Bottle Barometer

Part of Weather Forecasting

Building a functional water barometer from a bottle and tube to track pressure changes and predict weather.

How It Works

A water barometer exploits the same physics as mercury barometers but uses water instead. A sealed bottle is partially filled with water and connected to a tube open to the atmosphere. When atmospheric pressure rises, it pushes down on the water in the open tube, forcing water up inside the sealed bottle. When pressure falls, the trapped air in the bottle expands, pushing water out into the tube. The water level in the tube moves inversely with pressure — rising water in the tube means falling pressure and approaching bad weather.

Materials Needed

• One clear glass or plastic bottle (500 mL to 1 L works well)
• One narrow tube or hollow reed (6-8 mm diameter, 30-40 cm long)
• Sealant: tree resin, beeswax, clay, or pine pitch
• Water (clean, to prevent algae growth)
• A mounting board or flat surface

If you lack manufactured tubing, a straight hollow reed, bamboo section, or even a hollowed elderberry branch works. The tube must be watertight and narrow enough that small volume changes produce visible level shifts.

Construction Steps

Step 1: Fill the bottle approximately half full with water. Add a small amount of charcoal dust or plant dye if available — colored water is far easier to read.

Step 2: Insert the tube through the neck of the bottle so it reaches about 2 cm below the water surface inside the bottle. The tube should extend 15-20 cm above the bottle opening.

Step 3: Seal completely around the tube where it enters the bottle. No air must leak in or out except through the open top of the tube. This seal is the most critical part — test it by gently pressing on the bottle. If the water level in the tube does not change when you press, you have a leak.

Step 4: The tube must remain open at the top. Atmospheric pressure acts on the water through this opening.

Step 5: Mount the barometer vertically in a location with stable temperature. Temperature changes cause the air inside the bottle to expand or contract, producing false readings. Avoid direct sunlight, proximity to fires, and drafts.

Reading the Barometer

Water rises in the tube: Atmospheric pressure is falling. The air trapped in the sealed bottle is expanding relative to outside pressure, pushing water up and out. Bad weather is likely approaching.

Water falls in the tube: Atmospheric pressure is rising. Outside air is pressing harder on the water in the tube, compressing the trapped air in the bottle. Fair weather is likely.

Water level stable: Pressure is steady. Current conditions will likely persist for 6-12 hours.

Mark the tube with scratches or paint at the current water level. Check it every few hours and note whether the level is rising, falling, or stable. The trend over 6-12 hours is your forecast.

Calibration

On a day with clearly fair, stable weather, mark the water level as your “fair weather baseline.” Over the following weeks, note the level during storms, rain, and clearing. After 3-4 weather cycles, you will have reliable reference marks.

Create at least three marks:

• High mark — observed during the worst storm conditions (water highest in tube)
• Baseline — fair weather level
• Low mark — observed during the clearest, most stable weather (water lowest)

The Temperature Problem

This is the biggest limitation of water barometers. A 5 degree C temperature increase causes trapped air to expand as much as a significant pressure drop. Your barometer will give false “storm approaching” readings on warm afternoons and false “clearing” readings on cool mornings.

Mitigation strategies:

• Place the barometer in the most temperature-stable location available: a cave, root cellar, north-facing wall, or insulated shelter
• Read it at the same times each day (dawn and dusk, when temperatures are similar)
• Compare readings at the same time across days rather than within a single day
• If you have two bottles, place one indoors and one outdoors — the difference helps isolate pressure from temperature effects

Alternative: The Storm Glass

If you have access to ethanol and camphor (distillable from certain trees), you can build a storm glass instead. Dissolve camphor in ethanol, seal it in a glass container. Crystal formation patterns correlate loosely with weather changes. However, this is far less reliable than the water barometer and requires materials that may not be available.

Accuracy and Expectations

A well-built, temperature-compensated water barometer can detect pressure changes of 2-3 millibars — enough to predict major weather shifts 6-18 hours in advance. It will not match a mercury barometer’s precision, but it outperforms pure observational methods.

Check it twice daily at consistent times. Record readings with simple marks on a board or in a log. Over time, your records become a local weather database that improves every prediction you make.

Maintenance

Refill water as it evaporates — the level in the bottle must always cover the tube’s submerged end. Re-seal any cracks in the sealant as they appear. Replace water every few weeks to prevent algae buildup. In freezing conditions, bring the barometer indoors or use a water-alcohol mix to lower the freezing point.