How are trace amounts of precipitation accounted for in total precipitation?

Dear Tom,

I know what trace amounts of precipitation are and how they are measured. How are they accounted for in total precipitation, especially if there are many in the same time period?

Dave McAlister

Dear Dave,

Longtime Chicago weather observers Frank Wachowski and Paul Kubecka informed us that a trace of snow or rain is an amount too small to measure and is not cumulative from day-to-day. Hypothetically, there could be a trace of precipitation every day of the year and the annual precipitation would still be a trace. They did note, that on an hourly basis, traces can build into measurable precipitation (0.01 inch for rain and 0.1 inch for snow). If very light precipitation was falling, there could be a trace after one or two hours, but the total could eventually build to a measurable amount within a six-hour period before the rain gauge is emptied.

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