A satellite image of Typhoon Haiyan
Media playback is unsupported on your device

What causes a tropical storm?

The Philippines are hit by more than 20 tropical storms a year, says the BBC's Jay Wynne.

At least three or four turn into typhoons.

But why is the Philippines so susceptible to such storms? And what are the factors necessary for a tropical storm to form?

Go to next video: Typhoon Haiyan tears through Philippines