WIND, MONSOONS AND CYCLONES

See Bos 157C: what is the most dangerous month for typhoons in Japan?

Cyclones = tr… storms which start above warm seawater (> 27 C)

Haiyan:windspeed .>300 km/hr (34 mln Euro collected in Neth):

Saffi r Simpson Scale = category 1,2,3,4,5 ? Storm surge and tides (tsunami = tidal wave) Potentially, the most destructive phenomenon associated with

tropical cyclones that make landfall is the storm surge. Storm surge is a raised dome of water about 60 to 80 km across and typically about 2 to 5 m higher than the normal tide level. If the surge occurs at the same time as a high tide then the area inundated (fl ooded) can be quite extensive, particularly along low-lying coastlines.

See Bos 157C ; what proportion of the coastline of Japan is threatened by tsunamis ¼ - 2/4 -3/4 4/4

CYCLONES AND STORM SURGES

Tropical cyclones move with the tradewinds to the west (fi nish North/NE) (see sketch)

Tropical cyclones develop in Tropical Area above seawater 27C

Develop in the summer-autumn period of each hemisphere so: ( 21 june till 21 dec in Northern Hemisphere) or (21 ….. till 21 ……… in Southern Hemishpere)Storm develops because sea water starts to evaporate and

this evaporated air starts to rise: the rising air cools and condensates -> rain. As a result of this proces energy is released and the air can rise still higher.

At the surface of the water a very deep low pressure centre evolves (920 HPA air pressure (usu.1010): result: more air will be sucked into the this system and the storm will develop into a superstorm

TROPICAL CYCLONES

  Tropical cyclones use warm, moist air as fuel . That is why they

form only over warm ocean waters near the equator. The warm, moist air over the ocean rises rapidly upward from near the surface and becomes saturated (verzadigd) with evaporated moisture (vocht) . This means that there is less air left at the surface (i.e. low pressure). Air from surrounding areas with higher air pressure pushes in to the low pressure area to try to equalise the pressure. Then that “new” air becomes warm and moist and rises, too. As the warm air continues to rise, the surrounding air swirls in to take its place.

Trade winds (blow to the Equator) cause the moist air to spin inwards. As the warmed, moist air rises and cools off , the water in the air forms towering cumulonimbus (bloemkoolwolken) thunderclouds (because there is a huge amount of condensation). The whole system spins and grows, fed by the ocean’s heat and water evaporating from the surface. The diagram below shows a cross section system of clouds and wind through a tropical storm.

TROPICAL CYCLONES

TYPHOONS

TROPICAL CYCLONE

As the storm system spins faster and faster, an eye forms in the centre.

It is very calm and clear in the eye, with very low air pressure.

Higher pressure air from above flows down into the

eye

STORMS

SAFFIR SIMPSON

Tropical cyclones usually weaken when they make landfall (i.e. hit the land) because they are no longer being fed by the energy from the warm ocean waters. In the northern hemisphere they track (i.e. move) westwards due to the Coriolis eff ect.

The cyclone season starts at the end of het summer and in autumn. At that time the land is already cooling down more than the surrounding sea. This slows down also the power of the cyclones when they make landfall

TROPICAL CYCLONES

MAIN WIND SYSTEMS

Useful weblinks: BBC animation - the formation of a hurricane NASA animated guide to the development and structu

re of tropical storms Make a tropical storm - simulator Investigate how winds work together to determine wh

ere a hurricane will strike - simulator BBC Bitesize

revision notes about the formation of tropical storms   http://worldlywise.pbworks.com/w/page/25349490/Uni

t%202%20Section%20C%20-%20Causes%20and%20effects%20of%20tropical%20storms%20and%20responses%20to%20them (also animations of tsunamis and earhquakes etc!)

WEBLINKS