Many people like the feeling of the gentle wind in spring. Many like to see the falling leaves dancing in the wind in autumn. But sometimes, when the wind becomes a storm, it can be very destructive  . A series of such storms struck the US last month and caused very serious damage and human pain. Every year, major storms cause many problems around the world. There is nothing people can do to stop these powerful forces of nature. But new techniques are helping scientists to predict how, when, and where big storms will happen. The more exact scientists' warnings are, the better people can prepare for the storms. Predictions are improving. "We've gotten better over the years, especially the last few years," says Phil Klotzback, a scientist at an American university. How is a storm formed? Even if scientists know where a storm will happen, winds can suddenly change, carrying the storm to a new direction. "For a hurricane to happen, conditions have to be just right," Klotzback says. First, the ocean water needs to be warm enough so that it _ and rises into the air. As it rises, the air containing drops of water cools and turns back into liquid. This process gives off heat. This produces energy like an engine that causes winds to increase. It drives the formation of a hurricane. If wind speeds reach 40 miles per hour, the system is called a " tropical storm", and it gets a name. At 75 miles per hour, it becomes a hurricane. Hurricanes that hit the US start when a thunderstorm forms off the coast Africa. Storms also develop over tropical waters in other parts of the world. On average, 60 or 70 storms form off Africa every year. About 10 of them get names. There are usually about six hurricanes. Two tend to be very big, with winds of 115 miles per hour or a bit higher. The hurricane season lasts from June to November. Ninety percent of all hurricanes hit in August, September and October. According to the text, hurricanes usually   _  .
form off the coast of Africa and America hit parts of the world in summer and autumn cause sea winds to rise and blow over the sea strike the US but cause no damage