Wondering why it is sweltering hot this winter when it was biting cold same time last year?

Believe it or not, the reason behind this unusually balmy December is also the same reason the United States had a harsh winter in 2014: the infamous polar vortex.

Simply put, a polar vortex is an Arctic cyclone or a large mass of extremely cold air that sits over the poles all year round, and intensifies during the winter.

The behavior of the polar vortex is similar to that of spinning toy top. It is more stable when it is spinning fast, but wobbles when it becomes weaker.

According to meteorologists interviewed by NBC News, the polar vortex is quite strong this year, fencing all the cold in areas such as the North Pole, northern Canada and Siberia.

James Mitchell, a National Weather Service meteorologist in Buffalo, explained that the strong polar vortex is indeed preventing the cold air from going south.

"The jet stream has been to our north and we haven't seen it coming south. It's keeping all the cold air bottled up into Canada," Mitchell was quoted by NBC News as saying.

Conversely, when the polar vortex gets weaker it causes the cold to migrate south and stay during an extended period of time in the U.S.

Meteorologists also attribute the warm winter this year to the El Nino weather pattern, which is characterised by the significant rising of sea surface temperatures in the equatorial Pacific.

Because of these factors, temperatures of 10 to 30 degrees above normal should be expected across America — from the South through the Midwest and the East Coast.

Mitchell also said that snowfall should be expected only until late this week, and it would not be that significant.

"For sure, it's going to eventually get colder," the meteorologist added. "But for now, it's well above normal."