In general, you should avoid taking out your bag in extreme weather condition. Fog, rain, snow and sleet can lead to water marks or stain on the bag.
To clean your silk bag, make a solution of mild detergent and lukewarm water. Before dipping the bag into the solution, you may want to first apply the solution to a small patch on the bag.
It is better not to rub a silk bag as this can spread out the stain and wear out its fiber.