Why does it become difficult for balloonists and climbers to breathe when climbing up?

Atmospheric pressure changes not only with altitude. At any point, the pressure changes during the day. The pressure also depends on the air temperature. When heated, the air expands, 1 cu. m of cold air weighs more than 1 cubic meter. m warm. Warm air is light, it rises upwards, therefore, its pressure on the Earth’s surface decreases, that is, atmospheric pressure decreases.

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