CathInfo.com

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It certainly looks like they ALL have zero sense of humor.
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They try to make up for that by using smileys:    (That way, other flat-earthers know when to laugh.   )

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Yeah, like everyone is worried whether or not you think you have a sense of humor!  HAHAHAHA

All flat-earthers have to fear is sphere itself.                          

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No refraction is needed for an explanation.

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There doesn't seem to be any refraction going on, not unusual for cold climates.
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You're not using the curvature calculation correctly.
The elevation of the viewer is (supposedly) 500 feet (actually it's probably more like 550 feet but we'll ignore the 50' of error.)
The distance to Denali is not 140 miles (the correct distance is 120) because the flat-earther's scribbling on the photo is wrong.
Check the maps. It's 120 miles.
Therefore, using an accurate calculator that takes these into account, such as this one, you get:
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5,720 ft obscured by earth's curvature.
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Take that away from 20,310 ft and you have 14,690 ft visible -- well within reasonable in the picture shown, especially considering the base of Denali mountain range is right about the same as the obscured calculated. (The peaks of the Dutch Hills at Denali's base are 4,500 ft tall and they're not visible in the photo, since 5,720 ft are obscured under curvature at the horizon.)

The only thing flat-earthers have to fear is sphere itself.