The air above you weighs about a kilogram on every square centimetre

Sea-level pressure is about 101,000 pascals. That is the weight of a kilogram pressing on each square centimetre of you, from every direction at once.

A column of air from the ground to the top of the atmosphere has mass. Spread over a square centimetre, that mass is about a kilogram — 1.03 kilograms under the standard atmosphere, if you want the extra digits. Over a square metre it is about ten tonnes. The number was first measured by balancing a column of mercury against the air, which is why pressure still has an old name in millimetres of mercury. The modern figure is the same fact with a cleaner unit. You are at the bottom of a fluid, and the fluid is the sky.

A body does not register a load that pushes equally from all sides and is matched from within. Lungs, blood and tissue are at atmospheric pressure too. What you would feel is a difference: a vacuum against the skin, a rapid ascent, a cabin that fails. The kilogram is still there while you feel nothing. It is why a suction cup holds, why a straw works, and why the boiling point of water is a statement about this weight and not about water alone. Take the weight away, and water boils cold. The mountain entry is the same fact, thinner.