Disclaimer: Please note, that I write these articles to stimulate some thought. I will sometimes generalise a point or simplify an argument in an attempt to convey a message. Ultimately, it is just my point of view and of course is not the definitive word on the matter (as much as I'd love to think so!) ;-)
As much as I would love to fit both sky and ground into my compositions, I only have so much area within my frame to work with. So I have to figure out what is central to the story I'm telling in the image I'm composing. If I add a lot of sky in, then it should either compliment the foreground in some way (if both sky and ground have a relationship - by mirroring shapes or mirroring textures / patterns), or it should dominate the scene.
Picture 1
Despite many thinking this image is about the desert and the mountains, it's really about the sky. So much so, that the sky covers 3/4 the area of the frame, while the ground only covers 1/4 the area. My eye moves mostly around the sky with occasional visits to the two mountains - often in a triangular motion around the frame.
For me, it has to be either / or. You either have a lot of foreground and little sky in the frame, or you have a lot of sky and little ground in the frame.
We have to figure out what is it that we're trying to say, when we choose the proportions of sky and ground. Is the photo really about the desert and mountains? Or is the image really about the sky, and the mountains are in the frame to give some kind of context?
Settling for something in the middle often results in a confused message. The end result is that the viewer is pulled between both sky and ground and a competition race between sky and ground ensues for the viewers attention (see Picture 2).
But it's hard to choose the sky over the ground as a dominant subject. We tend to give foreground or ground subjects more prominence than they deserve because they are usually what draws us to a location in the first place. In the first image above (see Picture 1), I had that very circumstance. I was first drawn to this place by the strange rocks and mountains to be found there. We can be so wrapped up in what grabbed our attention, that we may fail to see other motifs or interesting patterns elsewhere (such as in the sky). Picture 1 is all the cloud shapes and the light scattering upon them. It is a 'sky photo' with some contextual mountains in the foreground. Not the other way around.
I've re-cropped the image so we can look at it from another point of view. In Picture 2, it would have been so easy for me to compose the shot in a more traditional format - with the sky only taking up 2/3rd's of the frame and the ground taking up 1/3rd. The net effect is that the horizon is more central and my eye is being pulled between two competing subjects: sky & ground.
Picture 2
In my original shot, the ground covered 1/4 area of the picture. In this crop where I've reduced the sky, the ground has now 'gained' and covers 1/3 the area of the picture. The result is that the sky has less dominance while the ground has gained in dominance and I find my eye is being pulled between sky and ground in a competition between the two for my attention.
My advice would be to do the following:
1. Set up your shot the way you want it and make an image for comparison.
2. Adjust the camera so that you get a lot more ground in the frame than sky, and make an image.
3. Adjust the camera so that you get a lot more sky than ground in the frame, and make an image.
4. Compare.
I guess I try to avoid central horizons where I can, because mostly they just make a composition feel less focussed. But there are times when going central does work. If the sky and the ground have a tight relationship (such as similar tones, or textures, or perhaps a pattern in the landscape that is also encountered in the sky). I also find I will go central when I feel that sky and ground have little to add to a photo - the main point of attention is to be found in the middle of the frame, By doing this, I can use the sky and ground equally to give a lot of space around a subject, as in Picture 3:
Picture 3.
The horizon is just slightly above centre. I can get away with this composition due to the requirement to have a lot of space around the island. Note however, that I still didn't go central, because that would introduce the feeling of the island sinking (see Fish Tank article from a few days ago).
Ultimately, often trying to have both sky and ground with equal attention leads to competition between the two, unless they are contextual (such as in Picture 3 above - where they just contribute a sense of space to the composition). I prefer to make things as clear as I can - either the image is about the ground (and therefore there is a whole lot more ground in the frame than sky) or it's about the sky (and therefore there is a whole lot more sky in the frame than ground).
Most often, you can't have both without it seriously compromising the strength of your composition.