Wayther Glossary
The words on your forecast.
Fifteen terms, what each one actually means, and where it appears on your screen. No thresholds, no advice, and no figures that are not sourced.
Temperature
Rain and visibility
Wind
More trips are changed by wind than by rain
- Wind speed
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An average, not a moment — and measured ten metres above the ground, which is where instruments are standardised. The air near the road is slower, because the ground drags it down, which is why a bridge or an exposed ridge is windier than the countryside around it.
In WaytherShown at every forecast point, so an exposed stretch is one entry in a list rather than an absence.
- Gust
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A short peak lasting seconds, sitting above the average. Services report the two separately because the average describes the air and the gust describes what pushed you — and for anything tall, the gust is the one that matters.
In WaytherShown beside the speed at every point on the route.
- Crosswind, headwind, tailwind
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The same speed is a different problem in each direction. A crosswind pushes on the flank and tries to rotate the vehicle; a tailwind from behind a high trailer loads the axle. This is why direction on a forecast is not decoration printed next to the number.
In WaytherWind direction is shown at every point.
- Why a tall vehicle feels more
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The force grows with the area pushing on it, and the turning effect about the wheels grows with the height it is applied at. A high-sided vehicle is a lever, not a block — so a figure on its own is not a decision until you read it against how exposed the road under you is.
In WaytherWind and gusts run the length of the route, so the exposed stretches are specific miles.
Temperature
Why the top of the hill is not the same as the bottom
- Lapse rate
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Air temperature falls as you climb — on average about 6.5 °C per 1,000 metres, which is the international standard atmosphere figure. It is an average rather than a rule, but the direction is dependable: a pass 1,500 metres above an 8 °C valley is not 8 °C.
In WaytherTemperature is shown per point, so a climb is a separate value from the valley.
- Freezing level
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The height at which the air reaches 0 °C. Above it precipitation falls as snow, below it as rain, and where a front lifts warm air over cold air the two meet. It is the ordinary reason one part of a route is being rained on and another is snowing.
In WaytherTemperature and precipitation are shown per point, so the change along the route is visible.
- Wind chill
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How cold exposed skin feels, and defined only below about 10 °C and above about 3 mph. It measures heat loss through skin, so it says nothing about how cold a cabin, a trailer or a loaded deck gets, and it cannot indicate frostbite when the air temperature is above freezing.
In WaytherWayther reports air temperature, not a wind chill figure — the right number for planning a vehicle. On a motorcycle in cold weather it is the more useful of the two.
Rain and visibility
What you can be inside, and what you cannot see
- Rain and showers
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Steady frontal rain does not care how long you drive in it. Showers come from convection, are patchy, and pass through — which makes one of them something you can sometimes drive under and the other something you cannot.
In WaytherPrecipitation is per point and timed to your arrival, so a shower is attached to the stretch of road it falls on.
- Freezing rain
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Rain that freezes the moment it touches something cold, formed when it falls through a shallow layer of air just below freezing. It is the serious one precisely because it is invisible in the air: the road freezes first and you find out by driving on it.
In WaytherExtreme temperatures are flagged along the route, which is the warning that it is possible on the stretch ahead.
- Visibility
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The distance at which something is still visible, reduced by fog, rain, blowing snow, spray and dust. It is one of the few numbers where the change matters more than the value: the step from 10 km to 2 is the difference between a drive and a crawl. The common fog is the dawn one, which forms on clear still nights, is thickest before sunrise, and usually clears within a couple of hours of it.
In WaytherCharted along the route and listed for every point.
Your route
The numbers on the screen, and what they attach to
- Forecast point
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A location along your route where the forecast is evaluated — not a town, a point on the line. This is the distinction the app rests on: the interesting part of a journey is rarely the start or the destination.
In WaytherThe map, the charts and the timeline are three views of one list of these.
- Departure and arrival time
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The moment you reach each point, worked out from when you leave. The forecast shown is the one valid at that moment, not the one for the day — and moving the departure moves every arrival on the route with it, which is the only way to find out whether leaving later gets you past the weather rather than through it.
In WaytherUp to 10 days ahead, by departure time or by arrival time.
- Day and night driving
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The split of a trip between daylight and dark, with sunrise and sunset on the route. It is there because an hour of delay turns a daylight crossing into a night drive over unfamiliar road, with the temperature on the way down.
In WaytherThe split is in the summary; sunrise and sunset are listed per point.
- Alternative route
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A different path between the same two points, with its own travel time, distance and forecast. For weather the useful comparison is not always the shorter one — the sheltered road is often also the scenic one, and sometimes the faster one.
In WaytherThe main alternatives are listed side by side, each with its own forecast.
- Severe weather alert
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An official warning, published by a weather authority rather than by any app. It is surfaced here because a warning that never appears on the route you are planning is one you meet unplanned — but the official warning remains the record of what has actually been declared.
In WaytherCalled out on the summary alongside high winds, extreme temperatures, heavy rain and poor visibility.
Where this comes from
Two figures, both sourced
Everything above is standard meteorological definition. The two numbers in it are not: the 6.5 °C per 1,000 metres lapse rate is the temperature gradient of the international standard atmosphere, agreed by the ICAO, the WMO and the ISO; and the wind chill limits — below about 10 °C and above about 3 mph, describing heat loss from skin — follow the index as defined by the national weather services of the United States and Canada.
Four figures you might expect are deliberately missing, because none of them is a physical constant: a wind speed at which driving is unsafe, a visibility distance at which driving becomes dangerous, a threshold for heavy rain, and a gust speed at which a bridge closes. The first depends on your vehicle and the road's exposure, the second is an advisory figure that differs by authority, the third is set by each forecast provider, and the fourth is a decision by whoever controls the road.
This page explains vocabulary. It does not forecast anything, and it is not a substitute for an official warning or a road closure.
See these terms on your own route
Every entry above is something Wayther puts on your screen, timed to the hour you reach it. Download the app and try every feature on a random route, free.
Forecasts come from Apple Weather and cover wherever Apple Weather is available, in metric or imperial units, up to 10 days ahead. Wayther plans a trip; it does not navigate, and this glossary explains words rather than reporting conditions. Keep your maps app for directions, and follow official weather warnings and road closures.