How does your Station calculate your Rain Rate.

[JennyLeez]

As I head to delete the Rain Rate from the Highs and Lows Spreadsheet I thought I would start this topic looking for comments so you would be able to see why.
In conversation with a member on and off concerning the RR it has become clear that the rain rate can not be reliably calculated, reported or compared in the Highs and Lows.

If you have a Davis you have the choice of letting the Davis report it or your software whether that Cumulus or WD or one of the others..
For non Davis:
WD reports the max for 1 minute. Multiplying this by 5 minutes or 60 to get the 5 min valve or the 1 hour value would produce an incorrect value in my opinion.
Cumulus reports the highest 5 minute average as its Max rain rate for the day. Dividing that by 5 to gain a 1 minute value or multiplying by 12 for an hour value would also return an unreliable value.

The stations using the WX templates and hence the testtags have me totally confused. I can not find any similarity with the rain rate value whatsoever.
Example:
Station 1 - 1.4 Max rain per minute = 139.4
Station 2 - 1.4 Max rain per minute = 215.1
Station 3 - 0.6 Max rain per minute = 124.0
If anyone could explain how these wx stations are calculating such high rain rates I would appreciate as this has me beat.

Anyways as you see from above, rain rate figures are all calculated and reported differently.
There is no reliable way for me to bring them all in line to summarise accurately. Unless anyone has some great ideas I shall remove from Highs and Lows and any summaries.

One thing I did read a few times in researching all this is that when the rain and wind is extreme, station sensors do not cope and rain rate figures reported would be incorrect and dare I say ' a stab in the dark'

I shall be interested in member thoughts regarding this as reading through the forums I have found a few 'mine is better than yours' topics regarding which is more reliable - That read or that calculated
JL

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[intrepid]

Mine does it by magic!

[Tony]

I thought it was the time between two bucket tips in minutes which gives you x
 60 minutes divided by x, the result is then multiplied by the bucket size which gives you the  hourly rain rate.

Then again... it could be magic!

[einzack]

I agree that as it is not consistent between brands and software, it should be removed.
I was curious about how Cumulus + La Crosse measured this a while back and when I found out I was confortable about using it to compare different rain events of my own, but realised that comparing my rates to anyone elses was pointless.

[JennyLeez]

Yours well may be Tony. All software appears to calculate differently.
With Cumulus 1 tip is classed as 3.6mm
Then for the day high. The highest average 5 minutes is reported for the day's high Rain Rate.

[TokWW]

Rates per hour are always a short period multiplied to the full hour equivalent.  A rain rate is the rate at which it comes down which could be in mm/min or mm/hr - it is the same rate, but over a short period of time, it is a fraction of the volume of that for an hour.

Consider km/hr...  are we going to record revs per second = m/sec or km/min or what and then try and relate that to km/hr, miles/hr, knots or whatever -- which is what is done by the weather stations - ie a small period number is mutliplied to create the full hour rate.

What an anemometer does is record the number of revs in a period - perhaps 2secs and then equate that to m/s or km/hr by using maths.  (Note that practically all units record it in knots and this is converted to km/hr... so Brian Hamilton says) So we know that there are 3600 secs in an hour etc, etc...   

What a rain bucket does is measure an amount per minute by counting the tips per minute.  If in a heavy downpour, I got three tips = 3 x 0.5mm = 1.5mm equivalent rain but in just one minute = 1.5mm/min, then that is at the rate of 90mm/hr - the same analogy as the wind.  If you wanted the real rainfall that fell in one hour, then it is not a rate but a volume but it becomes an average rate over the hour so it can have the same units.

But as you say, very few stations record the rain rate the same as the software and different software calculate it differently as well. 

The same can be said for measuring the average wind:

Wikipedia http://en.wikipedia.org/wiki/Wind

Sustained wind speeds are reported globally at a 10 meters (33 ft) height and are averaged over a 10 minute time frame. The United States reports winds over a 1 minute average for tropical cyclones, and a 2 minute average within weather observations. India typically reports winds over a 3 minute average. Knowing the wind sampling average is important, as the value of a one-minute sustained wind is typically 14% greater than a ten-minute sustained wind.

There are no strict rules for windspeed averaging, although Australian Bureau of Meteorology use a 10min average for average windspeeds.

I agree, there are too many variations on the ways that these rates and averages are calculated.

