Rain Rate query

[RayProudfoot]

Hi Steve,

We've had some beefy showers this afternoon and on the main Cumulus interface it shows a high rain rate of 1.41"/hr.

However, when I open a graph and view the rain rate there it only shows a max around 0.6" for the same time - 14:03. Any ideas why there is such a large discrepancy?

I'm attaching today.ini for info.

[daj]

Hi ray,

Can I suggest you attach the dayfile.txt file too, as this is what the graphs use to build their data. Don't attach the whole file as it may be huge but cut it down to the time around the rain period today.

http://wiki.sandaysoft.com/index.php?title=Dayfile.txt if you want details on what the file contains.

[RayProudfoot]

Hi ray,

Can I suggest you attach the dayfile.txt file too, as this is what the graphs use to build their data. Don't attach the whole file as it may be huge but cut it down to the time around the rain period today.

http://wiki.sandaysoft.com/index.php?title=Dayfile.txt if you want details on what the file contains.

Okay-dokey, here it is. As it's only 12kb I've attached the whole thing.

[daj]

Before anyone else shots me!!! DUH, I asked for the wrong file. The dayfile is a list of the days highs and lows. We need the monthly log file.

I really should read the Wiki -- you know, the article I wrote

http://wiki.sandaysoft.com/index.php?ti ... _log_files

Sorry Ray!

[steve]

VP2 data interval: 2.5 - 3 seconds.

Graph plotting interval: 60 seconds (while running) or at the data log interval, but only immediately after startup.

=> most readings aren't plotted.

[RayProudfoot]

No worries David. As it's the 1st of the month the file is small. The entry for 14:10 contains the highest value shown by the graph - 0.41" - but Cumulus shows a high rain-rate of 1.44" at 14:03.

My question is - shouldn't Cumulus record the highest rain rate since the last entry in the monthly log? It looks like it's taking a snapshot of the current rain rate which probably won't show the maximum.

[steve]

It looks like it's taking a snapshot of the current rain rate

That's all the graphs are - a snapshot of the values each minute.

Cumulus 2 stores all the highs and lows between each log entry, so it will (eventually) be possible to create plots of these.

[RayProudfoot]

VP2 data interval: 2.5 - 3 seconds.

Graph plotting interval: 60 seconds (while running) or at the data log interval, but only immediately after startup.

=> most readings aren't plotted.

Hi Steve,

For most of the weather variables peaks like this won't be an issue. Baro pressure or temperature won't have such peaks. But rain rate (and wind gusts) have instantaneous peaks so it would have been useful if those peaks could have been captured on the log and then shown on the graph.

The highest wind gust was also missed. Is there an easy way to ensure these high values can be captured for the log or can Cumulus only capture instant data from the console and not the highest value (for wind and rain rate) since the last log entry?

It's not a huge issue. At least I understand more about how the graphs get their data.

PS. Just seen your reply about C2 so I'm happy to close this discussion now.

[steve]

PS. Just seen your reply about C2 so I'm happy to close this discussion now.

I've just added 'peak rain rate' as one of the pre-defined graphs, just to make sure it works

[RayProudfoot]

PS. Just seen your reply about C2 so I'm happy to close this discussion now.

I've just added 'peak rain rate' as one of the pre-defined graphs, just to make sure it works

Thanks Steve! Top-notch service as usual.

[RayProudfoot]

Steve,

Sorry, it's me again with a slightly different question on rain rate. Again, we have some beefy showers passing through this afternoon. I now use Cumulus to upload my data to Wunderground at 5 minute intervals.

At 17:15 Cumulus logged a rain rate of 0.79"/hr (see below) but on my station data on Wunderground it only shows a rate of 0.09"/hr. http://www.wunderground.com/weatherstat ... =ISTOCKPO4. At 17:20 the rate stored on Cumulus again differed from that uploaded to WU.

Given that Cumulus is doing everything with a few seconds at each 5-min interval can you throw any light on why they don't match please? Is there a log that shows what is uploaded?

03/09/09,17:15,12.3,85,9.8,17.0,29.0,293,0.79,0.36,998.28,14.90,21.6,58,9.0,9.7,12.3
03/09/09,17:20,11.8,87,9.7,14.0,30.0,293,0.27,0.38,998.07,14.92,21.6,58,14.0,9.4,11.8

[steve]

Wunderground doesn't have a rainfall rate parameter on upload, just "rain today" and "rain last hour". So they must calculate the rate from the 'last hour' figure (presumably just using it directly as the rate). Cumulus uses the rate supplied by the VP2.

If you want to see what values Cumulus is sending to Wunderground, the messages get logged in the debug log when it's on.

[RayProudfoot]

Thanks Steve. That explains the quite large discrepancy. The next time a hefty shower hits me I'll turn on debugging and see what that says. Then I may raise a ticket with WU.

I wouldn't mind if the two rates were quite close but they're not. WU always under-calculates by quite a large amount.

[steve]

WU always under-calculates by quite a large amount.

Yes, it's bound to. The VP2 calculates rain rates over a very short period of time, so a short heavy shower will show a brief high rain rate. By using the 'last hour' figure as the rate, Wunderground are averaging out the rate over the previous hour, so to show anything like the same peak rate as the VP2 it would have to keep raining at a similar rate for a long time - up to an hour.

[RayProudfoot]

I've raised a ticket with them as their calculated rate will only be close to the actual when steady rain is falling over an extended period.

I'll report what they say but I'm not expecting miracles like them making changes to accept the actual rate.