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Storm Isaac to be downgraded

Tropical storm Isaac is forecast to become a tropical depression by the National Hurricane Centre later today. The Mayor of New Orleans has enforced a dawn-to-dusk curfew as Isaac continues to lash the city.

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What's the difference between a storm and a hurricane?

The National Hurricane Center (NHC) in the US has warned that Tropical Storm Isaac is likely to develop into a category two hurricane after it passes over the Florida Keys.

A storm becomes a hurricane when sustained winds reach a minimum of 74 miles per hour.

The NHC said Isaac will strengthen to hurricane status with "extremely dangerous" sustained winds of 105 miles per hour towards the middle of next week.

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