What a Chance of Rain Means
Three short lessons on the number in every forecast: what a "30% chance of rain" is a share of, how that one number is built from two, and how you can check whether a forecaster is any good.
For grown-ups, from what WeatherForge teaches children at Spark & Anvil.
The family forecast jar
You'll need: A jar, slips of paper, a pen, and a week or two of evenings.
- Each evening, everyone writes their chance of rain for tomorrow (0%, 30%, 50%, 70% or 100%) on a slip and puts it in the jar.
- The next evening, say whether it rained at home. Sort the slips by the chance written on them.
- After a week or two, look at each pile: did it rain on about as many days as the number promised?
- Talk about who was most often right, and who was most sure when wrong. Nobody wins; the jar keeps count.
A little history, and what nobody knows
Words or numbers
In November 2011 the UK’s Met Office replaced words such as “likely” with a percentage chance of rain in its forecasts.
How people read a 30% chance is not settled: the source notes it is widely misunderstood.
The same idea at every age
- Ages 3–5Rainy Days
- Ages 6–8Below Zero
- Ages 9–14Flash to Bang and 4 more
- Ages 15–1830% Chance of Rain and 3 more
Your child will meet this idea again at every age. The tools and courses here are the part for you.