<p>The yearly average Top Of Atmosphere (TOA) Outgoing Longwave Radiation (OLR) is around 145–345 W/m², as measured by the AIRS instrument:</p>

<p><img src="https://upload.wikimedia.org/wikipedia/commons/thumb/e/e0/AIRS_OLR.png/1024px-AIRS_OLR.png" alt="OLR" title="Outgoing longwave radiation"><br>
<sup><em>Source: <a href="https://commons.wikimedia.org/w/index.php?title=File:AIRS_OLR.png&amp;oldid=213002906" rel="nofollow noreferrer">Wikimedia Commons contributors, "File:AIRS OLR.png," Wikimedia Commons, the free media repository</a></em>. </sup></p>

<p>Now, this is an average that includes both day and night.  How much does it vary?  This depends on the location.  Based on GERB data, a 2007 paper estimates (please pay attention to the thick solid line only):</p>

<p><a href="https://i.sstatic.net/sZgjx.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/sZgjx.png" alt="OLR diurnal cycle"></a><br>
<em>Source: Comer, Slingo, and Allan (2007)</em></p>

<p>Even in places with a very large diurnal temperature cycle such as the Sahara, the difference between maximum and minimum is only about 10%.  In oceanic areas it is closer to 2%.  That means that if you take the mean (shown in the top figure) and subtract around 1–5% depending on the region (the drier, the more difference) you get a reasonable estimate of the nighttime outgoing longwave radiation.</p>

<p>If you want a more precise estimate, you can always download satellite data and calculate it yourself.</p>

<hr>

<p>Full source for second figure: Comer, Slingo, and Allan (2007), Observations of the diurnal cycle of outgoing longwave radiation from the Geostationary Earth Radiation Budget instrument, Geophysical Research Letters, Vol. 34, L02823, doi:10.1029/2006GL028229</p>

CC BY-SA 4.0
2020-02-06T08:55:21.797Z
2020-02-06T08:55:21.797Z
gerrit
<p><a href="https://en.wikipedia.org/wiki/CHEOPS" rel="nofollow noreferrer">CHEOPS</a> (also <a href="https://www.cosmos.esa.int/web/cheops/cheops-launch-and-operations" rel="nofollow noreferrer">1</a>, <a href="https://youtu.be/EAIUJaaVJ9U" rel="nofollow noreferrer">2</a>, 3) is a space telescope recently put in low Earth orbit to look for exoplanets around other stars. It uses a cooled detector array and so uses a heat shield to avoid thermal radiation from the Sun as well as the Earth directly below. </p>

<p>Thermal radiation from the Earth is a significant problem for spacecraft thermal management in low Earth orbit, so it's important to know how much thermal radiation Earth emits into space.</p>

<p>If we use <a href="https://www.jpl.nasa.gov/spaceimages/details.php?id=PIA18833" rel="nofollow noreferrer">an average emissivity <span class="math-container">$\epsilon$</span> of 0.8</a> and assume a temperature of 0 °C, then using</p>

<p><span class="math-container">$$P= \sigma \epsilon T^4$$</span></p>

<p>where the <a href="https://en.wikipedia.org/wiki/Stefan%E2%80%93Boltzmann_constant" rel="nofollow noreferrer">Stefan-Boltzmann constant</a> <span class="math-container">$\sigma$</span> is 5.67 <span class="math-container">$\times 10^{-8}$</span> W m<sup>-2</sup> K<sup>-4</sup> we can estimate it to be about 250 W/m<sup>2</sup>.</p>

<p>But what is it really, and how does it vary (on average) by the type of terrain under a satellite and time of day? </p>

<hr>

<p>From <a href="https://space.stackexchange.com/a/40557/12102">this answer</a> to <em>For a given 48 hour observing period, what fraction of the celestial sphere is available to CHEOPS?</em> (originally from <a href="https://sci.esa.int/documents/34375/36249/1567259940843-CHEOPS_EST_SCI_RP_001_RedBook_i1.0.pdf" rel="nofollow noreferrer">the CHEOPS Red Book ESA/SRE(2013)7
November 2013</a></p>

<p><a href="https://i.sstatic.net/Z5bQI.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/Z5bQI.png" alt="enter image description here"></a></p>

CC BY-SA 4.0
2020-02-06T10:01:03.550Z
When I think of "local variations of Earth's thermal radiation", I always think of this map: https://en.wikipedia.org/wiki/Earth%27s_internal_heat_budget#/media/File:Earth_heat_flow.jpg Then I realise Earth's internal heat is nothing compared to the solar contribution (we're talking mW/m$^2$ here)... But as a solid Earth scientist I couldn't help myself to post it anyway! :)
Jean-Marie Prival
<atmospheric-radiation><radiative-transfer>
How does Earth's thermal radiation into space vary by location and time of day?
uhoh
# How does Earth's thermal radiation into space vary by location and time of day?

