atmospheric pressure

The pressure exerted by the atmosphere on any surface as a function of its weight. For any point on the Earth's surface, that pressure is equal to the weight of the column of air, of unit cross-section, that extends up to the top of the atmosphere, which for practical purposes can be placed at 300 kilometers of altitude (exosphere). The mass of all the air contained in that atmospheric column - assuming it has a base of one square meter - is around 10,000 kilograms. The reason we do not die crushed while outdoors lies in the fact that the air above our heads not only exerts pressure downward, but in all directions - at any of the atmospheric levels we consider. In the International System of Units, atmospheric pressure is expressed in hectopascals (hPa). This unit is equivalent to the millibar, which is being used less and less. It is also common to express it in millimeters or inches of mercury (an inch [in] is equal to 25.4 millimeters [= 0.0254 m]), units linked to the measurement of this variable in barometers using that liquid. In honor of the instrument's inventor - Evangelista Torricelli (1608-1647) -, the pressure corresponding to one millimeter of mercury (mmHg) is, by definition, equal to 1 torr.