Oct 29, 2012 was a dark day for the American people. A huge hurricane (飓风) named Sandy hit the US, along with thunderstorms, heavy snow and flooding. It caused great damage in many places. Cities like New York were left with flooded streets and no electricity.

A hurricane is a kind of tropical cyclone (热带气旋) that is found in the northeastern Pacific Ocean, South Pacific, or North Atlantic Ocean. When it appears in the northwestern Pacific, it is called a typhoon. Southern China has the longest record of experiencing typhoons in the area.

So hurricanes and typhoons are the same thing; the only difference is where they happen. They form on the sea and are caused by temperature differences.

There is another kind of cyclone called a tornado. A tornado forms from the twisting (扭曲) of rising air. It's a violently rotating (旋转的) column of air that connects with clouds above it and the land below.

Tornados form on the land and often begin with thunderstorms. The US has the most tornados of any country. This is because of some of the special features of the North American continent. The continent spreads from the tropical north to the arctic (北极的). There are no large mountains from east to west, so it is easy for warm and cold air to meet. When this happens, thunderstorms soon follow.

As for Hurricane Sandy, scientists believe that it began due to unusually warm water. Then it joined with a large Arctic weather system, which increased its size and changed it into a winter storm with far more power.

On June 18 at 10:55 p.m., Mrs. Zhang was in bed in her home in Chengdu. Suddenly, from outside her house, she heard a loud voice counting “10, 9, 8, 7...”

It was an earthquake warning. Just a few seconds before the warning started, an earthquake had started more than 200 kilometers away.

Mrs. Zhang immediately got out of bed and ran downstairs to find a safe place. All over Chengdu, there are hundreds of speakers ready to give people early warnings and time to get out of danger.

Earthquake warning systems work because radio signals（信号）are able to travel faster than earthquakes do. If an earthquake starts, a warning is sent out by radio. Cities that are further away get a longer warning time.

According to earthquake research, just three seconds is enough time to save 14% of deaths from an earthquake.