If you're anything like us, you check the weather app before planning any outdoor activities. And when you see a chance of rain percentage listed, it's easy to take action and prepare accordingly. If the forecast says there's an 80% chance of rain, you probably cancel your plans. Or if you see 20%, you might think it's okay to go ahead with your afternoon hangout.

But despite the fact that we use this information all the time, it turns out most people don't actually know what the percentage really means. According to a study by Dr. Mark Williams, even meteorologists don't agree on how to calculate Probability of Precipitation (PoP). The result is that most weather apps don't give a full explanation.

The most common method is to use PoP as a probability of seeing measurable precipitation over a given area. That's why a "30% chance of rain" actually means 30% of the forecast area has a 30% chance of experiencing measurable precipitation.

Other methods include using a combination of the confidence level in the precipitation occurring and the expected areal coverage. This calculation is used by some TV stations, including Nexstar's KXAN in Austin, Texas. But it's not the most accurate way to measure rain chances, according to Dr. Williams' study.

In fact, it can sometimes be misleading because it doesn't tell you how long the rain is expected to last or how much total rainfall will occur. So if you want to be 100% sure about the likelihood of rain, you should use the Met Office's website.