How to Build a Water Barometer

Cut off the top part of the soda bottle and half-fill it with water. Tape the ruler to the outside of the bottle, standing upright so it measures height.

Chew on a piece of chewing gum until it's nice and sticky.

Take a piece of clear plastic tubing at least 12 inches long and place it in the bottle. Tape the tube to the bottle so that the end of the tube is suspended in the water but doesn't touch the bottom of the bottle.

Suck on the plastic tube like a straw until the water is about halfway up the tube. Now, quickly seal the end of the plastic tube with the chewing gum. This is the most difficult part--make sure you seal the tube without letting the water recede.

Mark the current water level on the ruler with your felt pen. Watch how the water level changes over the next few days. Compare the changes in your reading to the air pressure as recorded by your local weather station. When the water level in your barometer increases, it means that atmospheric pressure is rising, whereas a drop in level means the pressure is decreasing. The basic formula is as follows: P = P0 + pgh, where P is atmospheric pressure, P0 is the pressure of the remaining air in the top of the plastic tube, p is the density of water, g is the acceleration due to gravity (about 9.8 meters per second squared) and h is the height of the water in the plastic tube.