Why do most online weather forecasts only state the chance of rain as a percantage, without stating the expected rainfall in millimetres? If there’s a 90% chance of rain, it matters a lot whether it will be 0.5 or 5mm

Question

Why do most online weather forecasts only state the chance of rain as a percantage, without stating the expected rainfall in millimetres? If there’s a 90% chance of rain, it matters a lot whether it will be 0.5 or 5mm

In: 29

Answer 1

So percentage in precipitation is a bit confusing to understand, but there is a clear explanation. It’s not just the chances of it raining. It is actually, for a region, the chance of rain (forecasters confidence) multiplied by the percentage of area it is expected to receive.

So if there forecasters think, after using their fancy maps and cloud data and magic, that there is a 100 percent chance that it will rain in 50% of the area, the POP(probability of rain) = C x A (confidence x area) = 100 x 50% = 50%.

If they think there’s a 60 % chance of rain in 60% area, there Probability of rain is 36%.

So it may show you a hundred percent chance of rain, but you may never see a drop.