Frequently Asked Questions

MEASUREMENT NETWORKS MODELS
MEASUREMENT NETWORKS

What time reference is used: local o universal time?

    All dates shown in Oceanography and Meteorology section use Coordinated Universal Time (UTC) reference. Local time in the Iberian Peninsula and the Balearic Islands is UTC+1 in winter time and UTC+2 in summer time. In the Canary Islands the convention for local time is UTC+0 in winter time and UTC+1 in summer time. Therefore, to translate the dates shown on this web into winter local time, it is necessary to add 1 hour in case of the Iberian Peninsula or Balearic Islands, but nothing has to be done in case of the Canary Islands where UTC and winter local time are coincident. During the summer time use, the local time in the Peninsula and Balearic Islands is obtained by adding  2 hours to  the dates shown here and 1 hour for the Canary Islands.

I would like to know what is the sea state in a zone. Where do I have to connect and which are the most representative parameters?

   Click on the  Measurement Networks section of the web site. Once you are in, you can look up the distribution of the buoys in the maps of both buoy networks (Deep water and Coastal Network) and choose the one near to your area of interest. Then, clicking on the Latest Data link you will access the real time data section and check the evolution in the last days: choose the  T icon to display a data table or the G icon to visualize time series plots.
  The most representative parameters of waves are: significant height (Hs), mean period (Tm), peak period (Tp) and peak direction (Dirp, direction of the most energetic waves).
    SIGNIFICANT HEIGHT (Hs). It is one of the most widely used parameters of waves. Significant height represents the height of waves that a trained observer would determine with the naked eye from an observing position (not from the coast). It is accepted to be equivalent to the mean value of the 1/3rd  highest waves recorded in the measuring interval.
     MEAN PERIOD (Tm). Describes the mean value of the period from the waves recorded in the measuring interval.
    PEAK PERIOD (Tp). Is the period of the most energetic group of waves. The  more regular are waves, the more Tp and Tm look alike, though usually Tp is bigger than Tm .
    PEAK DIRECTION (Dirp). Describes the direction of the most energetic group of waves.

How does a buoy collect wave parameters?

    Traditionally the description of the wave conditions at a certain location and time is characterized by some parameters that define the sea state. Nowadays the sea state can be assessed through instruments like buoys (scalar and directional).
    In case of Puertos del Estado buoys, every hour they record instant elevations of sea level for a time interval in which the sea state can be considered to be constant. This duration (e.g. 20 minutes) depends on the buoy model and will be enough to obtain a representative sample of the wave conditions in that hour. Then the time series of instant elevations is processed using standard methods (zero crossing and spectral analysis) to determine the parameters that characterize the sea state in that hour: Hs,Tm,Tp, etc. Today, all this process is usually done on board the buoy and later the outputs are transmitted to land. Though, the scalar buoys of the Coastal Network still transmit the measured sample of elevations to a station on land where the analysis is performed.

Where are measured the highest waves in Spain?

   The strongest storms in Spain are usually detected in the Northwestern Coast (Galician coast). In this area storms mainly come from the north-est. They cause highest waves with largest periods  because they are generated in the North Atlantic and then propagated to the Galician Coast (when waves get older the period grows). These are the most characteristic storms in this area and can produce waves up to significant height of 10 meters with maximum wave height of 17 meters and wave periods up to 20 seconds.

I would like to know how to interpret the wave, wind and currents directions measured by the buoys.

    The zero angle indicates the geographic north and the angles increase clockwise. In general, when we talk about buoy-measured waves and wind data, angles indicate the "coming from" direction. However, in case of currents, direction angles usually  refer to the propagation direction or the "blowing to" direction.
    These criteria are different in case of model data:  modeled waves, wind and currents usually indicate  propagation directions.

Do you make any correction in the data directions due to magnetic declination?

        No, we do not make any correction due to magnetic declination because it is different in every position and it changes in time. But we take it into account, if necessary.

Is it appropriate to use buoy measured met-ocean conditions to a beach or a certain location on the coast?

    Deep Water Network buoys are moored in the open sea (more than 200 meters depth), far away from the coast line. Therefore, measurements are not affected by coastal and local phenomena and characterize the open sea waves in the area around the buoy. When using this kind of data for coastal studies, users mus be aware that the characteristics of waves at the sea side will be affected by the dominant direction of the open sea waves, but also by the sea ground and the coast line shape. In these cases, it would be advisable to propagate deep sea data to the coastal location of interest.

     Coastal buoys are moored in swallow waters, in the vicinity of the port facilities (less than 100 meters depth). In most of the cases, measurements are affected by the shape of the coast line and the sea ground. This means that they obtain information that is only representative of the local conditions. Therefore, these data must be used carefully to draw conclusions in areas away from the buoy position.

I would like to know what is the current sea level in a certain harbour. Where do I have to connect?

