Pictures are underexposed
When you are photographing scenes with mostly light objects (for example, snow, water, and sand), the picture is usually underexposed (darker than it really is). The camera meter registers the brightness of the scene and tries to set the camera lens and aperture for an exposure based on average brightness levels (18% reflectance) causing it to underexpose, as in the following picture.
When you are photographing scenes with mostly dark objects (for example, shade, shadow, and overcast skies), and very few light objects, the camera may overexpose the image, causing it to be too light. If you have a flash on your camera, you can compensate by adding "fill flash" for some extra light.
If your camera has an exposure compensation adjustment, you can increase or decrease the exposure to correct for these exposure problems. Increase the number to make the image lighter, and decrease the number to make the image darker. You may want to try a series of shots with different exposure compensation adjustments to get a feel for how much difference these adjustments make.