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Storms and Storm Surges - Frequently Asked Questions (FAQs)
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How do storms develop?
In order to give an answer to this question, different types of storms and their extension in space and time have to be distinguished. Extratropical storms are relatively large-scale phenomena, obtaining their energy from the general temperature difference between high and low latitudes. At the boundary between warm subtropical and cold polar air masses instabilities can occur, resulting in the development of extratropical cyclones the wind field of which eventually may evolve into a storm. Tropical storms (which are also called hurricanes or typhoons depending on the region) show a substantially smaller spatial extension. In order to allow for the development of tropical storms, special conditions are necessary which are met only in some regions of the earth. These include a minimum sea surface temperature of 26°C, weak vertical wind shear, conditionally unstable atmospheric stratification, and a sufficient distance to the equator in order to take advantage of the rotation of the earth. Other phenomena like tornadoes or local thunderstorms partly obtain their energy from other processes.
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What is a storm surge and how does it develop?
A storm surge develops when meteorological, astronomical and local effects interact in a way so that a dangerous rise of the coastal water level occurs. The meteorologial effects leading to a storm surge mainly consist of wind build-up and the rise of the sea level as a result of low atmospheric pressure (inverse barometric effect). Wind build-up means the rise of the water at the coast when onshore winds push the water towards the land. The amplitude of the water level rise produced this way is directly proportional to the square of the wind speed and the fetch but inversely proportional to the water depth. This means that the water build up the higher, the stronger the wind blows, the less the water depth gets and the longer the fetch is. These effects are particular dangerous when they nearly coincide with the high tide. Apart from that, a variety of processes exists which complicates this relatively simple picture in reality. In the German Bight, the highest storm surges usually occur during heavy storms from northerly to the nortwesterly directions. In the Baltic Sea, pronounced westerly winds first lead to higher water levels in the eastern and lower levels in the western basin. With the wind declining, the built-up water 'swashes' back and increased water levels occur at the west coast. This effect may be intensified by the additional on-set of easterly winds.
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Under which circumstances are ocean waves dangerous?
While storm surges primarily endanger coastal areas, ocean waves which come along with storms also pose a threat to navigation and other activities taking place at sea. It's particularly the steepness (the ratio of the height to the length of a wave) which may become potentially dangerous. Notorious are the so-called freak waves which pose a security risk through their sudden occurence, extreme height, and steepness. In the vicinity of the coast, waves during a storm surge cause erosion, e.g. the great scarps at the southern tip of Sylt. More on the topic of freak waves can be found here.
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Are storms, storm surges and extreme waves natural events, or are they man-made?
First of all, storms, storm surges and extreme ocean waves are natural processes. It is discussed to which extent the anthropogenic greenhouse effect may lead to an increase in frequency and intensity of such events and if extremes like tropical storms may occur in regions where they have not been observed so far. To the latter there are no conclusive answers for the time being. Model simulations indicate an increase in storminess in the region of the North Sea by the end of the 21st century. More information on this topic can be found here.
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Which influence will the climate change have on future storms and storm surges?
Information about the interrelation between climate change and the development of storms and storm surges can be found here.
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Are sea level changes and storm surges the same?
No. The observed water level consists of several contributions. Basically these are the tidal part, the meteorological part, and the mean sea level. While tides are caused by the gravitational forces of the sun and the moon, the meteorological part is due to the impact of wind and air pressure on the water surface. During a storm, the latter leads to increased water levels at the coast. The sea level rise is a slow change of the mean water level, e.g. caused by the thermal expansion of the sea water due to global warming, by the melting of inland ice or by the subsidence and the lift of land masses. Thus, at otherwise identical meteorological conditions, sea level rise leads to a higher build-up of water at the coast. However, meteorological factors may vary independently from the sea level rise and thus cause an additional change which is superimposed to this process.
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How will the sea level develop?
Scenarios of the IPCC assume a globally averaged rise of the mean sea level of approx. 10 to 90 cm by the end of the century. However, these changes may show regional differences. Up to now, little is known about the regional distribution of these variations. The observed rise within the past 100 years amounts to about 2 mm per year. More information on this topic can be found here.
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Will storms and storm surges become more frequent?
Although model simulations point towards a small increase for the region of the North Sea by the end of the 21st century, changes are not detectable so far. More information on this topic can be found here.
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Which is the most severe storm, or storm surge?
This question is not easily answered. In the region of the North Sea, a storm surge in January 1976 caused maximum water levels at many gauges but remained largely without effect. The floods of 1953 and 1962 caused significantly more damage. In the region of the Western Baltic Sea, the highest storm surge occured in 1872. One of the most severe tropical storms with a very high wind build-up developed in Bangladesh in November 1970. The Bhola cyclone was associated with a maximum recorded wind speed of 222 km/h and a storm surge height of more than 10 m at some places. It claimed lives of more than 500,000 people and represents the deadliest tropical strom known so far.
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Where can I get real-time information?
Information on the current wheater situation and the wheater forecast for Germany is provided by the Deutscher Wetterdienst (DWD). Large parts of the information are also available (in German only) at wetter.com. Another good source for weather information is WeatherOnline. Forecasts of storm surges and water levels for German coasts are provided by the Bundesamt für Seeschifffahrt und Hydrographie in Hamburg (in German only).
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