dr. radley horton is an expert on these super storms. he's a climate scientist at columbia university. welcome. >> thank you. it's good to be here. what's the definition of a hurricane? a hurricane is a storm with very strong winds. it's a storm that forms over the warm waters of the tropical oceans, and as soon as that storm's winds get up to about 35 miles an hour, we call it a tropical storm. and if if its winds get up to 75 miles an hour, it's a hurricane. >> how are hurricanes classified? >> hurricanes are classified by how strong their winds are. a category 1 hurricane, which is the weakest, has winds just above 75 miles an hour. category 5, which is the strongest, is winds at around 150 miles an hour. >> so, what causes these hurricanes? >> hurricanes are caused by a combination of factors. several things have to come together for you to get a hurricane. one of the most important things is a warm ocean. waters of the tropics. with the warm sun beating down, when temperatures get to be about 80 degrees fahrenheit, conditions are right for a lot of evaporation, which provides a lo