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
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Probably because most people care more about whether it will rain or not than about how much rain there will be. If you’re planning an outdoor activity or wondering whether to bring an umbrella, 5mm and 50mm are the same amount of problem.
Percentage refers to the entire forecast area. Volume can vary greatly from place to place within the forecast area.
Even as wrong as they often are, percentage is the more sensible forecast method that applies the the most people. I have a few friends with weather stations at their houses. We all live within 5 miles of each other. Even when we all get rain on the same days, we usually do not get the same amount.
I use the weather app Foreca. It uses localized forecast points and gives you the option to show volume instead of percentage. I prefer volume.
They kind of do. Its just more qualitative like heavy showers vs light drizzle in the forecast.
Any time you are talking “most” you have to consider that means “What do *most* consumers of this product want?” For a weather forecast. **Most** consumers of weather forecasts just want to know whether or not it’s going to rain, and about what time. How much is usually of little concern to them. They just want to know if they need rain gear, or if it’s a good day to be outside.
Those who want/need more details, like *how much* it might rain have specialized tools available that try and predict that.
With the exception of huge amounts of rainfall, like flood levels, which are given advance warning, I’ve never cared about the volume of rain ever in my life
I agree OP.
If it’s gonna rain for 5mins and be .5mm total – I’ll continue with my plans.
If it’s going to rain for 30mins (or 5 mins, but 6 different times), and be 5mm, I’ll change plans.
I guess it’s just a limitation of the science. Only say what you’re confident is true.
The precipitation forecast is usually also available when you have the forecast in more detail.
But generally you have more chances of getting rain if you have bigger clouds, and bigger clouds means more water to rain, so it’s a good starting point, if you need more info it’s generally available too. The probability is more like a headline
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.
I’d bet your weather agency probably does have amount of likely perception but you have to go look it up.
Weather forecasters have done a poor job of communicating what chance of rain means. 50% chance of rain means that if you take the geographic area in question and watch it for the period of time in question, half of the area will be rained on. Your driveway could be wet and neighbor’s could be dry… still 50%. It doesn’t mean “given these conditions, half of the time it WILL RAIN and half of time it WILL NOT RAIN. Meteorologist don’t have enough data, and it isn’t practical to narrow down specific locations, so they basically just say “Atlanta” has a 90% chance of rain instead of attempting to say “Main Street” will be rained on from 8:00am to 3:00pm.
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