Amazing Sunset Mirage Observed from San Francisco
April 17, 2017
Most everyone has seen a sunset where the round disk of the Sun slowly sets and eventually disperses below the horizon. As seen from San Francisco, the setting Sun is rarely round, as shown above, and sometimes it's rising instead of setting! Click here to see a video of one such rare and amazing sunset.
On this video, at first the Sun isn't really going down but is rather getting flatter. Then at around the 1:02 mark you'll see a thin red line at the sea's horizon. At this point, the real Sun is already below the horizon. What you're seeing is a mirage of the Sun. You'll notice that multiple images of the miraged Sun start appearing above each other. After a few more seconds, the Sun looks more like a nuclear explosion than our home star. Continue watching the video and you'll observe that the lower images of the Sun are connected to each other and take on a rectangular shape. At the 4:16 mark the lowermost images of the Sun begin to show the Novaya Zemlya effect, first commented on by polar explorers.
The miraged Sun isn't really setting; rather it's melting into the sky. Yet it's reluctant to disappear. Notice how some of the images of the Sun that seemed to have disappeared actually reappear again a split second later. For example, at 5:58 it seems that only the uppermost image of the Sun is still present, but at 5:59, the image of the Sun below it is reappearing and in addition is getting brighter. Also, note the weird clouds at upper right (they're much better seen after the 4:16 mark). They look odd because they're miraged as well -- a fata morgana in the clouds. This mesmerizing sunset was delayed by more than 6 minutes based on the ephemeris time for sunset in San Francisco.
So what's going on here? As with all mirages, temperature gradients are involved. However, in this case, a complex temperature profile over the sea, including a thermal inversion, wildly distorted the shape of the Sun as it set.