Why are the clouds white and Australia black in weather satellite infrared images? The question Is the sunlight on the GOES-16 Imagery reaching from the east? shows a visible light image and so in the night areas the Earth appears dark and in the day areas the Earth is a bit brighter and the clouds are the brightest.

We can have 24 hour imaging of clouds and weather in thermal infrared satellite images. Below is a a thermal infrared image at about mid day in Japan and Australia. I'd expect the tops of the clouds to be cold and the outback to be hot, and yet it's just the opposite!


The clouds are nearly white
Central Australia is nearly black
The ocean and all of the Asian continent are dull, almost featureless gray; if there weren't a superimposed outline it might be hard to even tell where the water ends and the land begins in some places.


Why is this? What's going on?



Himawari-8 IR1 image from Central Weather Bureau click for full size
I had a very similar question in a job interview!  The only difference is that it was an image from SEVIRI on Meteosat.

The imager on HIMAWARI is called the Advanced Himawari Imager (AHI).  The AHI IR1 channel is actually channel 13 with a central wavelength of 10.4 µm, which is in the window region (apparently it's called IR1 in reference to an older satellite).  The clear-sky atmosphere is mostly transparent in this region of the electromagnetic spectrum, so we can see the surface or clouds mostly unobstructed by water vapour, ozone, or other gases.

In an infrared image such as this one, high clouds are cold.  Forecasters who look at such images for a living like to see clouds as white.  I'm a physicist and I prefer to map values of high intensity to white, which would make clouds black, but forecasters are the most important users so they get their way.  :)

To display cold areas as white, hot areas must logically be displayed as black.  So what we are seeing in this image:


Central Australia is hot.  This is expected, because it is a hot desert with high emissivity.  The image is apparently taken at 12:00Z, which should be between 20:00 and 22:00 legal standard time in mainland Australia, so it's not long after sunset, when the land is still hot.  Australia should appear less "black" at an image taken 8 hours later.  The image is apparently taken at 12:00 "local" (140°E) time (03:00 UTC).  It's February and summer in Australia, and the northern hemisphere looks a lot less hot (it's also earlier in the morning there, compared to sunrise).
Cloud tops are cold.  This is expected, because they are in the upper troposphere.  Those white clouds are ice clouds.
There are grey-ish clouds too with little contrast to the ocean.  Those are lower, liquid clouds, they are warmer.  The lowest clouds may be difficult to identify in infrared images, which is why (for example) night time fog detection from satellites is difficult. 


(I got the job!)