For example, if Mt. Everest were located at the same latitude as Denali, the corresponding drop in barometric pressure would make an ascent without oxygen impossible. As for seasons and weather, air pressure is lower in winter than in summer, and a low pressure trough will reduce pressure. While temperature does not directly affect barometric pressure, the combination of cold stress and lack of oxygen increases the risk of cold injuries and altitude problems. The body undergoes numerous changes at higher elevation in order to increase oxygen delivery to cells and improve efficiency of oxygen use. These adaptations usually begin almost immediately and continue to occur for several weeks.
People vary in their ability to acclimatize.
Some adjust quickly while others fail to acclimatize, even with gradual exposure over a period of weeks. In general, the body becomes approximately 80 percent acclimatized after 10 days at altitude and approximately 95 percent acclimatized by six weeks. The respiratory rate peaks in about one week and then slowly decreases over the next few months, although it tends to remain higher than its normal rate at sea level. After 10 days, the heart rate starts to decrease. During the first week of adaptation, a variety of changes take place.