Because there is less oxygen, due to low air pressure.
When air pressure drops temperature also drops. Thus on high mountains low air pressure is accompanied by low temperature. This is true for high atmosphere also.
Air density decreases as you climb mountains because the air molecules become more spread out at higher altitudes. This means there are fewer air molecules in a given volume, leading to lower air density.
more mountains + more air equals more pressure.... its like sitting on a bag of air
At high altitudes on mountains, the air pressure decreases because there is less atmosphere above pressing down. This reduction in air pressure means there are fewer molecules of air per unit volume, resulting in thinner air.
the air pressure decreases
The air pressure is rising.
air pressure decreases
The air pressure is high when the air is cold, and it's sinking.
the air pressure increases
density decreases as air pressure increases
What is the center of low air pressure