Wayther Rain
The one hazard with edges
It does not look that way from inside a car in it. But an exposed ridge is exposed at every hour of the day, while a shower is not: it has a beginning, an end and a direction it is travelling in. The same fifty kilometres can be dry at seven and soaked at nine. That asymmetry is what makes rain the one condition on this site where moving your departure reliably moves you out of it.
The mechanics
Three of them change what you do behind the wheel. The fourth happens to the sky rather than to the vehicle, and it is the one that decides whether the trip is worth making at all.
Before a road gets slippery it gets hard to see. Falling rain scatters the light from other vehicles' headlights back at you, which is why night rain blinds sooner than day rain, and why the thing that changes your decision is rarely how hard it is falling but how far you can see down the road.
A wet road leaves a film of water where a dry one had texture, and grip comes from that texture. The film is why braking and steering feel different, and why the first shower after a dry spell catches people out: the water has picked up oil and residue that was sitting on the road through the dry weather.
This is the one that is not a driving problem. Rain arrives from a direction and leaves the same way, and between those two edges the sky over any given mile is doing something quite different from the sky a hundred kilometres away. No technique helps with that, because it is not happening to the vehicle.
Not a different weather — the same rain, with less light in it. A sight line that is adequate in daylight collapses when the only light is what is coming back off the spray, which is why the same stretch is a non-event in the afternoon and a problem at two in the morning.
The part you can act on
Standing weather is a wall: it is there when you arrive and it is there when you leave, and no timing gets you past it. A shower is an edge. It has a leading side you can be behind and a trailing side that will catch up with you if you dawdle, and both of those facts live in a forecast timed to your arrival rather than to a clock on the wall.
This is why rain is the easiest of the conditions on this site to plan around, and it is the one most people never try: shift the departure and the arrival at every point on the route shifts with it, so a band of rain that was going to be on you at mile 300 can end up behind you or still twenty miles ahead. Neither requires better weather. Both require better timing.
The same lever solves the night problem. An hour later is not just a different hour of rain, it is an hour with more light in it.
When the timing is fixed
A delivery window, a ferry slot or a campsite you have already paid for takes the departure time off the table. Rain is then a routing question rather than a timing one, and the useful comparison is not the shortest distance: it is whether the inland road, which is probably longer and probably flatter, sits outside the band altogether.
This is the case that turns up most often in how people actually use a route forecast. You were going to take one road, you checked, and you took the other one. That decision takes two minutes with the alternatives in front of you and is almost impossible to make from a forecast for your departure city.
Comparing alternatives on travel time, distance and forecast together is the only version of the comparison worth doing, because any one of the three on its own will mislead you.
The same thing, at 08:00 tomorrow
There is a band of rain sitting across the middle of your route and it looks, in the summary, like one solid block. It is not one solid block, and telling the two apart is most of the work.
Rain is the one condition here that has a beginning and an end on your own miles. The summary's heavy-precipitation block tells you how much; the map tells you whether it starts at 120 km and has cleared by 190, or starts at 120 and is still going when you reach the coast. Those are different trips and they look identical in the summary.
If it clears before you would arrive, then how hard it is raining is not the question — you are not driving in it, you are driving past it. If it does not clear, then the question stops being rain and becomes the road surface, which is somebody else's forecast to make and not this app's to guess at.
Not the middle of it. Arriving at the far side of a band as it clears is a different drive from being halfway through it, and one move of the departure slider is usually the entire fix. Look at several hours, because a band that has moved four hours may have moved off your road entirely.
A bypass that runs south of a band is a different forecast, not the same one later. That is worth the fuel more often than it is not, which is not true of the other three conditions on this site, and the reason is that rain has geometry: it has an edge you can get behind.
One band across a six-hour drive is a nuisance. Three is a different day, and the third one is the one that ruins it. They are all listed in the summary, so the count takes ten seconds here rather than four hundred kilometres.
Rain FAQ
There is no single answer to that, and no number on this page, because what matters is not how much rain is falling but whether you can see far enough ahead to react to what is in it. Visibility is the figure that changes the decision: the step from ten kilometres to two is the difference between a drive and a crawl. If you cannot see, slow down until you can, and if you still cannot, stop.
Because the water film the rain leaves has picked up oil, diesel and rubber residue that has been sitting on the road through the dry weather. It mixes with the water and forms a film over the surface at exactly the moment grip is already reduced by the wetting itself. This is why the first shower of a season catches people out and the third one does not.
No, it is easier, and for a specific reason: rain has edges. A shower has a beginning, an end and a direction of travel, so the same stretch of road is dry at one hour and wet at another. Wind is the opposite case -- an exposed ridge is exposed at every hour, and only the intensity changes. Rain is the one condition on this site where shifting the departure time reliably moves you out of it.
Not reliably, and this is where a forecast has to be read carefully rather than feared. Whether standing water on a specific stretch is likely depends on where that stretch drains, which is a property of the ground and the drainage rather than of the forecast. Wayther forecasts weather and does not report flooding or road closures. Flash flood warnings, where your area has them, are the authority on water on the road.
Precipitation and visibility at every point along the route, up to ten days ahead. Shift the departure and watch the band move off your miles.
Forecasts come from Apple Weather and cover wherever Apple Weather is available, in metric or imperial units. Wayther plans a trip; it does not navigate, and it does not report flooding or closures. Keep your maps app for directions, and follow official weather warnings and road closures.
