Whenever you include the sky in your photos, chances are your compositions will look better if there are clouds—the right kind of clouds, that is. Photographing clouds is really important for landscape photography in particular, but I find the presence of clouds to be useful with other types of outdoor photography as well.
Look for clouds with texture.
As I said above, you want the right type of clouds. Skies with clouds that lack definition (such as uniformly overcast skies) don’t generally help much. On the other hand, clouds with gaps, texture, and definition often work really well. When clouds have sufficient separation, light can break through the gaps to produce stunning and colorful results—especially at sunrise or sunset—even when cloud cover is significant.
Partly to mostly cloudy skies generally give you the best chance of getting colorful sunsets. Although you want a lot of clouds in the sky to catch the color of the rising or setting sun, too many clouds will block the light. Of course, the clouds will do what they will do, so it pays to be on location when mixed cloud cover is present. All you need is a tiny gap at the horizon, right where the sun is, to set fire to an otherwise completely cloudy sky. Clouds are also key to getting really great light on the landscape: if enough clouds light up, they can act as giant reflectors, bouncing intense and colorful light onto the landscape below.
Often, the best time to get great clouds is when storms are coming in or breaking up. Storm clouds are often very large, dramatic, and photogenic, and can transform even bland midday light into something much more photogenic, such as with this storm building over sandstone formations in the desert.