File:PIA22737-Mars-2018DustStorm-MCS-MRO-Animation-20181030.webm

https://photojournal.jpl.nasa.gov/catalog/PIA22737 This animation shows the evolution of the 2018 Mars global dust storm from late May to September.

The animation shows the optical depth tau -- a measure of how much light is being blocked by atmospheric dust as measured by the Mars Climate Sounder instrument onboard NASA's Mars Reconnaissance Orbiter.

The dust is mapped to two opposite hemispheres of Mars, giving a view of the full globe.

Certain features of the Martian terrain, including Olympus Mons, the three volcanoes (Tharsis Montes} in the equatorial region, and Valles Marineris, are also visible.

A tau of three indicates that only about 5 percent of the sunlight entering the atmosphere directly reaches the surface.