Dear Irving and WeatherCat estimators,
I am pondering over the calculation of rain rate (mm/h) from mm rain versus time data. Attached is a set of data from WC. Mine is a bucket-type rain gauge. Can someone explain how to make the calculation?
Well it really depends on what you want to use the rainfall rate data for. Strictly speaking, the rainfall rate is computed by your station by measuring the time between bucket tips. On a Davis station, you can measure 0.01" of additional rainfall per bucket tip. The smallest rainfall rate that Davis claims it can measure is 0.04" per hour. That's one buck tip every 15 minutes.
The situation is completely the opposite when it comes to heavy rains. A rainfall rate of 2 inches of rain per hour means that if it were raining continuously at that rate - your rain gauge would have tipped 200 times in an hour. 200 tips is more than once a minute. To find out how many seconds between tips divide 3600 (60 minutes per hour x 60 seconds per minute) by 200. The answer in one tip every 18 seconds.
Because rain can happen in brief bursts of heavy downpours, you can generate rainfall rates that are extreme. You can pick you 2 hundredth of an inch of rain in minute. That's a rainfall rate 1.2" of rain per hour if the rain kept coming! However, the downpour could stop in a minute.
The only rain data you have in WeatherCat is the total rainfall that fell between each sampling interval. That is only telling you the total number of bucket types in that time. If you had a sampling interval of 5 minutes (like I do,) if 0.02" rain fell during that past 5 minutes you would have no idea that it fell only in 1 minute instead of 5. That would lead to an incorrect estimate of the rainfall rate of 0.6" per hour instead of the 1.2" per hour that actually happened.
That's why weather stations compute the rainfall rate every time the rain gauge bucket tips. Even this has limitations. You can only measure changes depending on the size of the rain bucket. Still for an accurate station that's usually good enough.
Cheers, Edouard
