Atmospheric Pressure

On a daily basis, most of us do not think about the pressure outside; however, pressure plays a significant role on the weather.  Pressure itself is simply the amount of force per a given area.  Where pressure becomes noticeable is when there is a difference of pressure between to regions.  Before we discuss regions, let's take this to a local level.  Have you ever opened a door into a restaurant and the air rushes in?  Sometimes the door does not even close properly.  If the door is being sucked in, the air is being pulled from the outside to the inside.  This is a result of the pressure inside the building being negative in comparison to the pressure outside the building.  If the air is blowing towards you than the opposite is true.  The pressure inside the building is positive in comparison to the outside. Two adjacent areas of different pressures will try to achieve an equilibrium.

And how does this relate to weather?

If the region you live in is negative compared to an adjacent region; air will move from the adjacent region into your region (wind). The greater the pressure differential, the higher the wind speed.  By studying the atmospheric pressures at various weather stations and them comparing them, short-term predictions can be interpolated.  Basically you can plot the pressures and create maps displaying lines of equal pressure.  As a general rule, high pressure is associated with fair weather.  Low pressure is associated with stormy weather.

How is pressure measured and the history of the concept.

The grandfather of atmospheric pressure measurement was Evangelista Torricelli.  In 1640 he discovered how to measure the weight of pressure upon the earth at a given location.  Using a glass tube filled with mercury, he could measure just how high the mercury rose.  At sea level he measured 29.92 inches.  Galileo Torcelli and Blaise Pascal took the concept further and determined that there was a difference in pressure depending on the elevation the measurement was taken.  At higher elevation the pressure is lower and at lower elevations the pressure is greater.  This is why water boils at different times depending on the elevation.  Additionally Galileo and Blaise determined that minimal pressure difference had a significant influence on the weather.