Storm and Flood


What is a tsunami?



Tsunamis are tidal waves that are often caused by an undersea earthquake. Usually there is some warning of a tsunami because scientists can detect the seismic waves caused by the earthquake. The tsunami rushes along at up to 970 km/h, building into a wall of water 30 m high as it approaches land. Tsunamis are most common in earthquake zones, particularly around Japan.



Another form of tsunami is called a storm surge, in which giant waves are whipped up by a storm. In 1970 a storm surge and cyclone hit Bangladesh, killing 266,000 people. It returned again in 1985, killing another 10,000 people. 



Picture credit: google



 





 



How fast can the wind blow?



The fastest wind speed ever recorded was measured on a mountain-top in New Hampshire, USA. It reached 597 km/h. However, the wind inside a tornado probably blows much faster. Wind speed is measured according to the Beaufort scale, which was invented in 1805 by a British admiral. On this scale, the strength of the wind is measured by a series of numbers from 0 to 12. Wind speed 0 means that the air is calm wind speed 9 is a gale strong enough to damage houses. A wind of 12 on the Beaufort scale means a hurricane of over 118 km/h.



Picture credit: google



 

Storm and Flood


How big are hurricanes?



Hurricanes can measure between 320 and 480 km across. They travel at speeds of 16 to 24 km/h, growing larger and stronger as they move. Usually they travel west, and then swing east as they reach cooler regions, before gradually dying out. Repeated hurricanes occur during the storm season, and there may be as many as 15 in a single year. In the USA the National Weather Service tracks hurricanes coming out of the Caribbean and the Gulf of Mexico.lt forecasts the time when they will reach land and the strength of the accompanying wind. 



Picture credit: google



 



 





 



The eye of storm



What would you see inside the centre of a hurricane? If you stood in this area, called the eye of the hurricane, you would be in for a surprise. After the violent winds and torrential rain pass over, the wind suddenly drops. A wall of dark churning clouds, hundreds of metres high, surrounds you. There are light breezes and the Sun shines brightly. But you don’t have long to enjoy this peaceful sunny weather, because as the hurricane continues its destructive path it carries the eye with it. You will soon be enveloped in the storm once again. 



Picture credit: google



 



 



 





 



How are hurricanes formed?



Hurricanes are powerful tropical storms that can cause serious damage. They form near the Equator, where warm, moist air rises, drawing in cooler air below. The air moves in a spiral, eventually forming a whirling mass. Rising warm air continues to cool and drop down in the centre of the spiral to repeat the process. The winds inside a hurricane travel at speeds up to 200 km/h. Hurricanes form in the Atlantic Ocean, passing over the West Indies and the southern United States. Similar storms in the Pacific Ocean are called typhoons, whereas in the Indian Ocean they are called cyclones. 



Picture credit: google



 

Storm and Flood


Which world cities are at risk from flooding?



Many of the world’s cities are low lying and threatened by flooding. Bangkok, in Thailand, and Venice, in Italy, are typical old cities built by water because they relied on shipping. Both cities are built on mud and are gradually sinking, increasing the risk of flooding. Sea levels around the world are predicted to rise during this century, with the possibility of devastating flooding in places. Many cities in the Netherlands are below or at sea level. They are always in danger of flooding although elaborate defences help to protect them. The Thames Barrage has been built to prevent high tides flooding the city of London. 



Picture credit: google



 



 





 



What are tornadoes?



Tornadoes are violent destructive whirlwinds whose force is concentrated into a much smaller area than that of a hurricane. They are very common in the central United States, where they cause enormous damage. Tornadoes travel across the land at high speed, and the roaring noise they make can be heard 40 km away. A tornado happens when huge masses of clouds moving in different directions meet. The air begins to spin in a spiral, and a funnel of twisting air reaches out towards the ground. The low pressure in the funnel sucks up soil, dust and anything else it touches — even cattle.



