As this packs down to about the size of a tennis ball and weighs just over 100g, it can easily be stowed away in a jersey pocket just in case the weather changes.
It is made from Hydravent material, which is lightweight and windproof, but from our own experience we know it to be showerproof to a certain degree too. There are two mesh panels in the rear to aid venting, and the cuffs are elasticated with thumb holes in them. A drawstring base hem and drawstrong neck enable you to seal it up nicely against the wind. If you're after something that will fit in a pocket and provides just a handy barrier against those cold biting winds, then this is worth a look - but it's pricey for what it is.