- An essential part of calculating the average rainfall is the use of a rain gauge. Rain gauges are thin, cylindrical tubes with measurement marks along the side. When rain falls, a small portion of that rain is funneled into that tube, giving the total inches for a certain period of time (such as 24 hours).
- After 24 hours have expired, the amount of rain collected by the rain gauge is noted and recorded, and the rain gauge is emptied to ensure accurate future measurements.
- After a year, the total amount of rain that fell on a certain spot is added up, giving total rainfall for that year. That total is added to the previous totals from years past, then divided by the number of years the records have been kept. For example, if a mountain gets 100 inches of rainfall the first year, 110 the second year and 90 the third year, the total amount of rainfall (300 inches) is divided by 3 years (the number of years recording) to "average" 100 inches of rain each year.