Picture credit: google


Storm and Flood


What causes floods?



Flooding occurs when water cannot drain away fast enough in the rivers. In areas of non-porous rock, water runs off the land very quickly and streams and rivers soon overflow. Flooding also happens when winter snows thaw in spring. Huge floods cover parts of Siberia every spring, when snow melts while the rivers are still iced up. Low-lying coastal lands are vulnerable to flooding, especially when gales and high tides cause water to flow inland. Low-lying Bangladesh is particularly liable to this kind of flooding. In addition, melting snow in the Himalayan Mountains adds huge amounts of water to Bangladesh’s rivers, increasing the flood risk. 



Picture credit: google



 



 





 



 



Where were the worst floods?



Some of the most terrible floods known were in China along the Hwang-he, or Yellow River. The river burst its banks in 1931, killing more than 3.5 million people. For centuries, dykes have been built to prevent flooding, but these burst when the water flow is too great. Huge dams and sluice gates have been installed to reduce the risk of flooding. 



Picture credit: google



 

Oceans



 



 



What causes the tides?



Tides are caused by the gravity of the Moon. As the Earth spins round, the water in the oceans is ‘pulled’ towards the Moon slightly, making a bulge. There is a corresponding bulge on the other side of the Earth. Wherever the bulges are positioned it is high tide. In between the water is shallower and so it is low tide. High tides occur every 25 hours, because at the same time that the Earth is spinning on its axis, the Moon is travelling around the Earth once every 27  days. This means that high tide is about one hour later every day.



 



 



 





 



What is the continental shelf?



The shallow underwater plateaus of land surrounding the continents form the continental shelf. They slope gently down to a depth of about 180 m, after which they slope more sharply towards the deep ocean. The continental shelf can extend out from the coast for long distances, or it may be narrow, for example off the coast of Chile where the Andes Mountains plunge down into the Pacific with hardly any shelf. Usually the shelves are wider off low – flying regions. The continental shelf is a valuable resource, as most fish are found here. Also, there are plans to mine some of the minerals on the sea – bed in these shallow waters.



 



 





How deep are the oceans?



The depth of the oceans varies widely, but the deepest part found so far is called the Marianas Trench. This trench is a deep cut in the bed of the Pacific Ocean, and its depth is as much as 11,034 m. Even at this enormous depth, signs of marine life have been seen in the trench. In 1960 Dr Jacques Piccard and Lt Donald Walsh reached a depth of 10,911 m in a US Navy vessel, Trieste. The Pacific Ocean is home to nine of the ten deepest sea trenches. The exception is the Puerto Rico trench in the western Atlantic Ocean, which has a maximum depth of 9,220 m.



 



 



 





 



Why did sailors fear the Sargasso Sea?



Sailors feared that the Sargasso Sea was haunted by sea monsters and filled with seaweed that would entangle their ships and prevent their escape. The Sargasso Sea is a patch of ocean in the North Atlantic, about 3,200 km west of the Canary Islands. The ocean currents in this area are very slow, although they are surrounded by fast moving currents. This means that any seaweed floating in the Sargasso Sea will not be carried away. Patches of floating weed can be as large as 30 m in diameter. Despite the legends, they are never thick enough to entangle a ship.



 


Oceans



 



Why is the sea salty?



A mixture of salty substances is washed out of rocks by rivers and deposited in the sea. When rain falls on the land it gradually dissolves minerals. The dissolved salts slowly enter rivers and streams, and are carried down eventually into the sea. This process is happening all the time, and so the oceans and seas gradually become more salty.



 



 



 





 



 



 



What is the Gulf Stream?



The Gulf Stream is an ocean current that controls the climate and weather of the whole of Europe. The current starts in the Gulf of Mexico, carrying warm water across the Atlantic and travelling northeast until it reaches the European coast. Water evaporates from the surface of the Gulf Stream causing rainfall in Western Europe.



 



 



 


Continue reading " Oceans"