A quality running jacket should harness the ability to regulate your body's temperature, making training in cold and wet weather conditions far more manageable; mentally and physically. A key factor for runners in choosing the right jacket touches on the fabrics breathability - how well the jacket lets perspiration escape. Much of your decision will come down to personal preference, but it’s best to look for features such as a jacket made from a water resistant and breathable fabric over a fully waterproof shell that can be susceptible to excessive condensation build-up on the inside. Either way, look for a model with plenty of ventilation built into it, and a slim cut that allows freedom of movement without excess fabric getting in the way. A light mesh lining can help the jacket to feel less clingy, though an unlined option is better if you're seeking to minimise overall weight.