A 60% chance of rain: will it rain or not?

The Korea Meteorological Administration defines the chance of precipitation as the likelihood of rain or snow under weather conditions similar to the forecast. The threshold is 0.1 mm or more. Even a few drops landing on your palm count as a rainy day.

That is why the chance of rain tells you nothing about how much rain will fall or how hard. A 90% chance can mean a brief sprinkle, while a 40% chance can turn into a downpour once it starts. To see how much is coming, check the expected rainfall separately.

Mixing this number up is common. In 2005, a German-led research team asked people on the streets of Amsterdam, Athens, Berlin, Milan, and New York what a "30% chance of rain tomorrow" means. New York was the only city where more than half picked the meteorologists' meaning: rain on three out of ten similar days.

In the European cities, the most popular answer was that it would rain for 30% of the day tomorrow, followed by rain over 30% of the area. People looked at the same number and pictured different things. The researchers concluded that forecasters should say what the probability is counting.

If the chance of rain was 70% and the sky stayed clear all day, it is easy to feel the forecast missed. But a 70% chance also says that on three out of ten such days, it will not rain. A single day cannot tell you whether the forecast was right or wrong.

You can only judge a forecast by looking at many days together. If, across all the days marked 70%, it actually rained about seven times in ten, the forecast did its job.

In the end, you are the one who decides what to do with the number. On days when getting wet would be a real problem, like a job interview or carrying a laptop, tucking a folding umbrella in your bag at just 30% brings peace of mind. On days when a little rain is no big deal, you can wait for a higher number.

Checking the hourly forecast helps too. Looking at the chance of rain and the expected rainfall for the hours you will be outside makes it easier to tell when you will actually need an umbrella.