Them what explains the surface world not being able to see skyloft, if light passes through normally?
(or is it that they can only see the shadow of skyloft, that falls on the sealed grounds, and that's why the sealed grounds is so dark???)
Within the immediate vicinity of Skyloft (in the sky) there must a
literal cloud barrier. That is to say, a barrier of which is exempt from "magic" or what have you. This would imply that the clouds in that part of the sky are visible to the world below - blocking view of Skyloft and its satellite islands (Beedles' Island, Pumpkin Landing, etc.) However, if this is the case, I'm sure someone in the world below would find it suspicious that there's a part of the sky where there's ALWAYS clouds. Then there's the large cumulonimbus cloud formation known as The Thunderhead. I'm sure the inhabitants of the world below might wonder why it's always there. In the real world thunderheads are associated with thunderstorm as the name implies. More realistically, all this is assuming the Zelda universe actually adheres to the real-world's laws of science and weather phenomena, which isn't the case. lol
If anything, it might have to do with geography with regards to the location of the three provinces in relation to Skyloft. Perhaps Skyloft and its satellite islands, including the Thunderhead islands are simply out of view to their location in the sky. If you look at the SS map, which is conveniently placed in Cyg's siggy [noparse]
[/noparse], Skyloft floats in the center, but its true location puts it right above the Sealed Grounds. This combined with the aforementioned "literal cloud cover/barrier" and the fact that nothing inhabits the areas surrounding the Sealed Grounds might suffice as reasons behind the disparity in the view of the sky... They are simply out of the range of view.
On a slightly off-topic note: The "holes" in the clouds above the barrier with bright streams of light shooting from them are just there for the sake of gameplay. This explains why bird riders, knights, and the like don't get curious and try to fly there; they aren't visible to them in the game world. This or the idea that nothing exists in the world below is not a literal notion, but much rather a warning that nothing "of the Skyloftians' interest" exists there. They are probably aware that things "exist" there, but given the stories told to them about the world below, any down there is BAD. It might seem like I'm going a bit off on a tangent here, but what I'm getting at is the cloud barrier - as seen from
above the sky is more of a visual barrier than a solid one. From the world below, the rest of the sky might be visible, save for the aforementioned "literal" clouds surrounding Skyloft - barring its [the world below's] view of it.