Arm warmers are an ideal accessory for the transitional seasons of spring and autumn or on cold winter days when they can offer extra insulation under your jacket.
They consist of a simple fabric tube with their main advantage being that they are removable, so you can stow them in a jacket or jersey pocket when not needed. That way, you can ensure you are warm and cosy on early-morning training rides or during the initial stages of a ride, when your muscles are yet to warm up. When the sun comes out or when you are sufficiently warmed up, the arm warmers can be taken off.
Alpine tourers may additionally find them useful for long descents off the mountains, when all that thermal energy built up during the climb quickly dissipates in the face of a stiff mountain breeze.