Blow up the balloon to stretch it out, release air, and then use the scissors to cut the tip, or mouthpiece, off of the balloon. Start by slicing just the very tip off and stretch the opening of the balloon around the top of the jar. If it doesn't fit, cut more fo the balloon until you can stretch it across the mouth of the jar.
Secure the balloon snugly in place using the rubber band. Lay the straw on the jar with one-third of it hanging over the jar's edge. Secure the straw in place with tape.
Draw three lines 1/5 inch or 1/2 cm. apart from each other on the index card. Each line should then be labeled at the top with the words high, moderate and low. Tape the card onto the back of the jar, then point the straw so it lines up with the moderate line. Place the barometer on a flat level surface inside the house.
Check the barometer regularly each day for around a week and take note of where the straw is pointing. When the air pressure is low, it means the air pressure inside the jar is higher than outside it. This can mean bad weather may be coming. The balloon will then expand and push the straw downward. When the air pressure is higher outside the jar, the balloon will deflate into the jar and the straw will point upward. This means the weather should be fair.