Anyone who lives in the northern United States can tell you there's a big difference between a 30-degree day in October and a 30-degree day in February.
The former is bone chilling, the latter balmy.
It's not only a psychological state. There's also habituation, which means that after repeated exposure to the cold, the temperature of human skin drops less, leaving people warmer and more comfortable in the colder temperatures.
It's an advantage the Green Bay Packers will have over the warm-weather Dallas Cowboys on Sunday in the NFL's divisional round of the playoffs, small as that advantage might be.
"Part of it is just psychological to begin with, the fact that Green Bay always has played in cold-weather games at Lambeau (Field)," said John Castellani, a physiologist who studies human performance in extreme conditions for the U.S. Army Research Institute of Environmental Science. "But there is some physiological advantage. It's not huge but it's there."
Quantifying a cold-weather team's edge over warm-weather teams is impossible, and there's plenty that researchers in this young field of study don't know.
The Packers' recent history also argues against overstating the edge. In January 2002 the Atlanta Falcons came up from America's deep South to defeat the Packers 24-7 in a wild-card playoff game at Lambeau. And last January, the 49ers came from San Francisco's relatively moderate clime to beat the Packers at Lambeau 23-20.
It's also worth noting that this year's Cowboys have the kind of run-oriented offense with the NFL's leading rusher, DeMarco Murray, that is built to function well in bad conditions.
Still, you can't dismiss the effects cold weather could have, or not have, on each team Sunday.
"We always talk about the brain-body connection," said Matt Muller, an assistant professor of medicine at Penn State Hershey Heart & Vascular Institute and researcher in human performance in the cold. "If your hands are physically warmer because there's something going on in the blood vessels of the hands or you've adapted some way, your mind knows it, so both the brain and the body will be affected by the temperature."
The truth is, researchers know far more about human adaptation to heat than to cold.
Studies show that people adapt relatively quickly and well to hot temperatures and altitude, and that body metabolism actually changes. It takes anywhere from one to two weeks for human cooling mechanisms to adjust so, for instance, a person begins sweating more quickly and profusely in hotter weather.
The research to date on adjustments to cold are less extensive and definitive. Some studies suggest it can take as long a month or more to adapt to the cold, though others, including a recent one by Wouter D. van Marken Lichtenbelt of the Maastricht (Netherlands) University Medical Center, found that certain body fats burned more readily and coincided with less shivering after 10 days of exposure to cold.
Studies show that muscular performance diminishes when temperatures drop, starting probably at just under 50 degrees. As muscles get colder they lose strength and speed. Castellani estimated that for NFL players, the more pronounced responses to cold probably kick in at a little under freezing (32 degrees).
But no one has studied whether such major motor skills acclimatize, thus helping football players run faster or play more powerfully than those not used to the cold.
"It's unlikely acclimation will have any impact on that," Castellani said. "(But) it's interesting, no one's ever looked at that systematically."
The body's first reaction to the cold is to make sure its core temperature and internal organs remain warm. The hands and feet get cold first because the body automatically limits the blood supply to the extremities to conserve heat.
Football players generally don't have much trouble keeping up their core temperatures even in relatively extreme cold, because they're moving around, which produces heat. Their hands and feet can get cold, though, which can affect throwing, catching and gripping.
Habituation should help the Packers there, though the Cowboys players' extensive training in football skills could mitigate that edge.
I've always thought that the Packers' practicing inside or outside in the cold shouldn't make much difference, and that their edge came mainly from living in the cold day to day. But both Castellani and Muller said that habituation requires more than being outside for only a few minutes here and there.
With that in mind, both said coach Mike McCarthy's approach to practice might be optimal.
When the weather gets cold McCarthy usually has the Packers practice in the Don Hutson Center for individual and small-group drills, which are for honing skills, then go outside for 11-on-11 work. Former coach Mike Holmgren, on the other hand, usually stayed inside all practice because he wanted high-quality work and a lower injury risk.
"(The Packers should) be able to better execute the tasks you need to do," said Muller, who was a long-snapper in college at Kent State. "Probably most of that is driven by the psychology of being used to (the cold). But the bodily changes in my study were clearly there between people that worked in construction versus people that worked in offices (in a cold climate). So you can develop it, but it takes a long time. An NFL football season should be long enough for the Packers and Cowboys to have different bodily and mental responses."
Circumstance this year could mitigate the Packers' weather edge over the Cowboys, most notably the temperature in Green Bay on Sunday. Though the northern U.S. is going through a bitter cold spell this week, the forecast for Sunday calls for a relatively mild high of 19 degrees and a low of 4 degrees.
If the temperature on game day really is in the upper teens, it will be a comfortable day for the Packers. It will be less comfortable for the Cowboys, but it could be much worse and won't be anything like the Ice Bowl in 1967. For that game, the Cowboys woke up to a temperature in the mid-teens below zero and wind chill in the minus-40s.
This year, Dallas hasn't had a particularly warm winter. The average temperatures in Dallas for November were a high of 62 and low of 41; in December, 57 and 43; and so far in January, 46 and 31.
The high this month hasn't been below 36 degrees, so during the day it's been above freezing. But the low has been below freezing six times. Dallas' forecast for the rest of the week calls for highs in the upper 30s to low 40s, and lows in the 20s to low 30s.
It's still a significant drop for them to a high in the upper teens and low Sunday of 4 degrees. But there's only a 10 percent chance of snow, and the wind forecast for now is 5 to 10 mph, which is close to negligible.
"For the Cowboys, 15 (degrees) is something to think about because I don't think it's been that bad in Dallas this year," Castellani said. "I think they should be wary. And honestly I think it plays mind games with them."