Exposure meters assume that the scene has an average reflectance of 18%. When you shoot a subject that has a higher than average reflectance, like snow or sand, the meter will set the exposure to produce an under exposed photo. The opposite is also true. When you give the camera an 18% Gray Card to sense it will give you the right settings for the lighting conditions. Use those settings and your shot will be properly exposed and snow will be white instead of gray.
You can also put the card within the scene on a test shot and then use that in setting the gray point for the print of the final shot.