Differences in temperature causing differences in air density lead to differences in air pressure. As the temperatures of portions of the atmosphere increase, the air in these portions expands and rises, causing low-pressure regions. Meanwhile, as the temperature decreases, the air contracts and sinks, causing high-pressure regions.
These differences in atmospheric pressure are caused by unequal heating by solar radiation. Different regions receive different amounts of solar radiation and have different surface compositions. These compositions absorb, reflect and reemit radiation in different ways. The relative difference in atmospheric pressure between two regions can be measured with a spring or mercury barometer. Atmospheric pressure is also known as barometric pressure for this reason.
Pressure is defined as force per unit area, and the atmospheric pressure is consequently the weight of the air column perpendicular on a certain square area from that area to the top of the atmosphere. The value of this pressure is large, approximately 14.7 psi at standard temperature and pressure conditions. This pressure does not crush terrestrial organisms because their bodies are above this pressure, the reason why cuts bleed out instead of sucking air in. The value of atmospheric pressure decreases as altitude increases, because the height and weight of the pressure column from the top of the atmosphere to this higher altitude decreases.