        Click on the Measurement Networks section of the web site. Once you are in, select the Tide Gauge Network tab. You can look up the distribution of REDMAR tide gauges on the map  and choose the one near to your area of interest. Then, clicking on the Latest Data link you will access the real time data section and check the evolution in the last days: choose the  T icon to display a data table or the G icon to visualize time series plots.

What are the seal level references used by REDMAR tide gauges and where can I look them up?

    A basic aspect to work with sea level data is the reference or zero. In Spain, the Geographic National Institute (Instituto Geográfico Nacional, IGN) is responsible of establishing the national references of heights in land. In the Spanish Iberian Peninsula the reference is the mean sea level measured in Alicante in the decade 1870-1880, NMMA (Nivel Medio del Mar en Alicante). In the islands (Balearic and Canary Islands), the local mean sea level is used. This value is obtained using the sea level data from a tide gauge located in the island. For example, heights in Tenerife Island will be refered to the Mean Sea Level in Tenerife, and so on.

    Every harbour, on its own, define an adecuate reference or zero for dredge, operations, works etc. It  is known as the harbour zero and usually matches up with the lowest low tide.  All the sensors in the Spanish Harbour System Tide Gauge Network (REDMAR) use the harbour zero for the reference of sea level measurements. This reference value is supposed not to change, unless the  corresponding Harbour Authority demands a new definition of the harbour zero.

    The relation between the harbour zero and the National Reference of Heights (NMMA or equivalent on islands) is shown in our web site when available. This information comes from high precision levelings that have been made in cooperation with IGN using its methodology and is updated when a new leveling is made. To check the zero and bench mark information click on the Measurement Networks section of the web site. Once you are in, select the Tide Gauge Network tab. Choose the one you are interested in from the map to access the Description page. Then click on the Description of the System link.  

Is there any  statictics, historical or climatic information available online ?

    Yes, users can check and download information from our monitoring networks:
     1) Online access to data base. In the Data Base section, through clickable maps, users have access to waves, sea level and wind  data. The most representative information collected is available since the beginning of measurements in every station that has ever been part of our monitoring networks.

     2) Annual Reports. Periodically we process the data collected in every station for the meteorological year (December to November) to obtain plots and statistics. The resulting reports are posted on the web. To download them click on the  Measurement Networks section of the web site. Once you are in, choose the monitoring network you are interested in (Deep water, Coastal Network, Tide Gauge Network or Currentmenters Network). Then, click on the Data Reports link, and choose one station on the map. Automatically, a download menu will pop up.  

    3) Waves an Sea Level Long-term and Long-term Extreme values Reports are available when measured time series are long enough to be statistically representative. These reports are accessible  in the waves and sea level  links of the Data Base section.

    4) Data Files Request. If any of the above mentioned channels of information does not cover the user needs, we also deliver measured time series of data under request. In that case, contact the Data Bank Department. Data will be sent for free only for research purposes.

MODELS

What is the Mean Period, and the Peak period?

    A wave pattern on the ocean can be shown to comprise a number of groups of simple waves, each group with its own period. The Mean Period is the mean of all the periods and in Spanish is denoted by Tz and the period of the most energetic wave group is the Peak Period and in Spanish is denoted by Tp. The more regular the wave pattern is, the more similar are the Mean and the Peak periods, but normally the Peak Period is higher than the Mean Period.

What is the Significant Wave Height?

    The wave height is different from one wave to another so a useful concept is the Significant Wave Height that approximately corresponds to the averaged height of the one third highest waves. It is denoted by Hs or Hm0.

What is the meaning of wind sea and swell? (In Spanish Mar de Viento and Mar de Fondo)

    The wind sea is composed by irregular short waves that are being generated by the wind and are still growing. They usually have foam in the crests. The swell is composed by regular long waves that have been generated in a different place and propagate up to the area of interest. The total sea (in Spanish Mar Total) is the composition of wind sea and swell.
     In open waters it is easy to find swell coming from a place far away and wind sea that is being generated at the area of interest. Sometimes it can be observed two different swells at the same time, with different directions and periods. In these situations we will call "mar de fondo 1" to the most energetic swell and "mar de fondo 2" to the other.

Is the wave forecast applicable to a beach or to the coast line?

    This wave forecast is applicable to open waters. The waves that could be found at the beach will depend on the wave direction in the open sea, on the sea bottom and on the coast line. There is not a general rule to estimate the waves at the coast from the waves at open waters. Depending on the beach characteristics the waves will be different from a place to another. The most important parameter for this estimation will be the beach orientation and the wave direction in open sea.

Which is the interpretation of the forecasted wind value?

    The wind, which in this forecast is denoted by Vv, is the 10 meter high wind over the sea surface. This wind must be consider at open waters because at the coast the wind can be very affected by the orography and the terrain.

Which is the correct interpretation of the wave and wind directions?

    The zero indicates the geographic north and the angles increase clockwise. Sometimes the direction indicates the "coming from" direction and in other times the direction is the propagation or the "blowing to" direction. This aspect is always indicated in the table, map or graphic that is being consulted.