A UPF 50+ base layer blocks over 98% of UV radiation, which sounds excessive for winter—but it's genuinely worth considering in certain situations. UV rays are significantly stronger at higher altitudes (roughly 10-12% stronger per 1,000 meters of elevation), and snow reflects up to 80% of UV radiation right back at you. So you're getting hit from above and below.
UPF 50+ makes the most sense if you're spring skiing, riding at high-altitude resorts, or have fair skin that burns easily. During deep winter with heavy cloud cover and full layer coverage, it's less critical.
The good news is that UPF 50+ adds no performance downside—it doesn't affect breathability, wicking, or comfort. Many merino wool base layers naturally provide strong UV protection anyway. Just remember that a UPF-rated base layer only protects the skin it covers, so you still need sunscreen on your face, neck, and hands.