The biggest challenge when photographing snow lies in your camera’s metering system.
No matter how sophisticated the metering in an SLR camera is, it’s engineered to assume it’s reading “normal” subjects, of an “average” brightness. Photograph a landscape in summertime, with green grass, dark green trees, blue sky and white clouds, and the different brightness levels in the scene often average each other out. Much of the time, the meter will get these scenes absolutely correctly exposed. In simplified terms, it does this by averaging bright and dark parts of a scene, so the final exposure renders the overall brightness almost a middle shade of gray.
You may have heard the term “18% gray,” a middle shade of gray which, technically speaking, reflects 18% of the light striking it. This has been a universal reference point in photographic exposure meters for decades.
Just to clarify what that means: Middle gray is roughly the mid-tone on a gray scale — appearing to fall exactly between pure black and pure white. Subjects of this tone reflect about 18% of light (comparatively, white objects reflect nearly 100% and black objects reflect nearly 0%).
Simply put, most cameras assume that everything they photograph reflects 18% of the light, and expose accordingly. Put another way, your camera meters subjects assuming they should be photographically rendered as middle gray.
Freshly-fallen clean, white snow is obviously much lighter and more reflective than that, but the camera will automatically try to reproduce it as gray, by underexposing anywhere up to about two stops to correct for what it sees as a too-bright subject. This is exactly why many photographers will find their snow photos to be muddy and underexposed.
In-camera light metering works reasonably well with most subjects, in most lighting situations. However, there are tricky scenes that will baffle most meters — and snow is a classic example.