<p>You can measure the pressure of a gas by the line thickness of absorption or emission lines, the so-called <a href="https://en.wikipedia.org/wiki/Spectral_line#Pressure_broadening" rel="nofollow noreferrer">pressure-broadening</a>. That works independently of whether you can detect the thermal radiation from that gas layer.</p>
<p>The cloud layer is well-defined by the optical thickness getting larger than one (this is impenetrable by light, and parts are back-scattered to the satellite). So what you measure onboard of the satellite is thus the pressure broadening of the absorption lines of backscattered light from the cloud top in the atmospheric gases. Temperature measurements are not that readily available as they require much more sophisticated and expensive sensors for mid- to far-infrared corresponding to the temperature range found at cloud tops.</p>

CC BY-SA 4.0
2024-10-08T20:24:31.657Z
2024-10-08T20:24:31.657Z
planetmaker
<p>The cloud top pressure depends on the cloud top temperature. I think the reason for using the cloud top pressure, is because the main parameter the cloud type classification schemes are based on is the height, respectively the pressure levels.</p>

CC BY-SA 4.0
2024-10-08T13:18:35.677Z
2024-10-08T13:18:35.677Z
Vladimir Ivanov
<p>The ISCCP DEFINITION OF CLOUD TYPES is a table of cloud top (CT) pressure and optical thickness. I'm curious if anyone is able to explain the choice of CT pressure over temperature given cloud level categories seem to be more consistent with temperature over pressure.</p>
<p><a href="https://isccp.giss.nasa.gov/cloudtypes.html" rel="nofollow noreferrer">https://isccp.giss.nasa.gov/cloudtypes.html</a></p>
<p><a href="https://i.sstatic.net/QvfbEnZb.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/QvfbEnZb.png" alt="enter image description here" /></a></p>
<p><a href="https://i.sstatic.net/v86eCyzo.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/v86eCyzo.png" alt="low, middle and high cloud categories change with troposphere height that is determined by temperature" /></a></p>

<climate-models><clouds><weather-satellites><cloud-microphysics>
Why are satellite cloud type observations based on cloud top pressure instead of temperature?
Rey
# Why are satellite cloud type observations based on cloud top pressure instead of temperature?

(Asked by: username_0 on 2024-10-07)

The ISCCP DEFINITION OF CLOUD TYPES is a table of cloud top (CT) pressure and optical thickness. I'm curious if anyone is able to explain the choice of CT pressure over temperature given cloud level categories seem to be more consistent with temperature over pressure.

<https://isccp.giss.nasa.gov/cloudtypes.html>

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

[![low, middle and high cloud categories change with troposphere height that is determined by temperature](https://i.sstatic.net/v86eCyzo.png)](https://i.sstatic.net/v86eCyzo.png)

**Tags:** climate-models, clouds, weather-satellites, cloud-microphysics


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## 2 answers

### Answer by username_1 on 2024-10-08. Score: 1

You can measure the pressure of a gas by the line thickness of absorption or emission lines, the so-called [pressure-broadening](https://en.wikipedia.org/wiki/Spectral_line#Pressure_broadening). That works independently of whether you can detect the thermal radiation from that gas layer.

The cloud layer is well-defined by the optical thickness getting larger than one (this is impenetrable by light, and parts are back-scattered to the satellite). So what you measure onboard of the satellite is thus the pressure broadening of the absorption lines of backscattered light from the cloud top in the atmospheric gases. Temperature measurements are not that readily available as they require much more sophisticated and expensive sensors for mid- to far-infrared corresponding to the temperature range found at cloud tops.


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### Answer by username_2 on 2024-10-08. Score: 0

The cloud top pressure depends on the cloud top temperature. I think the reason for using the cloud top pressure, is because the main parameter the cloud type classification schemes are based on is the height, respectively the pressure levels.


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