(Asked by: username_0 on 2020-02-06)

[CHEOPS](https://en.wikipedia.org/wiki/CHEOPS) (also [1](https://www.cosmos.esa.int/web/cheops/cheops-launch-and-operations), [2](https://youtu.be/EAIUJaaVJ9U), 3) is a space telescope recently put in low Earth orbit to look for exoplanets around other stars. It uses a cooled detector array and so uses a heat shield to avoid thermal radiation from the Sun as well as the Earth directly below.

Thermal radiation from the Earth is a significant problem for spacecraft thermal management in low Earth orbit, so it's important to know how much thermal radiation Earth emits into space.

If we use [an average emissivity $\epsilon$ of 0.8](https://www.jpl.nasa.gov/spaceimages/details.php?id=PIA18833) and assume a temperature of 0 °C, then using

$$P= \sigma \epsilon T^4$$

where the [Stefan-Boltzmann constant](https://en.wikipedia.org/wiki/Stefan%E2%80%93Boltzmann_constant) $\sigma$ is 5.67 $\times 10^{-8}$ W m-2 K-4 we can estimate it to be about 250 W/m2.

But what is it really, and how does it vary (on average) by the type of terrain under a satellite and time of day?

---

From [this answer](https://space.stackexchange.com/a/40557/12102) to *For a given 48 hour observing period, what fraction of the celestial sphere is available to CHEOPS?* (originally from [the CHEOPS Red Book ESA/SRE(2013)7
November 2013](https://sci.esa.int/documents/34375/36249/1567259940843-CHEOPS_EST_SCI_RP_001_RedBook_i1.0.pdf)

[![enter image description here](https://i.sstatic.net/Z5bQI.png)](https://i.sstatic.net/Z5bQI.png)

**Tags:** atmospheric-radiation, radiative-transfer

> **Comments:**
> **username_1**: When I think of "local variations of Earth's thermal radiation", I always think of this map: https://en.wikipedia.org/wiki/Earth%27s_internal_heat_budget#/media/File:Earth_heat_flow.jpg Then I realise Earth's internal heat is nothing compared to the solar contribution (we're talking mW/m$^2$ here)... But as a solid Earth scientist I couldn't help myself to post it anyway! :)  

---

## 1 answers

### Answer by username_2 on 2020-02-06. Score: 2

The yearly average Top Of Atmosphere (TOA) Outgoing Longwave Radiation (OLR) is around 145–345 W/m², as measured by the AIRS instrument:

![OLR](https://upload.wikimedia.org/wikipedia/commons/thumb/e/e0/AIRS_OLR.png/1024px-AIRS_OLR.png "Outgoing longwave radiation")  
*Source: [Wikimedia Commons contributors, "File:AIRS OLR.png," Wikimedia Commons, the free media repository](https://commons.wikimedia.org/w/index.php?title=File:AIRS_OLR.png&oldid=213002906)*.

Now, this is an average that includes both day and night. How much does it vary? This depends on the location. Based on GERB data, a 2007 paper estimates (please pay attention to the thick solid line only):

[![OLR diurnal cycle](https://i.sstatic.net/sZgjx.png)](https://i.sstatic.net/sZgjx.png)  
*Source: Comer, Slingo, and Allan (2007)*

Even in places with a very large diurnal temperature cycle such as the Sahara, the difference between maximum and minimum is only about 10%. In oceanic areas it is closer to 2%. That means that if you take the mean (shown in the top figure) and subtract around 1–5% depending on the region (the drier, the more difference) you get a reasonable estimate of the nighttime outgoing longwave radiation.

If you want a more precise estimate, you can always download satellite data and calculate it yourself.

---

Full source for second figure: Comer, Slingo, and Allan (2007), Observations of the diurnal cycle of outgoing longwave radiation from the Geostationary Earth Radiation Budget instrument, Geophysical Research Letters, Vol. 34, L02823, doi:10.1029/2006GL028229


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