Being highly sensitive to light, your camera's metering system may react to the very bright reflected light in snow and beach scenes. What happens is that the meter automatically 'stops down' the lens aperture, and produces a dark, under-exposed picture.
When taking a. figure against snow or sand, move in as close as you can and fill the viewfinder with the subject.
If you're using a camera with manual and automatic facilities, and you want some of the background, a useful rule-of-thumb exposure guide is to shoot on manual mode, with a shutter speed of 1/250th a second, at f11 for ISO 100 film.
Alternatively, use your camera's exposure compensation or back light control (refer to the handbook).
