thousands of people along our nation's gulf and east coasts, those names are reminders of the incredibly emily tells us more. >> katrina in 2005, ike in 2008, and sandy 2012. all were powerful hurricanes that slammed into the u.s. in recent years. their winds, rain, and flooding caused tremendous damage. so, no wonder that the more we learn about hurricanes, the better. 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 above 75 miles an hour. category 5, which is the strongest, is winds at around 150 miles an hour. >> so, what causes these hurricanes? >> hurr

of soviet communism was called east berlin. this film by the u.s. government helps to explain what it was like to live in postwar berlin. >> two and a half million people, more than half of berlin's population, lived in the american, british, and french zones. but the whole city was surrounded by the soviet zone of germany. supplying the two and a half million people of the western sectors was a system of rails, canals, and roads. >> but the soviets wanted berlin all to themselves. so, they set up a giant blockade, cutting off the roads and railroads to west berlin. >> the suspension of all traffic successfully blocked all surface access to the city. the soviets claimed that technical difficulties caused the stoppage. the truth was that they were trying to force the western allies to surrender their position in berlin. and the weapon was hunger. to get in, the soviets expected the allies to pack up and leave. but they didn't. the united states and its allies refused to abandon the people to be starved into accepting soviet rule. there was a big problem, though -- how to feed more t