For most of the day either the land is darker than the sky or the sky is darker than the land. This is one of the biggest problems with getting a balanced photo during the hours that we normally are out and about.
There are, for obvious reasons two points in time where the two cross over. At sunrise and again at sunset there comes a point where the light above and below the horizon is roughly equal. This makes for some amazing photographic images if you happen to be around. One thing to look out for is that just before sunrise and just after sunset can give amazing images too. The shot above was taken just after the sun went down. There was enough light left in the sky to keep the shot interesting, and the reflections in the foreground balanced the light perfectly. Look for reflections at sunrise and sunset for that special image!