What is optical depth (AOD)?
Dust optical depth is a unitless number for how much sunlight dust blocks at a wavelength of 550 nm through the whole column of air. Unlike PM10, which describes the air near the ground, AOD also reveals elevated dust, so a high value can appear while the air at ground level stays fairly clear when the layer passes overhead.
How to read the colours
The scale at the bottom of the map starts at 0.5 in a pale colour, then 1 in orange and 2 in red, and ends in purple at 3 and above. Dark areas with no colour are below 0.5. Values above 1 mean a dense dust layer that dims the sun and turns the sky grey or yellow. Labels inside the image are in Arabic.