When the balloon is taut over the mouth of the jar, stretch a rubber band over the rim of the jar to hold the balloon in place. A glass jar is best, but you can also use a metal can. Whether you use a jar or can, the exact size isn’t critical. Just make sure that the mouth is not so big that the balloon can’t easily stretch over it.
Silicon glue will work well. You can use super glue, craft glue, or even a glue stick in a pinch, however. Make sure to let the glue dry before you move on. The longer the straw, the better (as long as it is a straight one). You can even insert the end of one straw into another to make one longer straw.
Stiff paper like cardstock or cardboard will stand up better, but you can use regular paper if that’s all you have. You can find lots of options at any store that sells stationery or office/school supplies. The pointer should be close to the paper, but not actually touching it.
If you like, you can label the pointer’s initial position “1” and then number each new position in order. This is a great idea if you want to use the barometer for a science project. The barometer works because high air pressure pushes down on the balloon, causing the pointer to move upwards, and vice versa.
Low pressure is usually associated with rainy weather. High pressure can mean mild or cooler weather.
You will probably need about 40 centimetres (16 in) of tubing to be able to stick up out of the top of the bottle. If your tubing isn’t long enough, cut the sides of the bottle so they are lower. Leave some of the tubing hanging loose.
Place your tongue over the end of the tubing when the water is in place, to hold the suction and keep the water from going back down. Be careful not to suck the water all the way up!
You have to work quickly to do this right! If you mess up, just try again.
You can also mark the changing positions on the ruler, if you like, or just measure how much the water rises and falls.
Since your water barometer has a ruler, you can also record pressure shifts as exact changes in inches or millimeters. Use this to notice even very slight changes.
