Why the first rain is the worst
A dry road collects a film of oil, diesel, rubber dust and brake dust. The first rain lifts that film into a slick layer on top of the water. Keep riding and the rain washes it off the road, so grip partly recovers while it is still raining. The longer the road was dry beforehand, the more there is to lift.
In practice: if a shower is forecast after several dry days, the start of it is the time to be parked for a coffee. An hour into steady rain, the road is wet but cleaner.
Wet is mostly a yes-or-no question
Most of the grip you lose in the wet goes in the step from dry to damp. The difference between drizzle and steady rain is much smaller than the difference between dry and drizzle. What changes again in heavy rain is standing water: once the road cannot drain fast enough, the tyre has to push water aside, and that is aquaplaning — a different and more sudden failure.
Paint, manhole covers, tar snakes and polished tarmac are far worse than the surrounding road when wet. No forecast can see them; you still have to read the road.
The rain stops, the road stays wet
A road does not dry when the rain stops. How long it stays wet depends on sun, wind, humidity and temperature — see how long a road takes to dry. A sunny, breezy summer afternoon can dry it within the hour; a cold, still autumn evening can leave it wet all night.
How RideWindow handles rain
The ride score weighs the chance and amount of rain, and a separate grip estimate follows the road through the rain itself: the drop at the start of a shower, the recovery as it continues, and the drying afterwards. Reduced grip caps the whole score, so a sunny hour on a still-wet road cannot read as a recommendation.
The ride windows on every forecast pick the stretches that are worth leaving for. For a longer ride, score the route for the hour you reach each stretch, so a shower on the far side of the hills shows up before you set off.