The French mathematician Blaise Pascal (1623-1662) took a barometer To the top of a mountain. After many observations, measurements and notes, he found that air pressure decreases with height. The air becomes thinner (decreases the number of molecules present in it), gradually, as the altitude increases.
From this and other experiments, scientists have concluded that most gases are compressed near the Earth's surface and that the air becomes thinner as altitude rises to a point where there is no more air - this is the boundary of the atmosphere of our planet. Advances in science and technology have enabled more knowledge about the atmosphere.
Sea level is used as a reference when calculating atmospheric pressure.
The higher the altitude, the thinner the air, and thus the less pressure it exerts on us.
Compressibility and elasticity
Notice what happens in the steps of the experiment below:
By plugging the tip of the syringe and pushing the plunger, the air inside the syringe is compressed and takes up less space. This is due to an air property called compressibility.
When the plunger is released and the compressing force of air is ceased, the air returns to its initial volume. This is due to a property of air called elasticity.