What’s in a colour?

Many of you will have seen posts of infrared satellite imagery, like this one, with a colour overlay on the clouds. But what do these colours mean and why are they important?

Satellites use information from different parts of the electromagnetic (EM) spectrum. An infrared satellite image uses the infrared portion of the EM spectrum. As the name suggests, this gives information about the temperature of the features being observed. This contrasts with visible satellite images, which use information from the visible part of the EM spectrum.

When viewing infrared satellite imagery, the temperature of the cloud tops is what’s most important. The colours on these satellite images highlight the coldest cloud tops. The colour key in this image goes all the way above -73°C, but cloud tops can be even cold than that!

As the old adage goes, you can tell a lot about a cloud by the temperature of its tops. Cold cloud tops can indicate high cloud, like the thick cirrus that often clouds over the day before a front. They can tell how high into the atmosphere the clouds go, which is crucial for aviation.

They can also show clouds that are unstable and have very strong updrafts within them. Cumulonimbus clouds, like the one in this image, are a great example of this, and can even give hints at where hail is forming within the cumulonimbus complex. All of this because of a couple of coloured pixels!

If you’d like to know more about how meteorologists use weather satellites, check out this blog post https://blog.metservice.com/SatelliteImages . For more satellite images over New Zealand, see http://bit.ly/NZVisible ^MM