The Science Behind Typhoon Haiyan

Less than a week ago, the Phillippines were devastated by Typhoon Haiyan. In the aftermath, the death toll and total cost of damages remain uncertain, although the casualty estimate currently stands at 2,500, with more than 600,000 others forced from their homes. The storm follows directly on the tail of a magnitude 7.2 earthquake that rocked the country in October.

Haiyan is not just another typhoon. With sustained wind speeds of over 310 km/h (195 mph), Haiyan is the most powerful recorded storm ever to make landfall. The previous record was held by Hurricane Camille, which hit Mississippi in 1969 with wind speeds of 300 km/h (190 mph). In comparison, Hurricane Katrina clocked in at 280 km/h (175 mph) and Sandy at 185 km/h (115 mph). Despite the record-breaking wind intensity, perhaps the most destructive force of Haiyan was the 5 meter ocean swell that accompanied it.

Paths of tropical storms 1985-2005

Typhoons, hurricanes, and cyclones all refer to tropical storms but in different geographic areas. The term hurricane is used in the Atlantic and northeast Pacific, typhoon in the northwest Pacific, and cyclone in the south Pacific and Indian oceans. They are massive low-pressure systems, spanning 100 to 4000 kilometers in diameter. Tropical storms are fueled by the evaporation of warm water from the surface ocean. As the water evaporates, it rises, cools, condenses into clouds, and falls again as rain, forming a continuous cell of strong rising and falling currents. In the center or eye of the storm, a strong downdraft prevents clouds from forming and creates an area of calm, with no wind or rain. The clouds directly surrounding the eye, known as the eye wall, are home to the strongest winds. The spin of the storm is a result of the rotation of the Earth on its axis. The Coriolis Effect, as it is called, causes storms to rotate clockwise in the Southern Hemisphere and counterclockwise in the Northern Hemisphere. The storm surge is the ocean swell caused by the strong winds.

Hurricane cross-section. Notice the rising currents throughout most of the structure and the falling current in the eye.

Satellite image of Hurricane Katrina. The ‘arms’ show the counterclockwise spin, and the eye can be seen in the center.

It seems that in the past decade, we’ve heard about more and more severe storms, especially in the Atlantic. Could climate change be the cause? Conceptually, scientists support this idea. Since warm water is the ‘fuel’ for tropical storms, rising surface ocean temperatures could well lead to more frequent and severe storms. However, establishing a causal link is much more difficult to do. What is clear is that storm frequency and intensity have been gradually increasing, particularly in the Atlantic. And while future trends are difficult to predict, a recent MIT study suspects that the rising trend should be expected to continue.

A view of Hurricane Isabel

This has been your daily dose of mostly harmless science.

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Cover photo courtesy of kakela, Haiyan photo from Mans Unides, and Isabel photo from Brad Gatlin, Flickr Creative Commons. Satellite images courtesy of Wikimedia Commons. Hurricane cross-section from CYGNSS, University of Michigan.