To dispatch a jet airliner under the FAA FARs it must be planned to arrival at the destination at a weight that will allow it to come to a full stop within 60% of the available runway. If the runway is wet, the available stopping distance must be 115% of the dry stopping distance. The rule is written to allow the use of dry stopping distance charts on a wet runway. Since most airports don't stretch their runways when they're wet, the easy way to think of this is the dry stopping distance must be ~52% of the available stopping distance if the runway is wet. Most operators consider the runway wet when the visibility is reduced below a certain value.