Why is Sandy unusual?

Atlantic City, New Jersey, resident Kim Johnson inspects the area around her apartment building, which flooded on Tuesday, October 30. Large sections of an old boardwalk also were destroyed by Superstorm Sandy. Nearly 11,000 people spent Monday night in 258 Red Cross-operated shelters across 16 states because of Sandy, the American Red Cross tells CNN. View photos of New York recovering from impact.

Cars float in a flooded parking area on Tuesday in the financial district of New York.

A power line knocked over by a falling tree blocks a street on Tuesday in Chevy Chase, Maryland.

Waves break next to an apartment building in Atlantic City, New Jersey, on Tuesday.

Workers shovel debris from the streets in Ocean City, Maryland, on Tuesday.

A U.S. Coast Guard helicopter flies over Central Park in New York City.

A man jogs near a darkened Manhattan skyline on Tuesday after much of New York City lost electricity.

Workers clear a tree blocking East 96th Street in Central Park in New York on Tuesday.

Rising water rushes into an underground parking garage in New York's financial district on Monday, October 29.

Taxis drive down a New York street where the power was out late Monday, October 29.

A firefighter speaks to a colleague while surveying damage caused by Sandy on Monday in New York.

Flooded cars line the streets of New York's financial district Monday night.

A truck drives by a flooded gas station in the Gowanus section of Brooklyn on Monday.

A flooded street is seen at nightfall during the storm on Monday in Atlantic City, New Jersey.

Firefighters evaluate an apartment building in New York that had the front wall collapse during the storm on Monday.

Heavy rains fall in Manhattan on Monday.

People walk through water on the beach near high tide Monday as Sandy approaches Atlantic City.

Two men run down Foster Avenue while dodging high winds and waves from the storm on Monday in Marshfield, Massachusetts.

An emergency vehicle plows through floodwaters on Monday in Dewey Beach, Delaware.

A person tries to cross the street during the storm on Monday in Atlantic City.

A traffic sign warns motorists west of Philadelphia on Monday.

A wall of water makes its way to shore as residents brave the storm Monday in Ocean City, Maryland.

A downed tree and fallen power lines lie over homes Monday on Harvard Street in Garden City, New York.

Two people shoot video along Brooklyn Heights' Promenade on Monday as Sandy approaches landfall.

Work crews push sand from a roadway in Fort Lauderdale, Florida, due to storm surge related to flooding on Monday.

Two women battle wind and rain with umbrellas in hand in Philadelphia on Monday.

Kira Brizill leads family members as high tide and winds flood the street on Monday in Freeport, New York.

John Edgecombe II, who is homeless, takes refuge from the rain and wind at a bus stop in Ward Circle in Washington on Monday.

Superstorm Sandy dumped a lot of rain, flooding a part of Greenpoint, Brooklyn

A Pennsylvania Department of Transportation truck slowly drives on the Pennsylvania Turnpike as Sandy approaches Bensalem, Pennsylvania, on Monday.

Buses at Frankford terminal in Philadelphia, Pennsylvania, sit idle after Mayor Michael Nutter ordered that all city offices be closed Monday and Tuesday due to potential damage from Sandy.

A woman walks down the promenade along the East River in New York City on Monday.

Jillian Webb, left, and Arianna Corso are pelted by wind and sand on Lighthouse Beach in Chatham, Massachusetts, on Monday.

Waves slam into the sea wall in Scituate, Massachusetts, on Monday.

Chris Losordo carries his father, Vin, across a flooded road in Falmouth, Massachusetts, on Monday.

A repair truck drives down a flooded street in Rehoboth Beach, Delaware, on Monday.

Superstorm Sandy dumped a lot of rain on West Side Highway in Manhattan, NY.

Floodwaters cover the streets of Rehoboth Beach, Delaware, on Monday.

Multiple waves hit the Cooper's Beach in Southampton, N.Y.

Waves crash against a previously damaged pier in Atlantic City, New Jersey, as Hurricane Sandy approaches landfall on Monday.

High winds broke part of a crane boom on this building under construction in Manhattan, causing several nearby buildings to be evacuated.

An emergency vehicle drives down Cape May, New Jersey's flooded Ocean Avenue on Monday.

A young boy runs along Rockaway Beach in the Queens, New York, on Monday.

A woman examines her storm-damaged porch as heavy rain continues to pour in Winthrop, Massachusetts, on Monday.

A lone figure makes his way down Seventh Street in Lindenhurst, New York, on Monday.

People brave high winds and waves in Winthrop, Massachusetts, as Hurricane Sandy moves up the coast on Monday.

A tree felled by the storm blocks Kramer Drive in Lindenhurst, New York, on Monday.

Waves crash over a street in Winthrop, Massachusetts, as Hurricane Sandy comes up the coast on Monday.

A police vehicle drives through a flooded area in New York on Monday.

The New York skyline is seen from the bank of the East River on Monday.

People walk on the boardwalk in Ocean City, Maryland, on Monday.

A man stands on the beach as heavy waves pound the shoreline Monday in Cape May, New Jersey.

The dome of the U.S. Capitol building is seen through a window as heavy rain hits Washington on Monday.

A member of the press takes a photo of a flooded street on Monday in Atlantic City, New Jersey.

A man takes a picture of the storm with his phone from the boardwalk in Ocean City, Maryland, on Monday.

A man stands on the sidewalk Monday as a vehicle drives up a flooded street in Atlantic City.

The Hudson River comes over the sea wall along the West Side Promenade in the Battery Park area in New York on Monday.

The owner of the Wilton House locks up his bar on Monday in Hoboken, New Jersey, as Hurricane Sandy approaches the area.

Two people stand near the edge of the boardwalk on Monday in Ocean City, Maryland.

People fight against the wind along Brighton Beach in New York on Monday.

A jogger runs along the East River in New York on Monday as a police car secures the area.

A man watches as the tidal surge pounds a pier in Ocean City, Maryland, on Monday.

A street on the shoreline of Milford, Connecticut, floods at high tide as Hurricane Sandy approaches on Monday.

A sailboat smashes on the rocks after breaking free from its mooring on City Island, New York, on Monday.

A lone tourist stands in Times Square early Monday as New Yorkers brace against Hurricane Sandy.

A satellite image taken at 12:25 p.m. ET Monday shows Sandy moving over the Northeast.

A restaurant on the boardwalk in Atlantic City, New Jersey, is boarded up in preparation for the bad weather on Monday.

A man walks down a flooded street in Atlantic City on Monday before the hurricane makes landfall.

Tourists wear plastic ponchos in Times Square on Monday.