I took the cog train up to the summit of Pike'sPeak one fairly clear day, and could see the curve of the earth in the horizontal plane out over the plains.If you run into somebody who thinks that the Earth is flat, have him or her watch one of these videos.
Out on Utah's Salt Flats, there are some views from the I-80 freeway where, if the factors(surface heating layer) that reflect the sky are not prevalent, you can see an arc on the horizon to the north, between the Newfoundland Mountians and the lakeside hills on the west.
Sailors can sometimes see the same sort of thing with the horizons if the surface layer of the air is clear enough.