What does 20% chance of rain really mean?

Question

What does 20% chance of rain really mean?

In: Other

Answer 1

According to National Weather Service rules, percent chance of rain is (what percent of your area will see rain) * (confidence in forecast.)

So, if half your area will see rain (.5) and you are 80% sure (.8) you would say 40% chance of rain.

The idea is to answer the question: If someone lives in this area should they bring an umbrella? (They calculate how risky they want to be given the percentage.)