For centuries it was believed that air has weight, and it can be felt only when it moves (ie in the wind).That was the view of Aristotle and for a long time it has been the law for scientists.
In the middle of the XVI century, a disciple of Galileo Evangelista Torricelli, solving the problem of the rise of water fountains discovered what is considered weightless air still has weight.As a result of the Torricelli invented the first mercury barometer by which managed to measure the air pressure on the earth's surface, and calculated its density.
However, the fact that the air is drawn underground and therefore presses down, could not serve as the answer to all your questions.In particular, it was found out that the air pressure is not limited to what is underneath, but in all directions at once
, including upwards.

known experience with the "Magdeburg hemispheres" - a metal sphere of two halves, the space between the evacuated - showed that the air pressure can be enough to make even a few horses could not separate from one another hemisphere.
Subsequently, it was discovered that this property is not only the air, but in general any gases.To find the answer to this riddle, it took another discovery - the theory of the molecular structure of matter.
molecules that make up the gas, are not linked and are in random motion.They constantly impinge on the wall of a vessel filled with gas.This impact is the gas pressure.
Since gas is attracted Earth, its pressure on the bottom of the vessel a little more than a wall and cover, but in most cases the difference is so small that it can be neglected.Only for the entire earth's atmosphere as a whole, the pressure difference at the surface and at high altitudes becomes noticeable.

In weightlessness gas pressure for all vessel wall the same way.
The pressure of gas depends primarily on the mass of the gas, its temperature and volume of the vessel.If the temperature remains constant, the increase in pressure reduces.At constant mass pressure increases with increasing temperature.Finally, at constant volume, an increase in mass leads to an increase in pressure.