KNOXVILLE, Tenn. (WATE) — In this week’s weather school, we’re going to talk about a device used to measure atmospheric pressure.

The amount of air over us is constantly changing and, as a result, the weight of that air, called pressure, is constantly changing as well. These changes in the air pressure are indications of changes in our weather. So, we use the measurement device called a barometer, which often measured in inches of mercury.

The first barometer was created by Evangelista Torricelli in 1643. Torricelli was trying to discover why atmospheric pressure was making water rise in a tube or a column. Torricelli knew that mercury in a liquid form was heavier than water. So, he didn’t need a large tube to measure this force of the air down upon his barometer. He discovered that the mercury would flow up from a reservoir into a tube, then he measured the height of the mercury on the side of the tube with a yardstick, in inches.

He discovered that a typical atmospheric pressure was about 30 inches and then it would fluctuate a few percentages due to fluctuation in the atmospheric pressure. This is because the column directly above the barometer pushes on that dish containing the mercury and it is forced up into the tube. The stronger the downward force, the higher the atmospheric pressure.

This is an example of a mercurial barometer, just one device used to measure the atmosphere that we use in our weather forecasts each and every day.