Author Topic: Is there an official way to calculate rain rate?  (Read 558 times)

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Offline DaleReid

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Is there an official way to calculate rain rate?
« on: March 19, 2022, 07:26:45 PM »
I have been home brewing a station and while other stations and software like Weather Display will calculate the rain rate, I'm wondering if there is official guidance on how to do that?

How far back does one look to average the rainfall?  5 minutes?  10 minutes?  Half an hour?

If the rain stops and is no more during this time frame, should the software routine set the rate to zero, or just let it wind down as time passes and no more rain comes into the gauge?

Is there a relatively easy way to calculate the rate?  My gauge, as most do, gives a tipper pulse for every 0.01" of rain. So I get a bunch of pulses which the datalogger counts up, but I'm going to have to figure out how much rain in previous minutes has occurred.   Is it possible to just figure out the time between tips and use that to figure the rate?
Dale
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Offline CW2274

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Re: Is there an official way to calculate rain rate?
« Reply #1 on: March 19, 2022, 07:44:31 PM »
Is it possible to just figure out the time between tips and use that to figure the rate?
I'm sure there is, but that's what the machine is for. With Davis, soon as that second tip occurs, you get a rate, which I believe is updated every twenty seconds. No idea if that helps you or not.
« Last Edit: March 19, 2022, 07:48:04 PM by CW2274 »

Offline davidefa

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Re: Is there an official way to calculate rain rate?
« Reply #2 on: March 19, 2022, 08:13:21 PM »
This is an extract of the davis an28 document:

Quote
RAINFALL RATE
Parameters Used: Rain Total (actually, rain rate is a measured variable in the sense that it is
measured by the ISS and transmitted to the display console, whereas all other calculated
variables are determined by the console from data received from the ISS.)
Formula:
Under normal conditions, rain rate data is sent with a nominal interval of 10 to 12 seconds.
Every time a rain tip or click occurs, a new rain rate value is computed (from the timer values)
and the rate timers are reset to zero.
Rain rate is calculated based on the time between successive tips of the rain collector. The rain
rate value is the highest rate since the last transmitted rain rate data packet. (Under most
conditions, however, a rain tip will not occur every 10 to 12 seconds.)
If there have been no rain tips since the last rain rate data transmission, then the rain rate based
on the time since that last tip is indicated. This results in slowly decaying rate values as a rain
storm ends, instead of showing a rain rate which abruptly drops to zero. This results in a more
realistic representation of the actual rain event.
If this time exceeds roughly 15 minutes, than the rain rate value is reset to zero. This period of
time was chosen because 15 minutes is defined by the U.S. National Weather Service as
intervening time upon which one rain "event" is considered separate from another rain "event".
This is also the shortest period of time that the Umbrella will be seen on the display console
after the onset of rain.
REFERENCES
"Surface Weather Observations and Reports ". Office of the Federal Coordinator for
Meteorological Services and Supporting Research, Washington, DC, 1998

Offline sky_watcher

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Re: Is there an official way to calculate rain rate?
« Reply #3 on: March 19, 2022, 08:37:26 PM »
Is there a relatively easy way to calculate the rate?  My gauge, as most do, gives a tipper pulse for every 0.01" of rain. So I get a bunch of pulses which the datalogger counts up, but I'm going to have to figure out how much rain in previous minutes has occurred.   Is it possible to just figure out the time between tips and use that to figure the rate?
Don't know what you are using as a datalogger, but it is (relatively) easy to measure the time of bucket tips using interrupts. Quite some time ago I started playing with IFD (Intensity, Frequency & Duration) rainfall measurements. An Arduino UNO measured the time of each bucket tip and output it for analysis.

You can have a look at the code if you want some ideas, although I haven't looked at it for some time. On the entry page there is an archive of the source code that you can download if you wish - no promise that it will work or do what you are looking for since I haven't looked at rainfall for a while. It measures bucket-tips with a time resolution of 1 second which was ample for my needs.

A bit of a warning: The code is a bit hard to follow as it allows for the optional inclusion of a multitude of different sensors at compilation, implements a software real time clock for timing events, automatically takes measurements while also accepting commands from the USB port.

http://brigadoon.power.on.net/project_dynamic/arduino/groundweather/html/index.html

Good luck on your measurements.
“The more a man knows, the more willing he is to learn. The less a man knows, the more positive he is that he knows everything...” ― Robert G. Ingersoll

Offline Cutty Sark Sailor

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Re: Is there an official way to calculate rain rate?
« Reply #4 on: March 19, 2022, 09:09:27 PM »
Rainfall intensity... from the time interval between 2 tips of bucket .
2 tips 50 seconds apart with a 2mm bucket, = 14mm/hr
BS (bucket size) times 60/tip interval seconds (TMR) times 60 minutes = intensity, in (mm) per hour.
BS x TMR x 60
as
.2mm x 60/50 per minute x 60 = .2mm x 1.2 x 60= 14mm/hr
Vantage Vue pro uses this formula, for example.So you get values like these:
For a 0.2mm bucket
@ 2.4 min interval = 5mm/hr
@  12 sec interval = 60mm/hr
@ 3.6sec interval = 200mm/hr
@ 1.4 sec interval = 500mm/hr 
Stephen Burt, "The Weather Observer's Handbook"page 158 
and the footnote on pages 305-306   
use tenths of inches, if want... whatever, as long as all units same system
« Last Edit: March 19, 2022, 09:18:27 PM by Cutty Sark Sailor »
 

Offline DaleReid

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Re: Is there an official way to calculate rain rate?
« Reply #5 on: March 19, 2022, 09:16:36 PM »
That gives me something to go on, and I had no idea there were so many documented sources for the info on how various stations figure this.

Thanks to each of you.   Dale
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