GRAND RAPIDS, Mich. — This week we visited Plainfield Child Care in Grand Rapids with a variety of younger-aged kids, so we kept things fun, simple, and hopefully entertaining. This experiment uses a glass, water, and a piece of cardboard. Everyone can do this, just make sure to have a bucket in case it fails...as ours did after a few moments.
Fill a glass about half to three-quarters full with water. Use a piece of cardboard and place it over the top of the glass. Holding the cardboard tightly against the rim of the glass, turn the glass upside down. When you let the cardboard go, it should stay in place. Science is not always exact, nor is it perfect, so make sure to do this over a bucket or over the sink. Our experiment held for just a few brief seconds, before cutting loose and the water spilling everywhere.
So what's happening: There's air pressure all around us all the time. In fact, it measures about 14.7 pounds per square inch (psi). If you were to measure that with a barometer (a device used to measure air pressure), it would read 1013.25 millibars. That pressure is always pushing on our bodies, we just never feel it since there's equal pressure pushing outward from our bodies. When we turn the glass over, air pressure pushes against the cardboard and holds it in place. In some fashion, it creates a "seal", almost water-tight but not always. There's also surface tension to the water that helps keep the cardboard in place. This is the same type of surface tension that allows us to fill a glass to the rim without having it spill over.
So in short, the force of the water pushing down on the cardboard is less than the force of air pressure pushing up on the cardboard. As a result, the cardboard seals, sticks, and stays in place and holds the water in. How much you fill the glass, and how stiff the cardboard is also affects the outcome, so try experimenting with conditions varied a little.
As noted, air pressure is all around us. When the barometer drops and air pressure lowers, we tend to associate that with formation of clouds and the chance of precipitation. When we measure higher atmospheric pressure, a high-pressure weather system is usually building into the region...hence the rising barometer. That tends to create sinking air or subsidence, which gives us dry, quiet, rain-free conditions with sunshine usually resulting.
So remember the next time you need rain, look for lower atmospheric pressure. The next time you need good weather, dry conditions, and sunshine, look for higher atmospheric pressure.