Atmospheric pressure falls as your elevation increases.
This is because as you climb higher, there is a lower mass of air pressing down on you.
When you fly, the cabin is artificially pressurized so that you stay comfortable up in the atmosphere where the atmosphere is so ‘thin’ (i.e. has less air pressing down on you).
The air pressure also changes over the course of the day due to changes in the temperature in the atmosphere as the Sun warms the air and changes its density.
That variation can look like this on Mars (with similar changes due to the Sun on Earth 😉
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