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Frank - you asked :  What do you think of this graphic? Is it surprising? It it correct?? It's certainly colorful.

It didn’t feel intuitive, but I used a spreadsheet to calculate for a given latitude the hours between sunrise and sunset on each day of a year. Totalling these hours for the year gives the average hours per day, and this was run for a number of latitudes.

It’s probably a bit crude as a model but it seems to suggest that the average hours of sunlight per day does increase slightly going north from the equator, as in the output attached.  I get a lower figure than is displayed on the graphic for the variation in average daylight between the equator and Greenland’s latitude. (I get about 9 minutes of variation as compared to about 35 minutes on the graphic).

[Frank - you said :  At the north pole, the Sun is above the horizon in 2021 from Mar 20 until Sep 22. That's 51% of the year. So per day, that's 51% of 24 hours, which implies 12h14m of daylight per day up there.]    My graph happens to look about the same as that 12h14m figure at 90°N. (noting the graph is decimal hours).

As compared to the variation calculated above by sunrise to sunset times, if “daylight” was to be defined as including when the sun is say a couple of degrees below the horizon, it seems to have a significantly bigger effect on the number of “daylight hours” per year in the far North than at the equator.

Regards, Adrian F