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Hi guys I have an small question here. At the NDBC website they have at most of the buoys the steepness of the sea at that time and date: http://www.ndbc.noaa.gov/station_page.php?station=46012

However this stations collect all the wave spectrum data and direction of it, so I suppose they use the cut frequency and then from then they decide how step is the sea at that moment at that place. At other places they don't get all the data, being more specific on the device: http://www.ndbc.noaa.gov/station_page.php?station=46214 they only have the dominant wave period and the wave height at the files: http://www.ndbc.noaa.gov/station_history.php?station=46214

The data in the file looks like this:

#YY  MM DD hh mm WDIR WSPD GST  WVHT   DPD   APD MWD   PRES  ATMP  WTMP  DEWP  VIS  TIDE
#yr  mo dy hr mn degT m/s  m/s     m   sec   sec degT   hPa  degC  degC  degC   mi    ft
2017 01 01 00 22 999 99.0 99.0  3.40 12.50  9.60 311 9999.0 999.0  12.2 999.0 99.0 99.00
2017 01 01 00 52 999 99.0 99.0  3.30 11.76  9.48 322 9999.0 999.0  12.2 999.0 99.0 99.00
2017 01 01 01 22 999 99.0 99.0  3.18 11.11  9.24 329 9999.0 999.0  12.2 999.0 99.0 99.00
2017 01 01 01 52 999 99.0 99.0  3.29 11.11  9.54 314 9999.0 999.0  12.1 999.0 99.0 99.00

Then using only that data how I can obtain the sea steepness?.

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The methodology to calculate wave steepness used by NDBC is explained in: http://www.ndbc.noaa.gov/algor.shtml

Steepness is defined as $\xi=\frac{2\pi H_s}{gT_z^2}$ where $H_s$ is the significant wave height and $T_z$ is the average wave period.

From your dataset, you have significant wave height and average wave period so you should be fine.

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