# Understanding the color coding of a weather map

I came across an infrared weather map like this

I couldn't find any legend on the colorbar's color convention. Initially, I thought that might be the color for reflected light's wavelength, but there was no way to find out. This image is obtained from https://mausam.imd.gov.in/imd_latest/contents/satellite.php#.

• The text in the link says it. What seems to be unclear ? – user20217 Jun 21 '20 at 9:44
• @a_donda I missed the text. The units are not clear to me. The values tell me that they are not temperature units. Can you shed some light on the quantity in the colorbar? – Galilean Jun 21 '20 at 9:52
• on second sight, there's more confusion: you posted an infrared image, but the link describes (and shows) visible light. I suggest you decide which one to take, and properly tell them apart. – user20217 Jun 22 '20 at 23:47

Unfortunately it's common for satellite data services to show these types of images qualitatively, omitting the units or even a colorbar.

This is an infrared image from the 10.8 micron channel (TIR1) of INSAT-3D. That website you link to is a bit misleading because (for me at least) following the link puts the text describing the 0.65 micron visible channel under the image. If you click "Infrared" in the sidebar of that page the text is replaced with the correct text about the 10.8 micron channel.

Given that TIR1 records are stored as 10-bit values (i.e., in the range 0 to 1023) and the color scale maxes out at 939, I suspect that they are just plotting the raw count data. This is normally transformed into radiances by applying a linear scale and offset to put the data in physical units.

I had a quick look at the equivalent 10.8 micron image from Meteosat over the Indian Ocean (see below) and I get a similar count range and a similar image when I apply their color scale. I also suspect that they've inverted their data, e.g., plotted count_inv = 1024 - count, to fit with the viewers' expectation that cold clouds appear white.

• Your image is an infrared image, the one in the question is not. – user20217 Jun 22 '20 at 21:59

Op: You have posted an infrared image, but your question and the link with its the legend is about visible light images. Though similar rules apply, the outcome is fundamentally different, e.g. dark sea and light landmass on visible, vice versa on infrared during day.

To the link: it is a visible light and not an infrared image. Though it is close to near infrared, it does not show a temperature equivalent, but from the legend one can take that the scale is (most probably) based on the amount of diffuse reflected light.

To teh image posted in the question: that'S an infrared image. Both show different things. I suggest you tell them apart, and decide if you want information about infrared or visible light.

The linked wiki article illustrates the reflectivity of different surfaces (visible light), cumulus clouds are very bright, stratus and cirrus gather around the middle of the diagram, while water surfaces reflect much less and so appear darker.

• "dark sea and light landmass on visible, vice versa on infrared during day" -> Note that these shades of grey are just conventions to represent temperatures, see earthscience.stackexchange.com/questions/19230/… – Jean-Marie Prival Jun 23 '20 at 7:58
• @Jean-MariePrival Right for IR, wrong for visible. OP posted an IR image, but asked for visible. In visible it's vice versa, dark for the sea, light for the land. Read the link they posted. – user20217 Jun 23 '20 at 8:28
• Like Deditos said it's just bad web design, if you click on Infrared-1 in the left side bar you get the proper text that goes with the IR image posted. – Jean-Marie Prival Jun 23 '20 at 8:36
• @Jean-MariePrival: It was not clear if the question is after IR (which is pretty self explaining), or visible, which needs a second look. It might well have been the other way round, that the question is after visible and the IR image posted accidentally. – user20217 Jun 23 '20 at 8:42
• Ok, I leave my answer there, for future reference. Now that's clear that it's not quite clear :-) – user20217 Jun 23 '20 at 11:45