Severity

The five ratings

Severity is how much a section’s weather affects the drive, on one five-step scale used everywhere in the app. A rating is never a colour alone: it is always a colour, a shape and a word together.

The shapes escalate with the rating — an outlined circle, a filled disc, a triangle, a diamond, an octagon — so they can be told apart without colour. The colours run from cool to warm and are checked for the common kinds of colour blindness.

How a section is rated

A section’s rating comes from one thing: the kind of weather forecast for it — rain, snow, fog and so on — and how intense it is.

1. Each kind of weather has a starting level. Rain starts at CAUTION, snow and fog at HAZARD, freezing rain at EXTREME; overcast sky is MILD.
2. Intensity moves it. Light weather is one step lower and very light two; heavy is one step higher and very heavy two. Light rain is MILD; heavy rain is HAZARD.
3. Some weather has a floor. However light, anything freezing — freezing drizzle, freezing fog, freezing rain, ice pellets — is at least HAZARD. Fog and thunderstorms are at least CAUTION. Tornadoes, funnel clouds and waterspouts are always EXTREME.

The rating is judged by what the weather does to driving rather than how dramatic it is. That is why very light freezing drizzle, a trace of it, rates as high as heavy rain: it freezes where it lands.

What does not change a rating

The rating is not adjusted for darkness, for the road surface, or for the individual readings in the section — wind, gusts, visibility, how much falls or the temperature. Those readings carry small marks of their own when they cross a threshold, and a section rated CAUTION with a wind worth flagging is still CAUTION. Conditions lists each reading’s thresholds.

The drive’s rating

A drive is as severe as its worst section. That sets the headline at the top of the timeline, such as “Weather that could slow you down”, and the WORST figure in your list of drives.

IPHONE & IPAD

ANDROID

What the rating is not

It is not an official warning, and it is not a road report. It says how the forecast weather should affect the drive, section by section; it does not know about closures, accidents or road treatment. Official warnings are shown beside it and never change it: see Official alerts.