- Your spot
- One time slot
- A chance, not a share
- Read it with the symbol
THE RIGHT READING
4 in 10 times
On about 4 of every 10 occasions with this forecast, at least a little rain falls where you are in that time slot.
See it as ten daysTHE EVERYDAY WEATHER QUESTION
40% chance of rain?
Here's what it means.
Not 40% of the day. Not 40% of the map. Something simpler, and more useful.
The percentage on your forecast is a probability for one place and one time slot. Here is how to read it, where it comes from, and how to decide whether the umbrella comes with you.
Plain English, with the sources linked.
4 IN 10 not 40% wet
See ten days like this
THE SHORT ANSWER
It's a chance at your spot.
A 40% chance of rain means rain falls
on about 4 of every 10 days like this.
NOT THE CLOCK
Not 40% of the day
It says nothing about how long it rains. A 40% hour could bring a five-minute shower, or nothing at all.
How to use the hourly view
NOT THE MAP
Not 40% of the area
Coverage can be one ingredient, but the final number is a chance for any single point in the forecast area.
Where the number comes fromA PICTURE OF PROBABILITY
Ten days
like this one.
Imagine ten days with exactly the same forecast. Choose a percentage to see on how many of them at least a little rain would fall at your spot during that time slot.
The picture shows how often, not which days, how heavy or how long. Forecasts are updated as the day approaches, so check again closer to the time.
WHERE THE NUMBER COMES FROM
Two forecasts,
one number.
The US National Weather Service explains its
percentage as confidence × coverage.
= 40%
80% sure × 50% of the area
The forecaster is confident showers will form but expects them to hit only half the area.
SCATTERED SHOWERS
= 40%
40% sure × 100% of the area
Rain would cover everywhere, but the forecaster is less sure it will arrive in time.
A BAND OF RAIN
Very different weather, same percentage. That is why the weather symbol and the hourly view matter too. Other forecasters, including the Met Office, use their own methods and thresholds, but the result is read the same way: a chance at your location in that time slot.
USE THE NUMBER
Four habits
for drier plans.
Check the hour you'll be out
A day's chance can look high because of one wet spell. The hourly view shows when it matters to you.
Look at the symbol too
The percentage says how likely; the symbol suggests what kind of weather is most likely.
Pick your own threshold
Decide in advance: maybe 30% for a long walk, 60% for a quick errand. Then the number makes the choice.
Check again closer to the time
Forecasts are updated through the day, and the chance for a given hour often firms up as it approaches.
RAIN FACTS
Five things worth
a second look.
Most people read it differently
A study of pedestrians in Amsterdam, Athens, Berlin, Milan and New York found that only in New York did most give the standard reading. Many took a 30% chance to mean rain for 30% of the time.
Read it with the symbol
The Met Office says the chance of rain should be read alongside the weather symbol. A 20% chance with a cloud symbol means mostly dry, with a small chance of rain.
Raindrops aren't tear-shaped
NASA explains that small drops are nearly round, while bigger ones flatten underneath like a bun as air pushes against them. Drops above about 4.5 mm break apart as they fall.
The smell has a name
Petrichor, the scent of rain on dry ground, was named in 1964 in the journal Nature from the Greek petra, stone, and ichor, the fluid of the gods in myth.
Some of the UK gets metres of rain
According to the Met Office, sites in Snowdonia, the Lake District and the Scottish Highlands each receive more than 4 metres of rain in a year.
QUICK QUESTIONS
Chance of rain, answered.
Short answers to the questions people ask most.
What does a 40% chance of rain mean?
It means that on about 4 out of 10 occasions with a forecast like this, at least a small, measurable amount of rain falls at your location during the time slot shown. It is a probability for one place and one period, not a promise of light rain.
Does a 40% chance of rain mean it will rain for 40% of the day?
No. The percentage says how likely rain is, not how long it lasts. A 40% hour could bring a five-minute shower or nothing at all, and an 80% day could still be mostly dry with one heavy burst.
Does it mean 40% of the area will get rain?
Not on its own. In the US National Weather Service method, expected coverage is one ingredient: the percentage combines how confident the forecaster is that rain will develop with how much of the area it is expected to cover. The result applies to any single point in that area.
How do forecasters work out the chance of rain?
One common method multiplies confidence by coverage. If a forecaster is 80% sure showers will develop and expects them to cover half the area, 0.8 × 0.5 gives a 40% chance. Being 40% sure of rain that would cover the whole area also gives 40%.
Is a 40% chance of rain high?
It means dry is still slightly more likely than wet. Whether that is high depends on what is at stake: for a picnic you might pack an umbrella, for a walk to the shop you might not. Check the hourly forecast for the time you will actually be out.
References
- Probability of precipitation explainedUS National Weather Service
- About our forecastsMet Office
- A 30% chance of rain tomorrowRisk Analysis, 2005
- Anatomy of a raindropNASA
- Nature of argillaceous odourNature, 1964
- How much does it rain in the UK?Met Office