How to Calculate the Chance of Rain

Look at all of your weather instruments and record the data in a notebook. Record the wind direction by looking at your weather vane. Record the atmospheric pressure using the barometer. The information is to be written down in millibars mercury. Write down the mercury levels located on the barograph. All of this information will help you determine your confidence level based on a scale of 1 to 100 in a later step.

Go online and find a weather map of your local area. These can be found on local news channel websites or on websites like weather.com. Measure the size of the entire cloud formation on the weather radar in inches. This can be done with a tape measure. At the bottom of the weather radar there will be a map scale. Use the map scale to convert the inches into miles. This will give you the size of the cloud formation in miles.

Look at a rain predictions table from the National Weather Service and decide from a scale of 1 to 100 how confident you are that it will rain in your local area. The information you gathered from step 1 will be used to help you determine your confidence number. The number you write down will be your confidence percentage.

Take the square miles of your city and divide it by the square miles of the rain cloud formation you got from step 2. Take the number from that calculation and multiple it times 100. This number represents the percentage of your local area that will receive precipitation.

Use the formula "PoP= C x A" to calculate the chance of rain. This formula is used by the National Oceanic and Atmospheric Administration. The formula stands for probability of precipitation equals the confidence percentage times the percent of the area that is to receive precipitation. This number will be the chance of rain in your area.