ASK TOM WHY: How can a rain gauge catch the rain when the wind is high?

Dear Tom,

 

My rain gauge reads low when it is windy. How can a gauge catch the rain when the wind is high?

 

Rob Collins

 

Dear Rob,

 

You have identified a very real problem. It is impossible to obtain accurate rainfall measurements in high-wind conditions. Even assuming nearby obstacles (buildings and trees, for example) do not block raindrops destined for the rain gauge, the body of the rain gauge itself acts as an obstacle to air flow. Air is deviated around and over the gauge and its speed increases. Raindrop trajectories are modified and smaller drops are blown away, resulting in an undercatchment error that can be as great as 80 percent in winds above 50 mph. Air foils to reduce wind speed over the gauge are helpful, but only below about 30 mph. Radar provides a reasonably accurate backup measurement of rain totals.

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