About 70% of a human's total energy expenditure is due to the basal life processes within the organs of the body (see table). About 20% of one's energy expenditure comes from physical activity and another 10% from thermogenesis, or digestion of food (postprandial thermogenesis)
That's a simple equation to remember:
70% = Organs
20% = Physical movement
10% = Thermogenesis + digestion
When your weight is stable, 70% of the calories you eat are burned through organ activity.
When it gets colder, your body will burn slightly more calories through thermogenesis but because that amount is already small (less than 10% of total energy expenditure in the human body), it won't be too high... Probably an extra 1-2% maybe.
The energy spent doesn't come only from the food, it comes as well from the water we drink and the air we breathe.
When climbers spend time in altitude, they will usually burn the extra fat and lose lots of weight because the amount of oxygen they breathe tends to be much lower, therefore, the body start taking energy from reserves.
This last theory that sounds right but is it true? Or is the weight loss related with a drop in appetite that people feel in altitude?
It could as well be related with lower amounts of food consumption related with the context.
It could as well be related with the amount of energy burned when making physical efforts.
This last one would be related again with the fact that there is less oxygen means that the body draws from reserves when making a physical effort.