Learning Lesson: Measure the Pressure - The "Wet" Barometer

 Overview

The amount of air over us is constantly changing. As a result, the weight of that air, called pressure, is constantly changing. These changes in air pressure are indications of changes in our weather. We measure this change using a device called a barometer (bar-meter or measurer).

This first barometer was created by Evangelista Torricelli in 1643. Torricelli was actually trying to discover the reason that water would rise no more than 33 feet up a tube through the use of a suction pump. He had first built a water barometer, but it required a glass tube 60 feet long.

In order to make the barometer smaller, he replace the water with mercury. Aware that mercury was 14 times heavier than water, he constructed a tube only 35 inches long. He inverted the mercury-filled tube into a bowl of mercury, causing the level in the tube to drop to around 30 inches and creating a vacuum at the top.

The top of the mercury column was observed to fluctuate by a few percent, due mainly to what we now know to be fluctuations in atmospheric pressure. This is because as the column of air directly above the barometer pushes on mercury in the bowl, it is forced up the tube. The stronger the downward push, the higher the pressure and therefore the higher the mercury rises in the tube. This is where the units "Inches of Mercury" are derived.

Procedure

1. Place the ruler in the glass and tape it to one side. (Make sure the numbers are visible.)
2. Tape the plastic tube onto the ruler in the glass. (Make sure the tube is not touching the bottom of the glass.)
3. Fill the glass about half way with water. Add a drop or two of food coloring and mix thoroughly.
4. Using the tube like a straw, draw some water about 2/3 into the straw.
5. Using your tongue, trap the water in the tube then cap the end of the tube with model clay or chewing gum.
6. Record the height of the water in the tube.
7. At the same time every day, for the next 5-10 days, record the height of the water in the tube, paying close attention to the change in the weather as the water level changes.

Discussion

What the students have constructed is a water barometer (also known as "storm glass"), and these types of barometers date from the 17th century. The actual change in pressure will occur too slowly for constant observation. The change in pressure will be most noticeable after a 24-hour period.

As a thunderstorm approaches, the mass of air begins to be lifted up into the upper atmosphere. We see (and measure) this as a decrease in air pressure. After a cold front passes your location, there is a net increase in the mass of air, which we observe as an increase in pressure.

Over and above the pressure changes associated with storms, there are four daily pressure fluctuations in the atmosphere. These diurnal changes are due to the sun heating the atmosphere. The amplitude of this daily change depends upon the latitude, season, and altitude.

The changes are greatest at the equator, decreasing toward the poles where it becomes zero. Also, the higher the altitude, the greater the daily change.