Graham Easterling wrote:

On Dec 15, 7:48*pm, Stephen Davenport wrote:
On Dec 15, 7:07*pm, Hugh Newbury wrote:





On 15/12/11 18:33, Gavino wrote:

"Stephen *wrote in message
...
It'll be nasty over the northern half of France. Gusts 60-70mph
possible, and 70+ in places, e.g. Brest Peninsula.

8 to 10 metre total sig wave in the Bay of Biscay.

A red alert has been issued in Spain for the Biscay coast of the Basque
region tomorrow. Waves up to 10m are expected.

Please, what is a "sig wave"?

Hugh

--

Hugh Newbury

www.evershot-weather.org

=====================================

Sorry... sig = significant. Total significant wave height -- combined
wind wave and swell. Multiply by about 1.63 for maximum wave.

Stephen.- Hide quoted text -

- Show quoted text -

I always understood it to be mean wave height (trough to crest) of the
highest third of the waves. Certainly, this is how my various books on
sea & surf define it, as do most of the sites showing swell
forecasts.


Yes, it's the average height of the one-third highes waves in the sea state.
For example, if you have a wave measuring device at a point and you measure the
crest to trough height of 300 successive waves passing that point the
significant height is the average height of the 100 highest waves. The
significant wave height is the standard way of describing a sea state.

The multiplier to get the most probable height of the highest individual wave
is dependent on the period of time considered. The multiplier of 1.63 is
applicable to about a 20-minute period whereas a multiplier of 1.86 is
applicable to a 3-hour period. In other words, for any constant level of
significant wave height the most probable corresponding maximum individual wave
height increases with increasing time. That is, of course, only the most
probable value. In the real world there's a great deal of variability and it is
not at all uncommon to find cases in measured data where the maximum individual
wave height is double the significant wave height, or even higher.

--
Norman Lynagh
Tideswell, Derbyshire
303m a.s.l.