My first attempts with Terragen convinced me it was an easy way to experiment with landscapes, with an intuitive interface, and quick results. However after the first few tries, once I started experimenting with atmosphere settings, the sky disappeared. I thought at first I had set the camera too low, or perhaps inside or touching some piece of the terrain, but it turned out to be the size and distance of the sky.

The settings for sky size and altitude are under "cloudscape" and under "atmosphere" are half-height settings for haze, atmospheric blue, and light decay.