Why did Typhoon Haiyan cause so much destruction?

Typhoon Haiyan, one of the most powerful and destructive tropical cyclones ever seen, has devastated parts of the Philippines, destroying homes and killing thousands of people.

How did this happen?

Tropical cyclones - or typhoons, or hurricanes... they are all the same thing - form over the ocean when warm sea temperatures and an unstable atmosphere together build a large, rotating storm.

A radar image showing the wind speed of Typhoon Haiyan as it hit the Philippines. The red and pink parts are the most intense. Note the calm 'eye' of the storm. Source: Philippines Government

In the Philippines, a tropical cyclone gets called a typhoon when wind speeds are above 118 km/hr. Typhoon Haiyan had winds up to 325 km/hr! Scientists are still looking at all of the data that was collected during the typhoon, but they think that typhoon Haiyan was the fourth most powerful tropical cyclone that we know about, and the most powerful to reach the land.

Haiyan passed directly over some large cities that were home to a lot of poor people, and their houses were not strong enough to protect them from the typhoon. If the typhoon had passed over land where not many people lived, or where the buildings were better built, then there would not have been nearly as much destruction and loss of life.

Typhoon Haiyan hit big cities like Tacloban which were not built to survive such a big storm.
By Trocaire from Ireland (DSC_0974) [CC-BY-2.0], via Wikimedia Commons

Strong wind can blow down houses, trees and other structures, but it can cause another problem too. When the wind pushes the ocean up against the land this is called storm surge. Storm surges can flood large areas. In the Philippines, the storm surge was estimated to be more than five metres high, and was big enough to destroy the two-story terminal building at an airport.

Hurricane Katrina was a powerful tropical cyclone that hit New Orleans in 2005. Much of the destruction there was caused by the storm surge, which broke the barriers that were built to hold the ocean back, causing massive flooding that lasted for weeks.

Hurricane Katrina caused massive flooding in New Orleans. Source: White House

Tropical cyclones also dump huge amounts of rain in a short amount of time, and this can lead to flash flooding.

Cyclones get their energy from the sea, so once they cross the land they tend to weaken until they eventually break apart and disappear. Haiyan caused the most destruction on the islands of the Philippines. By the time it reached the mainland of Vietnam and China it was much weaker.

Typhoon Haiyan crossed the Philippines and then continued on to Vietnam and China. The red dots are where it was most destructive.
By Keith Edkins [Public domain], via Wikimedia Commons

Scientists aren't completely sure what affect climate change is going to have on tropical cyclones, but they know that the ocean is getting warmer, and that warm water gives energy to cyclones, and so they suspect that tropical cyclones will get worse in the future as the world warms up.

Cyclonic Links
To get a scale of the destruction in the Philippines, check out these before and after photos at the ABC. The ABC also have a good page explaining about tropical cyclones.

Thanks to CSIRO's Science by Email for the links.