What Does a ‘Chance of Rain’ Really Mean?

With Bangkok in the middle of the rainy season, I find myself checking the weather on my phone more often. My weather app gives the forecast at different times of the coming day, the temperature, a little graphic showing whether it will be sunny, cloudy or raining, and a percentage of the chance of rain. This last piece of information was never taken very seriously by me, as I figured it was just sort of a roll of the dice by the meteorologists. How could anybody predict with any certainty what the odds were that it would rain other than by looking at the sky and guessing?

Probability of Precipitation

That’s when I found that the ‘chance of rain’ figure is more involved than simply guessing. The percentage that represents the chance of rain is properly called the “Probability of Precipitation” (PoP). The figure given represents the chance that the area covered by the forecast will see at least a small amount of rainfall somewhere within its boundaries.

This also means that before you can accurately use the figure to decide whether or not you should bring an umbrella with you when you go out, you need to know the size of the area your forecast covers. If your forecast area is a large one, it reduces the chances that rain may fall on you. Even an 80% chance over an area the size of Bangkok means you may stay dry all day. However, if you see only a 30% chance of rain, but the forecast covers only the street you live on, you should bring along that umbrella.

People who are aware of the weather patterns in their area can get pretty good at looking at the PoP for their area and knowing their chances of getting rained on. So, use the percentage of rain figure as a tool in deciding how prepared for rain you should be.