Rain must fall, but the only way to be sure exactly how much precipitation has accumulated is to use a precipitation gauge. It is commonly thought that the first precipitation gauges were used by farmers of the Choson Dynasty (now Korea) who used the data to determine the potential harvests of their farms. Like those farmers, you can make your own precipitation gauge, but for the most accurate readings, purchase one from a manufacturer who specializes in making them. (See Resources)
Prepare a spreadsheet on your computer. Create 12 column headers, one for each month of the year. Each cell in a column represents a day of that month.
Place your precipitation gauge outside, on or near the ground in an open area. Make sure the gauge isn't under any obstacles, such as trees, that would prevent you from obtaining an accurate reading.
Check the gauge daily at the same time each day. Determine how much precipitation has fallen for each 24-hour period by looking at the marks along the side of the gauge. Record each day's precipitation total in your spreadsheet. Empty the gauge every day.
Determine the precipitation for each month by adding the totals for each day in that month.
Add the monthly precipitation totals to determine the annual precipitation.