Materials Needed for Your Homemade Barometer:

• A glass jar (best) or a large metal tin container (like an empty and cleaned large can of baked beans) with straight sides and a wide top/mouth
  
• A balloon (do not use one you already blew up before)
• A few rubber bands
• A tube of silicon glue
• Adhesive tape
  
• 2 plastic drinking straws
• Small bit of thick paper (Bristol paper or cardboard paper), best use a nice red or other strong color
• A ruler
• A pair of Scissors

Directions on How to Make Your Own Barometer:

1.  Set the glass jar on a straight, firm surface.
2.  Cut the balloon so that you have a nice round part left, without the long end.
3. Stretch the balloon so it fits over the mouth of the jar nice and tight – the tighter the better. We do not want a loose, saggy cover. Jar must be sealed well.
4. Use the rubber bands to keep the balloon firmly in place over the mouth of the jar and to make sure the jar is sealed well, so no air comes in or leaks out.
5. Drip a bit of glue onto the balloon jar covering, in the very middle. Place the end of the long plastic drinking straw on the glue. Gently secure it in place with light pressure and then tape it down until the glue dries and holds the straw in place

 
Keep your barometer indoors. Place it near a wall. Use some adhesive tape to tape the ruler to the wall.
Watch your straw over several days and record what you see in a weather journal. Do a few readings each day at same time during the day, so you can compare changes more accurately.

Why Will Your Homemade Weather Forecasting Barometer Work?
At the time you sealed off the glass jar, you captured air inside it. The air pressure inside your jar was exactly the same as the air pressure outside of the jar. However, as the weather changes, so changes the air pressure in the atmosphere (the outside air).
Since your jar is nicely sealed, the air pressure inside it does not change. However, the air pressure outside changes with the weather. So this causes pressure on the balloon cover to increase or decrease.

Why it works!
Sunny, clear and warm weather raises in the air pressure around you. This means that outside air is going to press on the balloon cover, since it is stronger than the air inside the jar. And when the air presses upon the balloon cover, it makes it sort of cave in a bit, almost like the air in the jar is sucking in its breath… Because we have the straw on the balloon, it will sink down where it is glued to the balloon, therefore the other tip shall rise!
As a storm approaches your location, the mass of air around you decreases, causing the air pressure to drop (decrease). This will cause the pressure inside the jar to be higher and your balloon jar cover will bulge out. When this occurs, the end of the straw glued to the balloon goes up, while the end will go down!

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