DIY Jar Barometer: Predict the Weather at Home

A barometer measures atmospheric pressure. When air pressure rises (usually indicating fair weather), it pushes down on the membrane of this jar barometer, causing the straw indicator to rise. When pressure falls (rain approaching), the membrane bulges up and the straw drops. This project makes an invisible physical phenomenon visible and measurable.

What You’ll Need

• Wide-mouth glass jar or tin can
• Balloon (large)
• Rubber band
• Drinking straw (rigid plastic works best)
• Adhesive tape
• Index card or thin card for the scale
• Pen

Step-by-Step Instructions

1. Prepare the membrane: Cut the neck off the balloon. Stretch the remaining balloon piece tightly over the mouth of the jar. Secure firmly with two rubber bands so no air can leak in or out.
2. Attach the straw: Cut a straw to about 15 cm. Tape one end to the centre of the balloon membrane, pointing outward horizontally like a needle.
3. Make the scale: Tape an index card vertically to the wall or to a box placed behind the jar. The free end of the straw should point at the card. Mark the current straw position as a horizontal line labelled “Start” with today’s date.
4. Observe over days: Check the straw position morning and evening for a week. Mark each reading on the card and note the actual weather that followed.
5. Calibrate: After a few days you’ll see the straw rises before clear days and drops before rain. Mark “High” and “Low” zones based on your observations.

Safety Notes

• Keep the barometer indoors, away from temperature changes (heaters, open windows) — temperature affects air pressure inside the jar and will give false readings.
• Glass jars can break — store away from edges where they could fall.

Tips & Troubleshooting

• If the straw doesn’t move at all over several days, check that the balloon membrane is airtight around the rim.
• The barometer responds to relative pressure change, not absolute values — it won’t give you millibar readings, but it will show trends reliably.
• A longer straw amplifies small pressure changes and makes readings easier to spot.