We often talk about the ‘accuracy’’ of weather predictions, and it’s been mentioned forecasting becomes one

Question: We often talk about the ‘accuracy’’ of weather predictions, and it’s been mentioned forecasting becomes one extra day ‘accurate’ every 10 years as computing and understanding increases (e.g.8 day forecast). What does ‘accuracy’ of weather mean? How do we measure it?
- Keywords:
  accuracy,
  prediction,
  weather
Asked by Bryn to Sophie, Roz, Christina on 7 Jan 2020.
- Christina Pagel answered on 7 Jan 2020:
  
  There are lots of different ways of measuring the accuracy of forecasts but this is how I would do it: on days where there is an X% chance of rain, did it rain on X% of those days?
  
  So if in one year there were 50 days with an 80% chance of rain, did it rain on 40 of those days (80%)?
  
  You can get more sophisticated about it by adding in geographical constraints (e.g. did it rain where it said it was going to rain), rain totals (how much did it rain compared to how much was predicted) and so on.

Question: We often talk about the ‘accuracy’’ of weather predictions, and it’s been mentioned forecasting becomes one extra day ‘accurate’ every 10 years as computing and understanding increases (e.g.8 day forecast). What does ‘accuracy’ of weather mean? How do we measure it?