How to Calculate Probability for Weather

By Adam Benjamin

Many people wonder how the local weather forecaster comes up with the probability of precipitation. Most laymen call this percentage the chance of rain or snow. The percentage tells you the chance precipitation will fall at a point within your town or city. If you are interested in the coming weather, try a simple version of the forecast the weather forecaster makes.

Probability Calculation Instructions

Read your weather instruments. Look at the weather vane and take down the wind direction. Take a look at your barometer to see the current atmospheric pressure in millibars mercury. Read the recent mercury levels on the barograph and write down the trend. The National Weather Service uses nine trends: steady, rising or falling, rising or falling slowly, rising or falling quickly, and rising or falling rapidly.

Measure the size of any oncoming rain-cloud formation, or snow-cloud formation. Find a current local weather map. Try your local weather service, or look one up online. Locate the oncoming rain cloud formation, the set of dark clouds. Measure the area in inches. Using the map's scale for conversion of inches to miles, calculate the formation's square mileage.

Determine your confidence that it will rain in the area. Use a table from the National Weather Service that gives rain predictions derived from wind direction, atmospheric pressure and pressure trend. After you look up your current readings, give your confidence a number from 1 to 100. This number is your confidence percentage.

Estimate the percentage of the area that will have rain. Assume the oncoming cloud formation will come over your town or city. Divide the total square miles in your city by the square miles in the rain cloud formation. Multiple the number times 100. When the formation comes over the area, this number is the percentage of the area that will have rain or snow.

Calculate the probability of precipitation. The official formula from the National Oceanic and Atmospheric Administration is: PoP = C x A, where C is the confidence percentage and A is the percentage of the area with precipitation. Put your percentages into this official formula. Multiply your confidence times the area estimate. You have the probability it will rain at a point in your city or town.