The atmospheric pressure is the force exerted by the atmosphere above Earth's surface. The instrument used to measure the barometer in millibars or millimeters of mercury. The value of the normal pressure at sea level is 1013.25 millibars or 760 mm of mercury. This pressure variations suffers from the different temperatures recorded on land by the action of the sun. As it heats the air pressure decreases. Atmospheric pressure in a given area involves such factors as changes in weather and elevation above sea level. For example, when a tropical storm, the barometer for which normal value is 760 mm of mercury, down to 665 mm.
The Italian mathematician Evangelista Torricelli, who lived from years 1608 and 1647, in 1643 invented the barometer, an instrument that gave him fame.
The isobars are curved lines that join the areas which has the same atmospheric pressure at the same time . With them you can draw a map isobaric.