(no subject)

Jackie Fenton wrote:
>
>I would like to know if NOAA gives wave height from trough to crest
>or sea surface to crest in readings from the buoys.  I'll see what
>I can find out.


****************************************************************************

There are apparently several, not just one way, to measure waves.  As
explained in David Burch's book, <underline>Kayak
Navigation</underline>, p. 298, the Canadians publish wave data in a
measurement called "significant wave heights."  Swell is illustrated on
at least some of the canadian charts showing the direction and height
of the various components (i.e. a vector which illustrates the swell
from the cardinal points about the compass).  I could not find an
example of these vector diagrams, but I have seen them, I think in the
Canadian Sailing Directions (my copy got left in a motel room
somewhere).

Definition of "SIGNIFICANT WAVE HEIGHT" = The average height of the highest
1/3 of all waves.


Here are some definitions and URL's which appear on the web:

(1) Significant Wave height Statistics.

http://podaac.jpl.nasa.gov/topex/www/swh.html


HOW SIGNIFICANT WAVE HEIGHTS ARE COLLECTED (NASA):
>
>Significant wave height is calculated based on the shape of a radar
pulse after it bounces off the sea surface. A calm sea with low waves
returns a condensed pulse whereas a rough sea with high waves
>returns a stretched pulse.
>
>The highest waves generally occur in the Southern Ocean, where waves
over six meters in height (shown as red in images) are found. The
strongest winds are also generally found in this region. The
>lowest waves (shown as purple in images) are found primarily in the
tropical and subtropical oceans where the wind speed is also the
lowest. In general, there is a high degree of correlation between
>wind speed and wave height.


(2)THE SWELL WAVE HEIGHT CHART:

http://www.bom.gov.au/info/about_swell.shtml

>About The Swell Wave Height Chart
>
>Please note that this chart is automatically produced on a computer by
a numerical modelling program, which forecasts swell wave height and
direction based on surface winds which are also derived by
>numerical modelling. Please use the chart with care, because there's
no manual intervention or checking of the chart before you see it.
>
>The wave height contours represent the average height of the 1/3
>highest swell waves at that location; individual swell waves at that
>location may exceed the contour height.

Robert


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