A barometer is a device for measuring air pressure and can be used to forecast the weather within 12 to 24 hours. