Wind Gusts

[serowe]

How does Cumulus obtain the Wind Gust speed from a weather station? Playing around with graphs for historical data, I have noted that gust speeds are not present under somewhere around 3-3.5 km/h - see the following graph for an example:

http://mtb-images.dyndns.org:1088/weath ... dmonth.php

You will note that the gusts (the red dots) all have a base line around 3-3.5 km/h.

Is this a Cumulus calculation or derived from data on the station itself? If a calculation, why this cut off? (if you have a relatively calm day and recive a 'spike' of iwnd at 2 km/h that is, realistically, a gust - but doesn't appear to be recorded.

[GraemeT]

I don't know about the WM 918, but my LaCrosse WS-2355 has a rated resolution of 0.1m/s and my cumulus-generated wind graphs show gusts down to 1km/h or 0.27m/s.
http://weather.gktnet.com/cgraphs.php

[serowe]

Don't believe it is the resolution - if you look at the graph in that link I posted before, the normal speeds are shown below the gust speeds - that particular graph just happens to show a very definate bottom cut-off speed, but only for the gusts - thus my query.

The stated accuracy for this model is:

Between 2 to < 10 m/s - + or - 1 m/s
Range 10 to 56 m/s - + or - 10%
And
resolution 0.2 m/s (0.4 mph) (typical)

So the cut off I am seeing should be able to have been resolved by the station itself.

[GraemeT]

Is it possibly something in your graphing code that's the cause?

What do your cumulus-gen'd graphs look like?

[serowe]

The graphs are plotting the actual data from the minute by minute data and also the daily summary data - every record I have checked either has zero (ie no wind at all that day or minute) or, if ther is a gust the value is 4.3 km/h - no other values in the actual data between zero and 4.3. So, if you view that graph it will actually be updated to include from the time you ask for the data and backwards 28 days.

From what I can see on the generated graphs by C, it is pretty much the same (hard to tell though as they only show the past 48 hours).

Edit - have a look at http://mtb-images.dyndns.org:1088/weath ... =05&day=25 as an example - that is Wednesdays minute by minute data - no conversion or calculations are done on it - it is simply imported straight from the May11log.txt file into a MySQL data base table and then used for graphical and other displays.

[mcrossley]

You don't have a wind gust multiplier set in Cumulus do you?

[serowe]

Nope - from the configuration file:

Code: Select all

[Offsets]
PressOffset=11
TempOffset=0
HumOffset=0
WindDirOffset=0
InTempOffset=0
WindSpeedMult=1
WindGustMult=1
TempMult=1
HumMult=1
RainMult=1

This is why I am trying to determine where the gust figure comes from - the unit or from a calculation inside Cumulus.

[mcrossley]

Steve will have to chip on about the W918, but Cumulus normally obtains the gust figure from the station - it does with the FO and Davis anyway.

[steve]

The WM918 doesn't measure wind speeds below 1 m/s.

[serowe]

Which is 3.6 km/h! And it's resolution is 0.2 m/s or 0.72 km/h - so I should be at least seeing measurements of 3.6 km/h but I'm not in the data Cumulus is exporting.

So, again, this is why I am asking is Cumulus doing something to the data it is receiving? It appears as if it is throwing away the lower end readings the unit can produce.

[serowe]

Steve will have to chip on about the W918, but Cumulus normally obtains the gust figure from the station - it does with the FO and Davis anyway.

Thanks - this is all I was asking - didn't realise it was such a hard question