A midweight jersey (150-200 GSM) is built for those in-between temperatures around 10-18°C (50-64°F), making it ideal for spring and fall riding. The fabric has a slightly heavier feel, often with a brushed or thermal interior that traps warmth without feeling bulky.
You can layer a lightweight jersey, but a midweight handles temperature regulation more naturally. It breathes well on climbs yet holds enough heat for cool descents and steady efforts. Many riders find it's their most versatile piece for variable conditions where the weather shifts during a ride.
If you only ride in hot summer weather, you can skip midweight. But if you face cool mornings, breezy descents, or ride spring through fall, a midweight jersey is worth the investment. It's typically more comfortable than piling on extra layers, which can bunch up and restrict your movement on the bike.