"Ken Cook" wrote in message
ps.com...

Will Hand wrote:



Dave, it is the gusts that do the damage. Your gust was probably true as is
the
nature of gusts especially in the met. conditions of last night with long
lulls
then powerful gusts. For the record, my 3m agl mean at max gust time of 41
knots
was a mere 17 knots. The 17 knots, does of course correct up to 29 knots at
10
metres agl. So a force 7 with powerful gusts running at 2.4 above the mean
speed
(which was characteristic of the night I think). That was my site which is
fairly sheltered by a slope to my south and west, the open moor would have
been
wild!

Will.

Hello again, Will,

(Retired headteacher writing here and therefore not a meteorologist) -
humble mode off.
According to my bible - Met O Observers handbook,
MWS at 3 metres needs +20% for 10 metre conversion.
Thus your 17KT would come out at only 20KT at standard height.
Of course, a 10 metre pole is still not sufficient as it has to be an
effective height of 10 metres.
Met O told me that at least 15 metres would be needed here even in this
exposed location.

Full list for those interested,
Effective height Correction
1 - 2 metres add 30%
3 - 4 metres add 20%
5 - 7 metres add 10%
8 - 13 metres no correction
14 - 22 metres subtract 10%
23 - 42 metres subtract 20%
43 - 93 metres subtract 30%

The full formula is explained together wit reciprocals etc but I'm a
bit out of depth with that.
There is a separate table for wind over sea.
Gust speeds at reported wothout correction.


Thanks Ken, that looks familiar :-)

However, those formulae are very generalised and must assume a roughness length
that may or may not be representative of the site. Roughness length also varies
with wind direction/fetch. The general relationship (without roughness length)
is a logarithmic increase with height, now very roughly log(10)/log(3) = 1/0.48
= 2.08 . i.e. 10 metre wind = 2.08 x 3 metre wind, but this factor will be
reduced according to roughness. If we introduce a roughness length of 0.3 then
the factor becomes log (10/0.3)/log(3/0.3) = 1.53. The figures you quoted may be
OK for a well exposed open site with few obstacles but for rough terrain and in
built up areas they will be way out. I guess if you a proper MetO vetted site
then you would be well exposed. As for gust I can understand the reason for
using un-corrected values as in a well exposed site the momentum *may* not be
reduced much, but again in a "rough" site they will dissipate more.

I really must get that 10 metre mast!

Thanks,

Will.
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