Krishna Mukherjee, an assistant professor in the physics and pre-engineering department at Slippery Rock University, runs the planetarium at SRU. She said our winter solstice is the moment when the planet's Southern Hemisphere is tilted most toward the sun.

Winter officially begins today at 11:48 p.m. with the arrival of the winter solstice. So why has the weather been feeling more like April and what are the chances for a white Christmas?

After all, the winter solstice means today is the shortest day of the year, according to Krishna Mukherjee, an assistant professor in the physics and pre-engineering department at Slippery Rock University who teaches astronomy and space science classes and is in charge of the university planetarium.

“In the Northern Hemisphere, we will experience the shortest day of the year,” said Mukherjee, “It's the shortest day in the terms of the amount of sunlight. If you notice as the sun rises in the east and sets in the west, it moves progressively a little south.”

But added darkness doesn't necessarily mean increased cold, she said.

“People think the distance to the sun matters, but it's the height of the sun that makes the Earth warmer,” Mukherjee said. “During the winter, the sun is 26 to 27 degrees above the horizon. In the summer, it's 72 to 73 degrees up from the horizon. It varies throughout the year.”

She said the sun's height above the horizon is because of the tilt of the Earth's axis. The planet's rotation axis is not perpendicular to the path the Earth travels around the sun.

The winter solstice is the moment when the planet's Southern Hemisphere is tilted most toward the sun, Mukherjee said.

After today the planet's tilt will start moving back to the north, meaning for us each day's periods of sunlight will slowly grow longer until we reach the summer solstice on June 20, 2016.

So if the Earth is following its age-old pattern as it travels around the sun, what's keeping the temperature in the 40s and 50s and the precipitation in the form of rain, not snow?

“El Niño has something to do with it,” said Bob Smerbeck, a senior meteorologist with AccuWeather, the State College-based weather forecast.The phenomenon of warmer waters in the Pacific Ocean is causing the jet streams over the nation to shift and, for Western Pennsylvania, holding the cold at bay farther north.“Right now, it is keeping the air masses up in Canada,” said Smerbeck.At the same time, an air flow from the Gulf of Mexico is bringing warm air into Western Pennsylvania and keeping the mercury from dropping.Normal temperatures at this time of year feature highs of 36 and lows of 21, he noted.So, no white Christmas this year?“My inclination is to say, no, we are going to have a wet Christmas,” Smerbeck said.“The official forecast is mild and cloudy with a high of 54 on Christmas Eve. There could be some showers Christmas Day with a high of 66, but there could be a chance of snow showers later after that system goes by if enough cold air comes in behind it.”Smerbeck said any snow wouldn't be heavy, more of a dusting.If there is any snow, a full moon Christmas Night will make it look spectacular, Mukherjee said.Before that, she added, Wednesday, will be the best time to view the Ursid meteor showers, an annual event.The best viewing time will be between 3 and 4 a.m. when the meteors will be appearing between the Big and Little Dipper constellations, she said.Going into the New Year, Smerbeck said long-range scenarios at AccuWeather have higher-than-normal temperatures lasting through January.

The Earth is seen in its position in relation to the sun in each season.