It seems that whenever the challenge for evidence is put to flat earth believers, the flatness of the horizon, the infamous Bedford experiment and even the flatness of the clouds are often quoted as supporting the view.
Horizon
Imagine a microbe sitting on the surface of a snooker ball. In terms of scale that is a comparable to a human being on the surface of the Earth. Not exact of course so please don't bother quoting me on that but it serves as an illustration for the point. The microbe cannot see the whole of the surface of the ball for obvious reasons. In fact it will only be able to see a very small amount of the total surface directly. Furthermore since the surface of the ball is featureless, once the microbe starts moving over the surface it will have no means of being able to identify its point of origin again. However regardless of how far the microbe moves or indeed what direction it travels in the size or area of the surface that the microbe can see directly will always be the same. It could continue to wander over the surface for ever without ever reaching an edge. If we were to make some markings on the ball, such as different shapes then the microbe would now have some means of navigating over the surface and it would be able to revisit any shape any number of times. Doesn't that seem to reflect quite well our real world experience on Earth?
Back to the horizon. Since our microbe is looking out from a point on a spherical surface the distance he can see any direction will be equal. This will mean the 'horizon' is an equal distance away in all directions and that means he will have no direct perception of curvature. Any more than someone standing at the centre of a circle has any perception that the circumference of the circle is curved. To them it will just be a straight line. The only way our microbe could develop any direct awareness that they have been sitting on a ball is by increasing their distance above its surface to such an extent that they can see a distance away from the ball that is significant compared to its radius. This again follows the real world experience of those on the ISS or any orbiting satellite. But since flat earthers don't accept the existence of the ISS or any other satellites in orbit (for this very reason I'm sure), then they won't accept this last point.
Bedford Experiment
Hardly convincing and repeated experiments from the original location and others since has revealed very mixed results. Easily explained by the known properties of light refraction near the surface. The extent of light refraction is affected by changes in air density and temperature variations and this in turn explains the varied results.
I wonder how the results of a Bedford type experiment carried out on the Moons surface where there is no air would compare.
Flat bases of Clouds
Shouldn't need any explanation really but the base dimensions of typical cumulus clouds that show a 'flat base' are far too small to show any true sign of curvature.