Wind Speed and Gusts.

[Tom Gamble]

Hi,

Given my anemometer is not at at 10m it's all a bit academic. But I've
got a couple of questions.

I had a quick glance at http://en.wikipedia.org/wiki/Wind there I see
that the correct way to measure the wind is average it over a 10 minute
period. Does the W-8681 do any averaging?

The page also describes a Gust "one technical definition of a wind gust
is: the maxima that exceed the lowest wind speed measured during a ten
minute time interval by 10 knots (19 km/h)."

Sooooo to my mind a gust of wind can't be less than 10knots.

I found another definition
http://www.weather.gov/forecasts/wfo/definitions/defineWindGust.html
that states a gust has to be at least 16knots and > 9knots above the
lull.

So should we show the "gust" line on the graph? Perhaps it should be
labelled "max wind speed"

That said I see that the Reading University site does shows gusts in the
same fashion as pywws.

Regards,
Tom.

[Gina]

Hi Tom

My anemometer is at about 10ft rather than 10m - I guess I should see if
I can get it higher.

I too want a 10min average for the wind - both strength and direction.
Averaging wind speed is not difficult but the direction is circular
(polar) and not linear. If you describe the direction as an angle
clockwise from north, how do you average the wind when it's varying
around north?. I'm thinking of adding a longer "tail" to my direction
sensor but data averaging would also be good.

Regards
Gina

[Tom Gamble]

On Tue, 2010-01-26 at 21:51 +0000, Gina wrote:
> Hi Tom
>
> My anemometer is at about 10ft rather than 10m - I guess I should see if
> I can get it higher.
>
> I too want a 10min average for the wind - both strength and direction.
> Averaging wind speed is not difficult but the direction is circular
> (polar) and not linear. If you describe the direction as an angle
> clockwise from north, how do you average the wind when it's varying
> around north?. I'm thinking of adding a longer "tail" to my direction
> sensor but data averaging would also be good.
>
> Regards
> Gina
>

Well averaging the wind direction is not going to be easy. I have seen
my direction indicator do a full 360 in not much time at all. Just
thinking allowed and having done zero research these are my thoughts.

Take each reading and count the number of times the direction is seen.
The direction with the highest count is the starting point. then use
that as the datum and convert each direction to degrees +/- the datum.

So say for example you taking a reading every minute. you have ten
readings. lets say they are.

5 E
3 SE
2 NE

Then you get:-

5 X 90 = 450
3 x 135 = 405
2 x 45 = 90

Giving a total of 945 / 10 = 94.5.

If a similar set of readings around North you get

5 N
3 NE
2 NW

5 x 0 = 0
3 x 45 = 135
2 x -45 = -90

Giving a total of 45 / 10 = 4.5.

If the predominant direction were sat NNW

Then its

5 x NNW 337.5 = 1687.5
3 x NNE 382.5 = 1147.5
2 x WNW 292.5 = 585.0

Total 342.0 / 10 342 which is again NNW + 4.5

I'm sure one could find a set where the answer would be > 360 in which
case you simply subtract 360 to give a value in the range 0 to 359.

If two point tie say

4 NNE
4 NW
2 N

Then use the point halfway between them i.e. 315.

If there is no clear winner perhaps the best option is to say "Variable"

HTH.

Regards,
Tom.

[Gina]

Hi All

A little mod to Hourly.py which I've added to cure a problem - I thought
this would be helpful to others.

I've been editing the template and graph-template files to do what I
want. However, when testing gedit leaves backup files with names like
24hour.txt~. These are normally hidden but they get templated all the
same, often resulting in error messages after the error was corrected in
the working file.

To avoid this I have added code to detect hidden files and skip them :-

CODE
print "Templating"
for template in os.listdir(template_dir):
input_file = os.path.join(template_dir, template)
if not os.path.isfile(input_file):
continue
if template[-1] == '~':
continue

I added the last two lines above. This could be combined with the if
statement above, of course, for more compact code, but the above shows
the principle. This checks the last char in the template filename and
skips files ending in ~.

Regards
Gina

[Jim Easterbrook]

Gina wrote:
>
> I've been editing the template and graph-template files to do what I
> want. However, when testing gedit leaves backup files with names like
> 24hour.txt~.
>

> To avoid this I have added code to detect hidden files and skip them :-

Thanks for that Gina. As a vi user I hadn't had the problem, so hadn't
fixed it.
--
Jim Easterbrook <http://www.jim-easterbrook.me.uk/>