EQ2 – How are extreme weather events increasingly hazardous for people?-1.4 Tropical Cyclones Flashcards
Complete the Fact File for Tropical cyclones: Pressure; Rotation
Direction; Structure; Location around world; Oceans where hurricanes, typhoons and cyclones occur
A. Pressure; Rotation Direction; Structure; Location around world; Oceans where hurricanes, typhoons and cyclones occur
B. A cyclone is an area of winds blowing in a circular motion, usually centred on an area of low pressure. They circle clockwise in the southern hemisphere, and anti-clockwise in the northern hemisphere.
Cyclones often have different names, depending on where they occur. In the north Atlantic Ocean, they are called hurricanes, and they are often called typhoons in the northwest Pacific Ocean.
Cyclones can occur all year round in the southern hemisphere. The end of February and early March is the most common time for them to happen and is the height of the cyclone season.
Typhoons often occur on other planets, including Neptune and Mars. These storms have thunder, lightning and rain and can be many times stronger than those on Earth.
Tropical cyclones typically measure between 100 and 2,000 km across. The smallest occur in the eastern Pacific, while the largest usually occur in the northwest Pacific Ocean.
Tropical cyclone Bhola, which hit Bangladesh in 1970 may have killed half a million people. This densely populated area has always been one of the most active areas for cyclones.
In parts of Asia and the South China Sea, cyclones are called typhoons. Like hurricanes, they are named and are often given a name in the local language, based on where they formed.
Tropical cyclones can cause strong winds and heavy rains. Although they weaken once they reach land, they can cause heavy flooding up to 40 km from the coast.
In 1979, Typhoon Tip reached wind speeds of about 310 km per hour in the Pacific Ocean. In 1997, Typhoon Paka, which struck Guam in the Pacific, had estimated wind speeds of about 378 kmh.
There are usually between 75 and 100 tropical cyclones each year. Scientists believe that the strength of these storms is increasing, in some cases by about 15 percent.
Explain the formation of Tropical cyclones, include: temperature of sea, air pressure, condensation, energy, rising air, rotation
A. Temperature, sea, air pressure, condensation, energy, rising air, rotation, equator, north, south
B. They need a lot of heat to form and a sea surface temperature of at least 26.5°C, which is why they usually occur over tropical seas. They also need to be between 5 and 30° north or south of the Equator. It works like this:
When this warm and wet air rises, it condenses to form towering clouds, heavy rainfall. It also creates a low pressure zone near the surface of the water.
Rising warm air causes the pressure to decrease at higher altitudes. Warm air is under a higher pressure than cold air, so moves towards the ‘space’ occupied by the colder, lower pressure, air. So the low pressure ‘sucks in’ air from the warm surroundings, which then also rises. A continuous upflow of warm and wet air continues to create clouds and rain.
Air that surrounds the low pressure zone at the centre flows in a spiral at very high speeds - anti-clockwise in the northern hemisphere - at speeds of around 120 km/h (75 mph).
Air is ejected at the top of the storm – which can be 15km high – and falls to the outside of the storm, out and over the top, away from the eye of the storm. As this happens, it reduces the mass of air over the ‘eye of the storm’ - causing the wind speed to increase further. Some ejected air also cools and dries, and sinks through the eye of the storm, adding to the low pressure at the centre.
The faster the winds blow, the lower the air pressure in the centre, and so the cycle continues. The hurricane grows stronger and stronger.
Seen from above, hurricanes are huge circular bodies of thick cloud around 450 km (300 miles) wide. The cloud brings heavy rain, thunder and lightning.
In the centre is the eye of the hurricane, about 45 km across (30 miles) across. Often there will be no clouds in the eye. Seen from below it will seem calmer, with a circle of blue sky above. The eye is formed because this is the only part of the hurricane where cold air is descending.
In the northern hemisphere, the prevailing easterly tropical winds tend to steer hurricanes toward land - although their course is unpredictable. As hurricanes move inshore, their power gradually reduces because their energy comes from sucking up moist sea air.
Explain why tropical cyclones intensify in energy
A. Warm ocean, low pressure, strong winds, clouds, heavy rainfall
B. The air above the warm ocean is heated and once it reaches 27°C, the warm air rises quickly causing area of low pressure. It draws more warm moist air up from the ocean leading to strong winds. The rapidly rising warm air spirals upwards, cools and forms large clouds. These produce heavy rainfall.
Explain why sometimes their energy is lost (dissipated)
A. Lose strength, land, water, air
B. They dissipate (lose strength) when they move over land or cooler water because the energy supply from the warm water is cut off. Changes in windspeed, e.g. from meeting other weather systems, can also cause a cyclone to dissipate.
