On Mon, 8 Feb 2016 12:09:28 -0000, "Eskimo Will"
wrote:




Graham

I can see the site and the anemometer and vane using Google street view.
Mean speeds as high as 67 and gust to 86 mph will cause damage and look a
tad high to me.

It got me to thinking about cup size of my Vantage Pro. I've just checked
mine and realised that I've had it on 'small' cups for many years
(oops!), I had thought it would default to 'large' (wrong!). I don't know
how much difference that will make to the speed, probably not a lot.

The upper air station at Camborne (81 M amsl) was meaning (at 09 UTC) 44
mph with gusts to 65 mph, that still can't match the latest live mean
wind speeds from Trebehor (270' ASL).

I suppose the Vantage Pro just collects 1 minute mean speeds and graphs
those values in its own LCD display, and it's down to the display
software on the PC to take the average of 10 of those one-minute values
to produce a 10 minute mean. A one minute mean of 67 mph would be
significantly higher than a 10 minute mean (~x1.2 or more higher). The
length of that mean should be a user setting I would have thought and I
bet it's not 10.

Bruce.


Bruce,

The Vantage Pro console receives a wind measurement from the anemometer
sensorevery 2.5 seconds. That is the ever-changing figure that is
displayed on
the console. It calculates a rolling 10-min mean from those figures which
can be
displayed in the text bar on the bottom of the console screen. There are
other
options for that text bar.

A random 1-min mean wind speed may be higher or lower than a corresponding
10-min mean within which it is embedded.

--
Norman Lynagh
Tideswell, Derbyshire
303m a.s.l.
http://peakdistrictweather.org

Norman

Yes, of course you're right it could be. But what if the PC software people
use in conjunction with their AWS (to publish it on a website) has been
coded to select the highest 1 minute speed rather than the mean of the last
10 minutes?

All I'm trying to do is to compare like for like, i.e. mean wind speed
reported by people's AWS with those in UK SYNOPs, and it's not that easy!

Bruce.
========

In the real world it's gusts that do damage. And gusts can last for less
than a minute, so I'd say that peak 1 minute gusts will be most
representative of real life. Another issue is that we measure wind speeds at
10 metres above ground, so means speeds experienced by Joe Public will be
lower but 1 minute gusts will not be very different of course.

Will

It all depends on response time. Stuff around the house like chimney pots,
roofs, fence panels etc respond to very short duration events. For them, a
single very strong gust is what does the damage. The max gusts reported in the
SYNOPs are representative in this context. Traditionally, with the old Munro
anemometers, they were generally taken to be of about 3 sec duration. With more
modern equipment I suspect that it is possible to get much finer resolution,
perhaps down to 1 sec. This means, of course, that the same gust would probably
register a lower speed on a Munro anemometer than it would on more sensitive
instrumentation.

At the other end of the spectrum, the mooring system of a super-tanker moored
alongside a terminal responds to mean winds of 1 to 5 minutes duration. Single
very strong gusts have very little impact.

The reason why single very strong gusts can do so much damage to property is
that the force exerted by the wind is proportional to the square of the wind
speed. Therefore, for example, an 80-knot gust exerts about 30% more force than
a 70-knot gust

--
Norman Lynagh
Tideswell, Derbyshire
303m a.s.l.
http://peakdistrictweather.org