Flooding Caused by Storm Harvey

This is the extent of flooding caused by Storm Harvey. Drone footage over downtown Houston shows most of the city submerged in flood water. Several people have been killed and tens of thousands have been driven from their homes.

Rescue workers helped evacuees reach safety after hundreds were stranded in the catastrophic storm. Damage caused by Harvey is estimated to cost billions of dollars and rebuilding is likely to last years.

Why is Storm Harvey so extreme?

1. It intensified before making landfall - Some hurricanes in the Gulf of Mexico weaken as they approach land. But Harvey hovered over a warm eddy gathering power.

2. Warm sea temperatures - These have made more water evaporate, causing more rain. The Gulf of Mexico has been 1 to 2C warmer than usual. Some scientists say climate change is likely to be boosting temperatures.

3. The storm stayed put - This means it has dumped more and more rain on the same places. Areas of high pressure further afield have slowed its movement. And it's stayed close to the sea gathering yet more water.