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?

Question

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?

In: 2

Answer 1

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.