understanding buoy data
by John Newgard newgard@phys.ocean.dal.ca
Tz: AVERAGE WAVE PERIOD. Though the subscript z is new to me, judging
from the values given, it is clear to me that Tz is the average wave
period. Wave period is the interval between successive
waves. Picture the buoy bobbing in the water, with waves passing by and
the buoy going up and down. The wave period is the time taken for the
buoy to go from a wave crest, down into the trough, and back up to the
next crest that passes beneath it. The AVERAGE period is calculated by
adding up all the wave periods measured in say a 20 minute period and
dividing by the number of waves that went by.
Tz CAN be used to get a feel for swell quality. When Tz is much
smaller than the peak period (Tp) (say Tz = 7s and Tp = 15s), the swell is
made up of a broad range of wave periods, indicating that the swell is
really a mixture of short-period "sea" and the long-period waves we call
"swell". When Tz is closer to Tp (say Tz = 10s, Tp = 15s) the swell is
made up of a narrower band of wave periods (a "clean" swell). Tz
will always be smaller than Tp. Though the average period is somewhat
useful, the peak period is a more meaningful statistic (see below).
MaxWave: The highest wave measured in say a 20 minute record. This is
not the most useful stat; significant wave height (Hs) is the key reading
(more later). MaxWave will generally be say 1.8 x Hs. This stat can be
useful for gaging how well-developed (clean) the swell is. If MaxWave =
1.6 x Hs, the swell is quite clean (i.e. narrow band of wave heights),
whereas MaxWave = 2 x Hs tells you that the swell is not all that clean
(i.e. there is a broad range of wave heights).
Hs: SIGNIFICANT WAVE HEIGHT, i.e. the average height of the highest 1/3
of the waves. This stat is key, along with peak period (Tp). Hs gives
you a good feel for the height of the surfable waves, though you will
have to come up with your own fudge factor in translating Hs at the buoy
to the height of the sets at your local breaks).
Tp: PEAK PERIOD. This is another way to express the time interval
between waves. Tp is the wave period associated with the peak of the wave
spectrum, i.e. the wave period at which the highest wave heights
occur. In a given swell, there will be a range of wave periods all
mixed in together, with the set waves having the longest periods. In my
experience, Tp corresponds to the period of the set waves and is a good
indication of swell quality (i.e. the longer the period, the more
powerful and hollow the waves can get for a given spot and conditions).
Hmo: Zero-moment wave height. This is a spectral parameter, essentially
the same as Hs.
Direction: The direction from which the waves are travelling. A
southwest swell will read 225 degrees, west swell 270 degrees, etc.
