Problem with maximum rain rate

It’s difficult to know how the maximum rain rate in WD is arrived at. I have had WD stating on the normal screen that the maximum rain rate was 360mms/hour and when I look at the logfiles which occurred at the same time, the rainfall in 1 minute was 1.1mms which should give 66mms/hr. I’ve seen this happen often and don’t understand how WD arrives at this answer. I know it’s possible to measure the rainfall in a shorter time slot than 1 minute but it then becomes subject to errors because of the resolution of WD which only records to 0.1mms, even if the rain gauge has a finer resolution.

I hope an explanation of this is possible. I am using WD version 10.37Q build 46 in client mode and I feed all the meteorological data (including the rainfall) into WD every 2 seconds.

John

the rain rate, mm/min, is calculated on the fly every second, i.e a real time value
it will start to go down if the next tip of the rain gauge was a longer period than the last time between tips
the mm/hour is an extrapolation of the mm/min value

the value stored in the logfiles is just the amount of rain fallen between each minute update (i.e when the clock changes to a new minute)

OK, thanks Brian, that’s a good explanation. So the maximum value reported by WD is dependent on the rainfall tip which is not the actual rainfall rate?

The reason I ask is because a local resident is having a dispute with his insurance company about paying up for flooding damage. If I quote 360mms/hour (which is what WD reported) I don’t think they would believe it, and rightly so. The figure obtained in the log file (mms in the last minute giving a maximum of 1.1mms or 66mms/hour) seems to give a much more believable value and when I plot this figure against time I get a gradually increasing rate which peaks and then falls afterwards.

So I plotted out the result and we will see if the insurance company respond to that. They claimed that during the period concerned the maximum rate couldn’t have been greater than 16mms/hour. They obtained that figure from the local Met Office site which is at sea level and 15 miles from here whereas we are at 600 feet above sea level.

John

the 360mm/hour is an instantenous peak rate
if it stayed that heavy for 1 hour, then you would have got 360mm
but of course the rain does not stay as heavy as that peak
so you are better to look at the amount fallen over a time period

a good way to do that in WD, not long after the rain event, is go to view, real time graph
then left mouse click at the start of the rain in question, then hold and move the mouse to the end of the rain in question then let go of the left mouse click
the amount of rain fallen and the time taken is then shown

OK, Brian, thanks for that. I hadn’t thought of using the real time graph. If it’s set to 5000 maximum records, does it continue to log data, or does it stop once 5000 records have been received? The problem is that one does not always know that a record of some parameter (in this case rain rate) is going to occur and if the real time graph can be left running at, say a sampling rate of every 10 seconds, that would be fantastic and would show exactly what is of interest. In the meantime I’ve set it running at a sample every 30 secs and I will see what that produces.

This is one more facet demonstrating the versatility of your software. I wish I had known about it sooner!

Best wishes,

John