More than 350 years ago, the Italian scientist Torricelli proved using invented the mercury barometer not only in the nature of emptiness, but that air has pressure. Potted at one end of the glass tube he poured mercury and flipping it open end down, dropped into a container of mercury. The mercury column under gravity partially lowered in the tube, but not until the end, keeping the height of 760 mm, the height was adopted for the standard normal atmospheric pressure. Acting in this way mercury barometers are used to this day.
Today, however, more frequently used metal (aneroid) barometer. This thin-walled metal box where the deflated air. With increasing atmospheric pressure the bottom of the box is pressed, while reducing spreads. Changing the position of the bottom with the help of the lever is transmitted to the arrow oscillating above the dial. If the readings of the barometer does not change, it means that the weather will remain unchanged. When the pressure starts to fall down arrow is rejected, you can expect windy and rainy weather, if it happens quickly, then coming storm or rain. If the pressure increases rapidly, you should expect short-term improvement in the weather, and if slowly, it soon will become a stable good weather.
In many countries today a unit of atmospheric pressure is considered hectopascal. One hectopascal (one hundred Pascals) corresponds to one millibar is still the unit of measure of atmospheric pressure. Therefore, the numbers on the scales barometers are not changed. If, for example, the arrow indicates 1000 millibars, meteorologists say 1000 hectopascals.
Atmospheric pressure is measured by a barometer.