Roger Musson wrote:
"The paper quotes some other studies that model a projected increase in
wind speed in a cyclone of only a few percent, given a rise in sea
temperature of a few degrees, and anyone can appreciate that if you have
a cyclone with wind speeds of 100 mph, it won't be that much worse if
they increase to 105 mph."
Technical point - wind pressure is the square of the speed, so a 5%
increase in speed leads to a 10% increase in pressure. 10% may well be
significant for a structure that is near its design limit.
See the entry on "wind pressure" in the AMS Glossary of Meteorology. The
formula is
p = 1/2 (1+c).rho.v**2
Where p is pressure, rho is density, v is velocity normal to the
surface. c is a structure constant which depends on the shape of the
surface. For a large flat plate c is about 1 which indicates that the
pressure on the plate is the sum of the wind load on the facing side
plus an equal suction on the lee side.
In terms of damage to buildings, the suction is often more destructive
than the load, since it is relatively easy to construct a roof or window
to withstand the direct pressure, but much more difficult to stop roofs
lifting or windows being sucked out of their frames. This is
particularly a problem with shallow-pitched roofs where the lee side
acts like an aerofoil.
For a cylinder, c is about -0.3, but then other factors operate,
particularly resonance because a cylinder will shed vortices on
alternate sides, which is why metal chimneys are often constructed with
a spiral to disrupt the vortex shedding and prevent a destructive
resonance building up.
Another consequence is that if flying debris breaches the windward side
of a building, the pressure will be transferred through the interior and
double the load on the downwind surfaces. The building may then explode.
Hence in any strong wind situation, one of the first preventative
actions should be to secure any loose debris, old planks, roofing sheets
etc.
Wind, like water flows, is not uniform, it is a turbulence phenomenon
where peak speeds may be much higher than the mean. Also there will be
funnelling through other structures, again giving much higher speeds.
Stephen Palmer
Technical Co-ordination Manager
Met Office FitzRoy Road Exeter EX1 3PB United Kingdom
email: [log in to unmask]
http://www.metoffice.gov.uk
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