always some moisture in the air even in the desert.
It results from the ground heating up in the daytime, which then loses heat by conduction, convection, and radiation, into the atmosphere where the IR is repeatedly absorbed and re-emitted in all directions by greenhouse gases (including water vapour).
A warmer atmosphere from whatever cause, natural or enhanced, will result in greater downwelling IR.

Future posts will look at the relationship between solar radiation, downwelling IR, and temperature.

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8 Responses to “Downwelling Infra-Red Radiation and Temperature: Part 1”

  1. Bill In Oz Says:

    Ken, In what way is this significant to the long term temperature record ?

    • kenskingdom Says:

      Not immediately, but you will see from the previous post last year there has been no increase in IR for 27 years. A couple more posts coming.
      I should point out that some of my posts are just about interesting stuff.

  2. Bill In Oz Says:

    The fact that there has been no increase in IR for 27 years is interesting as it undermines the whole idea that CO2 is causing the earth to warm…

    And yes, I get that some posts are just about interesting stuff..Just a surprise Ken..

    • John in Oz Says:

      We know that the Earth has warmed since the Little Ice Age and this has continued over the past 27 years (albeit a small amount).

      The questions is -‘if not CO2 as claimed by catastrophists, what is causing the rise?’

      More monies to study the real cause might be of benefit

      Bill – we have still not connected up in Mt Barker

  3. Bill In Oz Says:

    BTW Ken I do not have the expertise to make an informed comment on this IR issue.. My general thinking is that global climate is too huge a subject to establish any clear patterns of causation.. Eg We know that the ENSO & IOD have a huge impact on weather & thus climate..But I have NOT seen anywhere a coherent explanation of why these phenomena happen ?

  4. WXcycles Says:

    A timely discussion Ken, thanks for your focus on this.

  5. Terence McDonald Says:

    What atmospheric emission % did you use for your RAT calcs?

  6. kenskingdom Says:

    No calculations at all: data shown are as downloaded from the bureau’s website at
    so atmospheric emission does not come into it.

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