It's a Monday afternoon in Happy Valley and students are spreading out blankets to sit in the sun and relax. Could it really be the middle of January? Abnormally high temperatures have pervaded Pennsylvanian winters in recent years and especially this year.

Many people, warm and cold weather lovers alike, are wondering what is causing this change.

"It's not global warming," said Sam Perugini (graduate-meteorology).

Perugini said Earth is in an interglacial (warmer) period. During interglacials, average annual temperatures rise for many years until they level off for a short time.

They then begin a normal decline into another glacial period or "Ice Age." Most interglacials last about 40,000 years, according to The Earth System with Global Change Update. Earth's current interglacial period, called the Holocene Epoch, began about 10,000 years ago.

"If you look on a larger scale, the climate has large fluctuations naturally," Perugini said.

Lee Grenci, instructor of meteorology, also said the ongoing trend toward warmer average temperatures can not simply be attributed to global warming, or any single climatic factor.

Since the weather is highly variable in nature, it is difficult to pinpoint exactly what is causing the warm weather trend, he said.

Both Grenci and Perugini agreed that the extent of human impact on the global climate to date is uncertain.

Temperatures in Pennsylvania have risen about 0.25 to 0.40 degrees Fahrenheit per decade during the last half-century, according to information from the Climate Prediction Center's Web site (www.cpc.ncep.noaa.gov).

This might not seem like a lot, but again, on a larger scale the changes become more apparent.

During the week of Feb. 3 to 9 of this year, average daily temperatures in State College were 3 to 7 degrees above normal, the site said. And for the month of January, average temperatures in State College were about 7 to 9 degrees above normal.

This record accounts for the cold snap at the beginning of the month -- which brought the only snowstorm Happy Valley has seen thus far this winter -- as well as the unseasonably high temperatures at the end of the month.

Many people refer to that weeklong warm spell as a "January thaw." But Grenci is quick to dismiss this moniker.

"It wasn't really a thaw because the typical cold weather [of January] didn't occur," he said.

But what has been responsible for temperatures in the 50s and 60s when they should normally be in the 20s and 30s?

"What we're seeing this winter is mostly westerly air flow in the atmosphere," Grenci said. This means the air in the atmosphere flows nearly straight from west to east, he said, referring to the map at left.

The lines on the map represent contours of equal heights, around 18,000 feet above sea level, at which a certain pressure is found, and atmospheric airflow is generally parallel to these lines, Grenci said.

Normally, the lines have much larger amplitudes, curving in a "serpentine" manner and displaying ridges and troughs that extend much farther north and south, he said. This allows cold air masses from Canada to move southward into our region in the winter. But this season's "zonal," or west-to-east, air pattern has been preventing this process from occurring.

"We are experiencing milder Pacific air masses instead of frigid Arctic air masses," Grenci said, adding that the zonal pattern also is responsible for the minimal amount of snowfall in State College this winter. In the same way that Arctic air hasn't been able to move south, moist air from the Gulf hasn't been able to move north, so State College has not seen a lot of heavy snow, he said.

Furthermore, local climate records beginning in 1948 indicate the temperature has not reached the single digits yet this year, and it has not gone below zero in the past six years.

"The 1990s were overall the warmest decade," Grenci said.

Perugini said that although no monthly high-temperature records have been set thus far, the winter of 2001 to 2002 might set a seasonal record high.

But even with this knowledge, Perugini said the unseasonably warm weather does not surprise him, and does not make forecasting noticeably more difficult.

"The weather just naturally fluctuates," he said. "This season is unusual, but not unprecedented."