If the high and low tides differ in height, the pattern is called a mixed semidiurnal tide. When the two highs and the two lows are about the same height, the pattern is called a semi-daily or semidiurnal tide. In general, most areas have two high tides and two low tides each day. Three basic tidal patterns occur along the Earth’s major shorelines. Unable to move freely around the globe, these tides establish complex patterns within each ocean basin that often differ greatly from tidal patterns of adjacent ocean basins or other regions of the same ocean basin (Sumich, J.L., 1996). The large continents on the planet, however, block the westward passage of the tidal bulges as the Earth rotates. We recommend that you use the water level forecast in these areas.If the Earth were a perfect sphere without large continents, all areas on the planet would experience two equally proportioned high and low tides every lunar day. The difference in height between low- and high tide is small in these areas, and so the weather effect may dominate the water level. In the south of Norway, the tide tables may differ substantially from the observed water level. Areas with small differences between low- and high tide The red curve shows the predicted tide (tide table). The green curve shows the water level forecast, which includes predicted tides and predicted weather effect. The blue curve shows the observed water level (may be observed at a permanent tide gauge further away, corrected with a time lag and height correction). WATER LEVEL: Example from the tide gauge at Helgeroa (Larvik). These forecasts are added to the Norwegian Mapping Authority’s predicted tides, and we can provide a water level forecast five days in advance. With the help of models, the Norwegian Meteorological Institute predicts the effect the weather has on the water level five days ahead of time. For the Norwegian coast, it is mostly winds from the south and west that cause this effect. The wind can push water towards the shore and into the fjords, creating an excess of water concentrated along the coast. Wind also plays a crucial role in how high the water level gets. A powerful low pressure system of 960 hPa will cause the water to rise approximately 50 centimetres. Generally, if the air pressure drops by one hectopascal (hPa) or one millibar (mb), the water level rises by one centimetre. Low air pressure leads to higher water levels, and high air pressure leads to lower water levels. The meteorological contribution can cause water levels to deviate more than a meter higher or lower than what the tide table says. In addition to tides, the water level is affected by meteorological effects, especially variations in air pressure and wind.
Changes in air pressure and wind can change the water level The closer to a permanent tide gauge the zone is, the more accurate the data are. The tide in each zone is related to the nearest permanent tide gauge, using a time lag and a conversion factor for the height.
High tide graph series#
The coast is divided into zones that have roughly the same tidal conditions, based on water level measurements from the permanent stations and many shorter measurement series from other locations. Tide tables, or predicted tides, are also called astronomical tides because they result from gravitational effects from the sun and the moon. The water level measurements are used to calculate the tide tables. The Norwegian Mapping Authority has 24 permanent tide gauges that registers the water level continuously. Water level forecast for the following five days, observed water level in real time and tide tables.