As gravity pulls the atmosphere towards Earth's surface, the weight of the air creates a force that we call atmospheric pressure. To measure this force at a given location, you simply weigh the air above you with an instrument such as a barometer. Atmospheric pressure isn't the same all over the planet. It varies from place to place according to its relationship with several environmental factors, including geography, altitude, and temperature.
Alternating bands of high and low pressure form along Earth's surface from the poles to the equator. In addition, atmospheric pressure changes with altitude. At higher altitudes, there is less air pushing down from above. Thus, atmospheric pressure is lower on a mountaintop than it is at sea level. With less pressure pushing the air molecules closer together, the air is less dense. As a result, the higher you go, the thinner the air feels. Though the concentration of oxygen in the air is the same throughout the troposphere (the layer of the atmosphere in which we all live), there is less total gas per unit of volume in air at the top of a mountain than in air at sea level. This means that each breath of air a mountain climber takes contains fewer oxygen molecules.
Air temperature is also related to atmospheric pressure. At higher altitudes, the relationship is different than at sea level, where air pressure is higher at colder temperatures because colder air is denser. The reason some climates are cold is because the land and water lose more heat to space than they retain at the surface. Thus, as relatively warmer air rises through the atmosphere, it expands, becoming less dense, but also cooler, with altitude.
Knowing that temperature and pressure drop together with altitude more rapidly in cold climates makes it easier to understand why one sister, standing on Denali (Mount McKinley), will find it more difficult to catch her breath than her twin, standing at a similar altitude on Mount Everest. Although both Everest and Denali are located in areas of relatively low atmospheric pressure, Denali is further from the equator. Thus, air conditions there are typically much colder than on Everest. This sister's physiological altitude, then, is several hundred meters higher than her twin's actual altitude: the atmospheric pressure is lower and the air is thinner.