A long sleeve cycling jersey is your best bet for cool weather riding—think fall and spring shoulder seasons, early morning starts, or winter layering. The full-length sleeves trap warmth and block wind better than short sleeves, keeping you comfortable when temperatures dip below about 60°F (15°C).
But warmth isn't the only reason to go long. Long sleeves provide excellent sun protection for extended rides, especially if you burn easily or ride at high altitude. Lightweight long sleeve jerseys with UV-rated fabrics can actually keep you cooler than bare skin in direct sun by shielding you from rays while still breathing well.
Mountain bikers also favor long sleeves for the extra protection against brush, branches, and trail debris. Just note that long sleeve jerseys offer less ventilation than short sleeve options, so they're not ideal for hot, humid conditions unless you choose a lightweight model built specifically for sun coverage.