The National Aeronautics and Space Administration said La Ninas occur when lower than normal sea surface temperatures form along the equator in the Pacific Ocean.

"With this La Nina, the sea-surface temperatures are about two degrees (lower) than normal in the eastern Pacific and that's a pretty significant difference," said David Adamec of NASA's Goddard Space Flight Center in Greenbelt, Md. "I know it doesn't sound like much, but remember this is water that probably covers an area the size of the United States. It's like you put this big air conditioner out there -- and the atmosphere is going to feel it."

The lengths of La Nina events vary as well, some lasting for several years. NASA said it will monitor the phenomenon by satellite to determine if the current La Nina is a normal event and begins to weaken during February.