Friday, 2 January 2009

Temperature, Heat Capacity and Heat Conductivity

Temperature, Heat Capacity and Heat Conductivity The heat capacity of seawater is 3134 times greater than that of air. Water has insignificant heat conductivity. That is why distribution of heat to greater depths is very slow and is mainly achieved through convection.
The highest temperature of water is registered to occur between 3 and 4 p.m., and the lowest – a couple of hours after sunrise. There are three temperature layers of seawater: surface layer (epilymnion), intermediate layer (metalymnion), and deepest layer (hypolymnion). The thickness of the former two layers varies with the weather, season, and currents. The temperature of the surface layer is almost constant, being between 19 and 25ºC in the summer. As the deepest layer begins, temperature drops by a few more degrees and it remains constant thereafter (7–9ºC). That is the temperature of sea depths and it does not depend on the season.
Water Motion Water motion constitutes sea currents and waves. The reason for the formation of currents might be the different density of water, constant winds, etc. Ocean currents are usually caused by constant winds, whereas local ones are mainly due to the character of coastlines. According to he direction of their flow, currents can be classified as vertical or horizontal. There are three main types of waves: wind waves, standing waves, and seismic waves.
Wind is the main reason for the formation of waves. The process of wave formation can be divide into different stages. When the speed of wind is less than 1m/s, air motion does not affect the surface of the water. If wind intensifies, these rows of waves become irregular and peaks appear, which are due to the different pressure at the front and at the back of the wave. At a greater speed of wind large waves are formed, running in parallel rows. Th largest waves reaching hundreds of meters continue even when the wind has ceased. They create the so-called dead drift.
Standing waves are formed when the level of the water rises at one coast and in the same time drops at the other. A sudden decrease of atmospheric pressure at one of the coasts, appearance of strong wind or heavy rain can all be the causes for standing waves. The fluctuation of the sea level may reach 80cm, which is dangerous for vessels at the harbors.
Seismic waves are formed because of underwater earthquakes. A vessel that is nest the site of the earthquake experiences a hydraulic blow which is why old maps frequently contain non-existent reefs. Seismic waves are often present in the Hawaii region where they have the special name zunami. Such waves are formed in the Pacific Ocean, the Mediterranean, the Caribbean Sea, and the Malaya Archipelago as well. Sometimes, these waves reach the height of 35m and are dangerous not only for the ships but also for the native population because of their destructive power.Waves change their form when they reach shallow regions. When the depth becomes equal to the height of the wave, the water particles no longer move in a circle: their orbit becomes elliptical. The length of the waves decreases and the height increases. The front slope of the wave becomes vertical, the top is inclined forward, then it falls and eventually destroys the wave. This phenomenon is called a surf. Its force may reach up to 38 tonnes/m2.

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