Wait until the weather is mild before building your barometer. Use scissors to cut off the part of the balloon you blow through.
Remove the lid from the glass jar and stretch the balloon over the opening. Stretch it until it is taut but no tighter. Stretch four or five rubber bands around the neck of the jar to keep the balloon in place.
Stick an inch of tape against the neck of the jar with half protruding from above the top of the neck. Poke a hole through here and stick the straw through so that the end rests on the middle of the balloon and about an inch protrudes from the side .
Cut a section of card 4 inches wide and the about the height of the jar. Fold it into a square tube and tape closed. Tape the tube to the side of the jar beside the straw.
Mark on the side of the tube where the tip of the straw is level with. Make observations. When the air pressure is low, the air in the jar will expand, tilting the straw and moving the tip next to the card down below the mark. The converse will happen when the air pressure is high.