Waypoint 1: Reading Warning Levels

Why warnings come in levels

A forecaster's job is part science, part communication. They might know that a storm has a 60 percent chance of bringing damaging gusts to one coastline and a 20 percent chance of flooding inland. Millions of people can't read a probability map before breakfast, so warning services compress that knowledge into a small number of levels that anyone can understand at a glance.

The trade-off is simplicity versus precision. Too few levels and every warning sounds the same; too many and nobody remembers what level four means. Most services settle on three or four steps, and almost all of them now ask the same two questions: how likely is this, and how much harm could it do?

Colours describe impact, not only weather

The UK's Met Office issues yellow, amber and red warnings. Each warning is placed on an impact matrix that crosses likelihood with potential impact. That's why a yellow warning can mean two quite different things: a high chance of low-impact weather, or a low chance of something more serious. Reading the text tells you which one you're facing.

Across Europe, Meteoalarm — coordinated by EUMETNET, the network of European national weather services — gathers national warnings onto one map using green, yellow, orange and red. Green means no special awareness is needed. Yellow asks you to be aware, especially if you're outdoors or travelling. Orange means dangerous weather is expected, so be prepared. Red signals very dangerous, often exceptional weather: follow the instructions of the authorities.

The important habit on this stretch of trail: don't treat a colour as a forecast of wind speed or rainfall. Treat it as a forecast of disruption to your life.

Watch, warning, advisory

In the United States, the National Weather Service uses words instead of colours. A watch means conditions are favourable for a hazard such as a tornado or hurricane — keep an eye out and get ready. A warning means the hazard is occurring or imminent — take protective action now. An advisory covers conditions that are less serious but can still cause trouble, like dense fog or minor flooding.

A handy memory trick from fellow walkers: a watch is "watch the sky," a warning is "go now." Japan's Meteorological Agency adds a further tier. Since 2013 it has issued emergency warnings when an event is expected to be so severe that it may occur only once in several decades. When you hear that phrase, it is the top of the ladder.

Reading the small print

Once the colour or word has your attention, slow down for the details. Nearly every warning answers the same questions:

• What: wind, rain, snow, ice, thunderstorm, heat or coastal flooding.
• Where: the counties, regions or postcodes affected.
• When: a start and end time — the warning is not valid outside them.
• Impacts: likely effects, such as power cuts, closed roads or flying debris.
• Actions: what the issuer wants you to do.
• Who: the issuing authority, so you know the warning is genuine.

Warnings are also living documents. They can be upgraded, downgraded, extended or cancelled. If you read a warning in the morning, check again before you travel in the evening.

Practice at this waypoint

Before you move on, open your own national weather service's warnings page on a calm day. Find the legend that explains its levels and note down, in one line each, what every level asks you to do. Stick that note on the fridge or save it in your phone. When a real warning arrives, you won't need to translate — you'll already know the language.

Remember: this guide explains how warnings work. The warning itself always comes from your official weather service.