An inferior mirage is a phenomenon in which atmospheric refraction bends light rays to produce a mirrored image below a real object. It occurs when a hot surface heats the layer of air above it, placing it below a colder & denser layer of air.
Flat-Earthers like to use inferior mirages to “explain” how a distant object can appear partly obstructed if the Earth is flat. In reality, an inferior mirage cannot produce an appearance similar to an object partially obscured by Earth’s curvature.
An inferior mirage occurs if the surface of the Earth is heated by the Sun and produces a layer of hot air of lower density just above the surface. Grazing rays bend back up into the denser air above. An inferior mirage is unstable. Because hot air rises and cooler air descends, the layer will mix, giving rise to turbulence, or the shimmering effect.
An inferior mirage only appears close to the horizon, usually not larger than about half a degree. Other parts of the scene are unaffected by the presence of the inferior mirage.
An inferior mirage only occurs if the weather allows it. It does not happen all day, all the time. Flat-Earthers like to cherry-pick scenes when an inferior mirage is occurring, and conveniently ignore when it is not occurring. A distant object can still disappear bottom first without the presence of an inferior mirage, with a clearly defined horizon line.