One of the first factors to consider is the waterproof rating of the jacket. This is usually measured in millimeters (mm) and indicates how much water pressure the fabric can withstand before moisture begins to seep through. A jacket with a rating of 5,000 mm will keep you dry in light rain, while one rated at 10,000 mm or higher is suitable for heavy downpours. For regular use in rainy conditions, a jacket with a rating of 10,000 mm to 20,000 mm is advisable.