They don’t always form at the same height, but when it comes distances like that, with cloud base typically being a few thousand feet above ground level, the human eye isn’t very good at judging distance. You might get some idea if you know the size of the object you’re looking at, but clouds vary in size.
Clouds do vary in height; in fact one of the criteria used to classify clouds is how high they are. See [this summary](https://cloudatlas.wmo.int/en/useful-concepts.html) from the International Cloud Atlas.
Clouds often form as a result of air being lifted, which causes it to cool. If it cools enough, moisture in that air can condense. The rate at which air cools as it rises, called the adiabatic lapse rate, is fairly consistent for unsaturated air. But even then, the level at which you reach the dew point, the temperature at which the moisture would start to condense, depends on the starting temperature and moisture content of the air.
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