Rain chances are not timers

Use place, time, temperature, wind, and rain chances to make a plan.

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A forecast predicts conditions; an observation records what happened. Check location, valid time and units. Rain probability describes a chance at a point during the stated period, not how long rain lasts. Update your plan as forecasts change.

Worked example

At the practice station, morning is 12 C with 20% rain chance; afternoon is 19 C with 70%. Afternoon is warmer and has the higher rain chance, not a guarantee of rain.

Fictional forecast table: morning 12 C and 20 percent rain, afternoon 19 C and 70 percent rain, evening 14 C and 40 percent rain.

Would you believe it?

A 40% chance of rain is not a promise of rain for 40% of the day. Probability is not a stopwatch.

National Weather Service: rain probability

Question 1 A 40% rain chance means rain for 40% of the day. Is that right? No Yes Only in winter

Question 2 What is the afternoon-minus-morning temperature difference? 31 C 12 C 7 C

Question 3 Which value is a probability? Afternoon 70% 19 C

Question 4 Does 70% guarantee rain at your location? No Yes It guarantees exactly seven drops

Question 5 What extra information helps plan outdoor activity? The color of your socks Only yesterday at another city Wind and official alerts

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