Pressure is the measurement of a force (weight) on a per unit area. The barometer measures the force our atmosphere exerts on a per unit area of surface. The air has weight and changes in pressure can be measured and help the forecaster even today. The atmosphere exerts a weight of about 14.7 pounds on every square inch at sea level. Our bodies have pressure inside that equals the pressure we feel from our environment so we do not feel less weight on our shoulders when the pressure is low. Our atmosphere at sea level can support a column of water 10.4 meters high (34 feet high) in a barometer using water. Imagine constructing a barometer over 30 feet high. Mercury was the liquid chosen thanks to its density, and reduced the length of the barometer to less than a foot.
The word barometer originates from the Greek 'Baros' meaning weight and 'metron' meaning measure. Evangelista Torricelli is credited with the invention of the mercurial barometer in 1641. As with many great discoveries more than one person is involved in the process of invention. Before Christ, Aristotle believed that the air had weight, circa 323 BC. . A main stumbling block for Aristotle and eventually Galileo was the belief that a vacuum could not be created by man. There was even religious controversy surrounding the existence of a vacuum. The church excommunicated anyone that defended the idea that a vacuum could exist, and could be reproduced by man. The existence of a vacuum is important because the mercury (or whatever liquid was chosen) must have dead air space in the top of the tube so it can move freely up or down depending on the atmospheric pressure acting upon it. If a vacuum could not exist then the air at the top of the glass tube would have additional force acting upon it, thus rendering its readings ambiguous. A vacuum is easily produced with a siphon. In the 1630's a scientist by the name of Gasparo Berti produced a vacuum in one of his experiments as he siphoned water from one point to another. It is amazing that before the widespread use of running water and electricity that some folks were debating about the existence of a vacuum and the weight of the atmosphere.
The two most commonly used scales used for measuring pressure is ' inches of mercury ' and millibars. When the pressure is 29.00 " of mercury that means that the atmosphere will support a column of mercury 29 inches high, so the "inches" are not technically the units of pressure because they do not correspond to the dimensions attributed to the force per unit. The 'Weather Report" will give you the barometric pressure in inches or millibars. The conversion rate from inches to millibars is 1 inch = 33.86 Mb. High pressure systems, with their sinking air puts more pressure, or exerts more force, on the bottom of the atmosphere causing the mercury in the tube to rise thus causing the barometer to rise. Conversely, Low pressure systems, with their rising air, exerting less force, from the bottom of the atmosphere (at the surface) causes the mercury in the tube to fall, thus reducing the pressure and lowering the barometer. High pressure systems are associated with fair weather, i.e. no precipitation because the air is sinking unable to rise and cool and condense. High pressure systems may bring very hot or very cold weather. The highest barometric pressure readings will always occur in winter with a strong, continental, polar air mass overhead. Cold air is more dense than warm air resulting in higher pressures in the winter when these strong polar air masses move in. Low pressure systems are associated with precipitation because warm, moist air is rising from the surface cooling and condensing into clouds, thunderstorms, or large scale storms such as hurricanes. In the northern hemisphere the circulation around a high pressure is clockwise and is counter clockwise around a low pressure system. Where the 'Highs and Lows ' are located in terms of your location will determine the wind direction.
To demonstrate that the air has weight, weigh, on a good science classroom scale, a ball (basketball) that is filled with air and then weigh the ball after all the air is forced out. The inflated ball will weigh more than the deflated one. To demonstrate that air has volume fill a container with water about 3/4 full. This container should be about 6 - 12 inches deep. Then take an empty paper cup and turn it upside down inserting into the container with water at a perpendicular angle to the water. No water fills the cup. (If water does get into the cup you have allowed for some air to escape making room for the water, you will see a bubble if you do let this happen.) Now punch and small hole in the cup bottom of the cup and turn it upside down and insert it into the container. Now the cup fills with water because the air can escape through the hole at the bottom of the cup making room for the water to rush in.