How to read a weather forecast

The symbol is only a summary

A single cloud-and-rain symbol for a day can mean a wet morning and a dry afternoon, or a dry day with one heavy shower. It is chosen from the most significant weather in the period, so a day with an hour of drizzle can carry a rain symbol while being mostly fine.

That is why our daily rows put the rain amount and the number of hours it is expected to rain next to the symbol. Two millimetres in about two hours is a passing spell of rain. Fifteen millimetres over ten hours is a wet day whatever the symbol looks like.

What the chance of rain really means

A 40% chance of rain does not mean it will rain for 40% of the day, or over 40% of the area. It means that, in weather situations like this one, rain falls at that spot on about four occasions in ten. It is a statement about likelihood at a point.

The useful trick is to read the chance and the amount together. A high chance with a small amount means drizzle or light rain is likely but will not add up to much. A low chance with a large amount is the signature of showers or thunderstorms: most places stay dry, but where a shower hits, it pours.

Beyond a few days we replace the percentage with the number of model runs that give at least 1 mm. Seeing that 38 of 51 runs bring rain on a Saturday tells you both the likelihood and how much the models disagree, which a single percentage hides.

Wind: always look at the gusts

The average wind speed is what blows steadily; gusts are the brief peaks that slam doors, topple bins and catch high-sided vehicles. On a blustery day the gusts can be half as strong again as the average. For anything wind-sensitive, from a bike ride to a garden gazebo, plan around the gust figure.

Direction matters too. Our arrows point the way the wind is blowing to, so a south-westerly wind has an arrow pointing north-east. In the UK, south-westerlies usually bring mild, damp air; northerlies and easterlies bring the cold.

Feels-like temperature

The feels-like figure adjusts the air temperature for wind, humidity and sunshine. On a raw winter day with a strong wind it can be five or more degrees below the actual temperature, which is what decides how many layers you need. In summer, high humidity can push it above the air temperature.

How far ahead to trust it

Forecasts lose detail as they go further out, but they do not all lose it at the same rate. Temperature holds up best, then whether a day is broadly wet or dry, then the timing of rain, and last of all the position of individual showers.

As a rule of thumb, the next two to three days are good enough to plan around in detail. Days four to seven are good for the pattern. Beyond about a week, look at the spread of possibilities rather than one number. That is exactly what our confidence marks and the ensemble chart are for: they show you, day by day, when the forecast stops being a forecast and becomes a range.

Model forecast or observation?

Almost every number you see on a weather site, including the figure labelled now, comes from a forecast model rather than a thermometer. We label ours as model forecasts for that point. Local effects such as a frost hollow, a sea breeze or a city centre's warmth can make your street a degree or two different.