Frequently Asked Questions: General Queries
For a more in depth discussion of wave measurements and standards, we
Coastal Engineering Manual (CEM), published
by the United States Army Corps of Engineers' Coastal and Hydraulics
- What do Hs values represent?
The wave height (Hs) represents a 30-minute average of the 1/3 highest
waves at a sensor. The height is the distance between the trough
and the crest of the wave. Statistically, the highest wave during the
measurement period is likely to be approximately twice the reported
wave height (1.8*Hs).
- What are Tp and Dp? Why are there different values given in
The period of waves is the time it takes two consecutive crests to pass
a single spot, and the direction is the compass angle (0-360 degrees clockwise from true North) that the
waves are coming from. In the ocean, however, no two waves are perfectly
identical - they are constantly coming from different directions at
Since there is never just a single direction or period for ocean waves,
we can only measure the peak period (Tp) and peak direction (Dp).
The peak period is the most common period between consecutive waves, while
the peak direction is the most common direction. To come up with these
values, all of the wave energy for a station over a specified period of
time - approximately 30 minutes, in most cases - is grouped into different
bands. For instance, in CDIP's 9-band products, all waves with periods
from 6 to 8 seconds go into the 7-second band, those from 8 to 10 secs
go into the 9-second band, etc. After all of the wave energy has been
divided into these bands, the band with the most energy is selected as
the peak band.
The cutoffs between bands, however, are arbitrary, and changing the
cutoffs and/or number of bands affects the resulting Tp and Dp values.
At CDIP we use two formats for analysis, 9-band and parameter.
In the the 9-band products, all of the wave energy is divided into
9 broad bands. In the parameter products, the energy is split into 64
(or 128) narrow bands. As a result, the 9-band Tp and Dp give more
general, broad values, while the parameter Tp and Dp identify finer,
So which Tp/Dp values are better? It depends what you're looking for.
For instance, the broad bands of the 9-band values are better for
addressing general questions about the sea state (e.g. which is
currently predominant - local seas or ground swell?). To pick up more
subtle features - like the arrival of long-period swell from a distant
storm - the parameters values may be more helpful.
- Although the wave height is staying pretty stable, the peak period
and peak direction are jumping all around. Which of those Tp and Dp
values are really correct?
Over any given period, the waves at a point in the ocean are actually
coming from a number of different sources. There may be short-period waves
from local winds, long-period waves from one or more distant storms, and a
range of other wave fields originating from different weather systems.
The peak period and peak direction describe the strongest of all the
sources of wave energy. When one source becomes stronger than another,
the Tp and Dp values can suddenly change dramatically. And when two sources
of wave energy have near-equal strength, the Tp and Dp values may
bounce back and forth between them.
For instance, in summer on the US west coast we often have long-period
swell from large storms in the southern hemisphere and short-period seas
from local winds off the coast. Whenever the south swell is the stronger
the peak period could be 18 seconds and the peak direction 180 degrees.
Whenever the seas are a bit stronger, the Tp could be 5 seconds and the
Dp 290 degrees. And if these two sources have near equal energy, we may
find that from hour to hour the Tp and Dp values jump up and
down repeatedly, showing whichever of the two sources is briefly the
- Are your sea temperatures correct? At the beach the lifeguards
are reporting 63F, but the buoy says 70F!
There are often big differences between inshore and offshore temperatures
due to various phenomena. In the surfzone, for instance, mixing may result
in temperatures much colder than in calm surface waters offshore. Our
buoys are all located offshore and measure sea surface temperature using
a sensor located about 18 inches directly below the buoy.
- What causes wave sets?
Very often waves from different parts of a distant storm arrive here at
the same time. Their period (or lengths) can be nearly the same size.
For a while the crests of the two wave trains begin to coincide. The
two trains adding together result in waves that grow bigger. Later, the
troughs of one begins to coincide with the crests of the other. The
combination wave grows smaller. We see this as a "set" of large waves,
followed by an interval of smaller waves.
- Why are wave heights sometimes underestimated on windy days?
Local winds generate very short-period, high frequency waves, and not all
of CDIP's instruments can measure these waves effectively. This is
especially true of pressure sensors postioned deep underwater. At Kings
Bay, for example, the sensors are mounted at a depth of 55 feet. Due to
attenuation, these sensors do not feel high frequency waves, and so on
windy days the high frequency energy will not be reflected in the
- Why are the times on your buoy data often an hour or
more old? Can't the data be more up-to-date?
To determine the wave conditions, you can't simply look at the ocean for
a few seconds. Instead, you need to sample data over a long period.
For most of CDIP's wave calculations, a data sample of approximately
30 minutes is used. And unlike the NDBC and other data providers who
use sample end times on their spectra, CDIP uses sample start times.
This means that when we update our site based on a data sample that
ended just a few minutes ago, the time assigned to the data - the
start time - will already be more than 30 minutes old. For Datawell
buoys, at the end of a 30-minute sampling period, the sensor calculates
a wave spectrum and starts to transmit it; it's repeatedly sent in
4-minute blocks over the next half-hour.
This means that the when a CDIP station updates, the new spectrum will
have a time that is at least 35 or 40 minutes old (30 min sample + 4 min
transmission + a few minutes to process). But sometimes newly-updated
data may have a start time that is up to 1 hour and 10 minutes old;
it simply depends on which point in the half-hour cycle the buoy is
contacted. (I.e. CDIP only grabs the data from stations once each
half-hour, and it may happen that we're grabbing the spectrum near the
end of its transmission cycle.)
- Is the wave direction from the SF/IB Nearshore Buoy correct?
All the buoys show a northwest swell, but that buoy is reporting
the same swell from the west-southwest!
When looking at buoy data, it's important to distinguish between
deep-water measurements and shallow-water measurements. In deep
water, swell direction is primarily determined by the location
of the fetch that produced the swell. So deep-water buoys on the
West Coast register swells from the Gulf of Alaska as NW swells,
and swells from the South Pacific as S or SW swells.
In shallow water, on the other hand, the swell direction is
determined primarily by the local bathymetery; swell is refracted
such that wave crests approach the coast parallel to shore.
I.e. whether it's a NW swell or S swell, at your local beach
the waves always come in and line up at nearly the same angle,
with only slight shifts to the N or S.
For example, the San Francisco Bar buoy is a shallow-water buoy
at a location where the shore normal points to the WSW. Thus all
long-period swell, regardless of source, has been refracted to
the WSW at that spot. So the readings you see are correct given
the water depth (15m) at the buoy's location.
- Sometimes the swell and wind wave heights reported on the NDBC
site for CDIP buoys aren't consistent with neighboring NDBC buoys.
The CDIP buoys report larger wind waves, while the NDBC buoys
report more swell. Why is this the case?
The relative amounts of swell and wind waves (or seas) reported for
a buoy depend upon the cutoff period used to distinguish swell from
seas. NDBC buoys include anemometers and use wind measurements to
determine the sea/swell cutoff period based on current conditions.
CDIP buoys, on the other hand, do not measure winds, and instead
of a dynamic sea/swell cutoff they use a fixed 10-second cutoff
when reporting swell and wind wave measurements to NDBC.
Sometimes there are significant amounts of wave energy with periods
in the 8 to 10 second range. Depending on wind conditions, NDBC may
report this energy as swell, whereas CDIP buoys will always report
it as wind waves. Hence the NDBC reports will show more swell,
and the CDIP reports will show more wind waves. By looking at the
swell and wind wave periods, however, it should be clear when this
sort of discrepancy occurs; the NDBC buoys will be reporting a low
swell period, one that falls below CDIP's 10-second cutoff.