Should be possible to determine amount of snow on ground using ultrasound or maybe even laser range finder? At the very least, this would help determine distance between bottom of sensor and ground, which could change over time (eg. leaf build up in wilderness, snow, foliage, etc).

Ground-facing UV sensor could be compared to sky-facing sensor of same type; when snow is on ground, more UV should be reflected? If not, would it work with IR instead of UV?