Aspects of the disclosure relate generally to detecting road weather conditions. Vehicle sensors including a laser 310, 311, precipitation sensors 340, and/or cameras 320, 322 may be used to detect information such as the brightness of the road, variations in the brightness of the road, brightness of the world, current precipitation, as well as the detected height of the road. Information received from other sources such as networked based weather information (forecasts, radar, precipitation reports, etc.) may also be considered. The combination of the received and detected information may be used to estimate the probability of precipitation such as water, snow or ice in the roadway. This information may then be used to maneuver an autonomous vehicle 101 (for steering, accelerating, or braking) or identify dangerous situations.