[Brooklynweather]

mmm interesting subject

[iomkiwi]

Yes - it even made me go and read my Davis Console Manual 

I believe that WD keeps its own rain figures seperate from the console figures. Maybe the /min figures are from WD and the /hr are from Davis? (if you have the box ticked in station setup)

The Davis manual also says that rain rate calculations are based on the interval of time between each tip.
So it is not multiplying a /min volume out to 60 min. It doesn't give a spec for /min rain rate so may not measure/record it?

If the bucket tips 7 times in a minute WD would(?) report 1.4mm.

But it would only take a couple of those tips to be 5 seconds apart and the Davis would report a much larger /hr rate (by my maths around 144mm/hr - a tip every 5 seconds is 12 per minute or 720 per hour, multply by tip volume of 0.2mm)

[Weather Display]

WD uses the time between bucket tips
and the rate then starts dropping when the next time is longer than the time before (that data is updated every second)
so WD is providing an instantenous rain rate
the mm/hour is just that mm/min exptrapolated out to 1 hour (i.e *60)
that is different to the rain in the last immediate 1 hour period
a rain rate in WD of 2mm/minute is heavy rain
note that the Met service classivy sustain rain totals of 10mm every hour as heavy rain and enough to warrent warnings (120mm in 12 hours or so at that rate)
but you get times when the rain rate is much higher, i.e if 2mm/min was sustained then that would be 120mm in 1 hour
I have seen the instantenous rate rate (i.e time between tips) each 6mm/minute

the WD rain rate generally works very well for the Davis stations because that station udpates the bucket tip info frequently
other station types the rain gauge info is passed along at much less frequent intervals, which makes the time between tips not as usefull and in accurate

[JennyLeez]

Thanks heaps for those explanations.

I feel the error in reporting is the valve is not being qualified as to whether per min, per 5 min, per hour etc.
WD clearly states the value is max per minute but is that derived from WD calculations or Davis.
Cumulus I now know calculates the Max from the highest 5 minute interval. But again this would vary depending whether using a Davis or not.
The WX template using the testtag $maxrainrateyesthr I now find this is the Max hourly rain rate although this valve is not qualified in the output. So for the last few months I have been uploading a summary of the rain rate comparing a mixture of 5 min Max and 60 min Max data * shakes head *

To many variables and to those who have asked me to continue summarising the Rain Rate I will think about it. Too many calculations need to be made to bring the rate into an across board comparison.
( I should add here I am already calculating a rough Wind Run for WD and WU users. Maybe in the future Mr Weather Display might take pity on me and add this valve in for me   )

I also find myself constantly suspicious of what is being reported and this has happened again today which is good timing for this topic.

Is a Rain Rate Max of 790mm per hour possible.
Thats 13.2mm per minute?

[Weather Display]

what and where is the wind run data needed, and I will add that

re

Is a Rain Rate Max of 790mm per hour possible.
Thats 13.2mm per minute?

is that from WD you mean, and if so, what weather station type, and what PC speed,version of WD etc?

bouncing of the spoon mechanism during heavy rain and with wind could lead to fast tips to get an instantenous high rain rate like that (i.e it might be actualy be that high)
even debri in the rain gauge suddenly flushing out could cause a problem

[Tony]

For what it's worth;
I take a slightly different approach and record the amount of rain in the previous 60 minutes rather than a rain rate.
On reflection I probably shouldn't post this as rain rate figures but as "Maximum recorded hourly rainfall" or something similar.

Tony

[JennyLeez]

If possible could the Daily Wind Run Total go under the Maximum Gust Speed
This would be great as at present I calculate this off the daily average wind speed.
http://www.weather-display.com/windy/gb/May2012.htm

I am ever hopeful to have a script some day to extract the data I need. But early stages with this at present.

Many many thanks
Jenny

[Weather Display]

I take a slightly different approach and record the amount of rain in the previous 60 minutes rather than a rain rate.

WD provides that info as well as the rain rate

[Weather Display]

I have added that daily wind run total now
to build 25 .zip update
ready now
( I have updated my web site data (via action, update av/ext now, to show that new data )
http://www.weather-display.com/windy/gb/May2012.htm

[JennyLeez]

Thank you
Going to save me heaps of time. Hard case though, I save here but it appears I shall lose again calculating these rain rates.
Guess I need to do some emailing with download link huh.
Again thanks