Let us take a closer look at the troposphere. The pressure at sea level is approximately 1 bar or 1000 millibar (mbar or mb). If you manage to get below sea level, without diving beneath the sea surface, the pressure is higher. For example, on the coast of the Dead Sea in Israel, at 430 m below sea level, the air pressure is just above 1060 millibar.
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As you go higher in the troposphere the pressure drops. This happens because the higher you are, the less the atmosphere or air column there is above you, pushing down on you. At the altitude of the peak of Europe’s highest mountain, Mt Blanc (4807 m), the air pressure is only about 430 mbar. That is less than half that at sea level. On top of Mt Everest (8848 m) the air pressure is only about 340 mbar. And at aircraft cruising altitude of about 12 km it is only about 200 mbar, only 1/5 that at sea level.
The troposphere cools with altitude, i.e. the higher you go, the colder the air is. On average, the air is cooler by about 0.7 °C for every 100 m higher you go. So, while it may be a pleasant 25 °C or more on the surface, it is -50 °C or below at aircraft cruising altitude at 10 km altitude or more. The gradual change of the temperature with height is called a vertical temperature gradient or temperature lapse rate.