Atmospheric pressure varies due to altitude and temperature and these changes in pressure affect flight and human performance. At higher altitudes, the pressure (the weight of the air) becomes less and take offs and landings require more distance. Lift must be established to get the plane off the ground, and since the air is thinner at higher altitudes, the distance needed to get to the necessary speeds for take-off are increased. The less dense air also makes the engines less efficient, and the lack of oxygen at higher altitudes is why cabins are pressurized and have oxygen systems.
Because of the changes in pressure due to temperature, pilots continually update their altimeter, an instrument that measures the distance above a fixed level.