Landscape photography is a very difficult subject to handle. The camera's today are marvelous pieces of technology. But, they are limited in their dynamic range and only record a certain amount of information. For example, during a sunrise or sunset, a camera will be able to either record a properly exposed image of the background(sky) or foreground(earth). It is nearly impossible for a camera to record all the information in a scene in one image. It can be done, but without digital manipulation, the image with either appear to dark or too light. A photographer may be able to increase that dynamic range with split neutral density filters, but the problem will still remain. It will be improved, but there will be darker areas and lighter areas on the image that will not be seen unless recovered using photoshop or some other comparable software program to digitally enhance the image.
One of the key factors in this equation is the light available from the sun. If it is cloudy, great. I can record the image on the camera and avoid dark shadows. I can also do this at twilight, but a longer exposure is necessary. The worst time to take photographs is when the scene is very contrasty and has no mid-tones to speak of!