Ken Cook wrote:
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.
Best wishes,
Ken
Copley, nr Barnard Castle, Teesdale, County Durham
http://copley.mysite.orange.co.uk
Hi
In response to some of the comments, my anemometer is set at 5m agl
and quite exposed.
Between 0340 and 0500 GMT this morning, gusts of 50mph, 51mph, 54mph
55mph and 58 mph were recorded. Mean wind speed during this period was
at 33mph.(corrected to the notes above gives 37mph)
As stated it is the gusts that cause damage. During this period a few
trees and fences were brought down. There were two large flashes in the
sky, no doubt caused by power lines, as there was no lightning.
Power went off for about 10 minutes.
Alan
www.wincantonweather.org.uk