ASK TOM WHY: What does "trace" amounts of rain or snow mean?

Dear Tom,

 

I sometimes hear of “trace” amounts of rain or snow. Just what does that mean?

 

Roger Reilly, Milwaukee

 

Dear Roger,

 

Liquid precipitation is measured in hundredths of an inch (0.01″) and trace amounts are defined as less than half that amount (0.005″). Snow is measured in tenths of an inch (0.1″) and trace amounts are defined as less than half that amount (0.05″). When very light precipitation occurs, a weather observer must determine if the measured total equals or exceeds the trace amounts. The water content of snow is determined by collecting the snow in a rain gauge, melting it and then measuring it as if it had fallen as rain. Snow depth is measured to the nearest tenth of an inch of accumulation on the ground before melting. Snow that falls when temperatures are well above freezing and melts upon contact with the ground is recorded as a trace accumulation.

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