Typhoon Koppu has killed at least 20 people in a series of devastating floods and landslides on northern Luzon Island in the Philippines, causing extensive damage to many homes and businesses and, in particular to the the region's rice crop, which was close to being harvested. The storm initially struck the east coast of the island on the morning of Sunday 18 October 2015, bringing with it 210 km per hour winds, and torrential rains as well as a substantial storm surge, causing widespread flooding and damage to crops, however local authorities had arranged the evacuation of around 60 000 people from low-lying areas in the path of the typhoon, keeping casualties to a minimum. However after crossing the island Typhoon Koppu then suddenly veered to the northeast on Monday 19 October, sweeping across the northwestern tip of the island. It is here the majority of the casualties are casualties are thought to have occurred, since while the storm was much weaker having dissipated much of its energy as it passed across the island, the people here were much less well prepared.
Flooding in Baguio City on Luzon, caused by Typhoon Koppu. Harley Palangchao/Reuters.
Tropical storms are caused by solar energy heating the air above the oceans, which causes the air to rise leading to an inrush of air. If this happens over a large enough area the in rushing air will start to circulate, as the rotation of the Earth causes the winds closer to the equator to move eastwards compared to those further away (the Coriolis Effect). This leads to tropical storms rotating clockwise in the southern hemisphere and anticlockwise in the northern hemisphere.These storms tend to grow in strength as they move across the ocean and lose it as they pass over land (this is not completely true: many tropical storms peter out without reaching land due to wider atmospheric patterns), since the land tends to absorb solar energy while the sea reflects it.
The path of Typhoon Koppu to date (thick line), and its predicted future path (thin line, circles represent margin of error. Tropical Storm Risk.
The low pressure above tropical storms causes water to rise there by ~1 cm for every millibar drop in pressure, leading to a storm surge that can overwhelm low-lying coastal areas, while at the same time the heat leads to high levels of evaporation from the sea - and subsequently high levels of rainfall. This can cause additional flooding on land, as well as landslides, which are are a common problem after severe weather events, as excess pore water pressure can overcome cohesion in soil and sediments, allowing them to flow like liquids. Approximately 90% of all landslides are caused by heavy rainfall.
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