# What does it mean in weather forecasting when the weatherperson says “40% chance of rain”? What factors are being considered to come to this prediction?

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A lot of times on the news, the meteorologist says, “today there is a 40 percent chance of rain” or “80 percent chances of snow”. What factors go into making this decision?

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It’s not that there is a 40% chance of rain. It’s 40% of the forecasted area will get rain, they just don’t know 100% sure where exactly. Obviously the further out the timeframe is then the less accurate the prediction.

As others said, the “40%” means they think that 40% of an area will get rain. They cannot be certain if it will be one neighborhood or another though.

And as for what goes into that, weather predictions are based almost 100% on taking the current weather conditions that you have, and comparing it to past data and what happened in the past.

That is how weather models on computers work, they take the current weather data and compare it to DECADES of data that has been collected. So if the computer sees that condition XYZ is happening now, and that if the past 10 times XYZ happened it rained 8 of those 10 times, well it’s probably gonna rain again then.

And removed that the “XYZ” condition involves taking multiple kinds of data from multiple sources.

Models to predict weather rely on historic weather data. The models will look for times when conditions today are similar to conditions seen in the past. Now, weather has a lot of factors that play into it. Local temperature, pressure, and humidity as well as the factors of nearby regions that may cause weather fronts to move and change. As such, conditions today are likely not to be exactly like those seen in the past. So models will take the closest examples and examine how often those examples resulted in precipitation. For example, if 40% of similar conditions in the past resulted in rain, the weather prediction will report a 40% chance of rain.

From Weather.gov:

>To summarize, the probability of precipitation is simply a statistical probability of 0.01″ inch of more of precipitation at a given area in the given forecast area in the time period specified. Using a 40% probability of rain as an example, it does not mean:

>(1) that 40% of the area will be
covered by precipitation at given time in the given forecast area or

>(2) that you will be seeing
precipitation 40% of the time in the given forecast area for the given forecast time period…

>If a forecast for a given county says that there is a 40% chance of rain this afternoon, then there is a 40% chance of rain at any point
in the county from noon to 6 p.m. local time.

It means they are predicting that 40% of the area in which they’re discussing will be getting some rain. They watch weather patterns and use past data to make their best guess.