# Measuring and calculating waves

Wave measurements have been made by SMHI since 1978. For many years the instruments used were placed on the sea floor and worked like an upside-down echosounder. Some simple buoys were also used. More recently, advanced buoys have been used that can also measure wave direction.

For practical reasons early buoys had to be placed quite close to the coast, often close to a caisson lighthouse to provide them with electricity. Sending data via the mobile phone network also limited their range. These instruments measured wave height and period.

More recently, more advanced buoys have been used that can also measure the wave direction. They have also provided more exact measurement and access to these data has become much easier while they can be placed almost anyway instead of being limited by practicalities.

Lower resolution data is collected from the buoy via satellite every hour while the high resolution data is stored in the buoy until it can be manually retrieved. Forecasts of the wave field are made using mathematical models driven by weather forecasts. These models provide a more complete picture of the wave field in time and space – but the wave buoys are still needed to check that the models are correct. This validation is done continuously (link to http://produkter.smhi.se/OceanWeb/html/waveobs.html).

## Wave height and wave length

The wave height is the vertical difference between a wave crest and a wave trough. The wave length is the horizontal distance between two consecutive wave crests. The wave period is the time interval between two consecutive wave crests. The wave velocity (celerity) equals the wave length divided by the wave period.

Sea reports give the significant wave height. This is calculated from the height of all the waves during a 20 minute period. The significant wave height is the average height of the highest third of these waves. The term came into use when trying to relate the height of waves reported by observers on ships to those measured by oceanographic instruments. It was found that the wave height reported by a skilled observer was equal to the average height of the highest third of the measured waves.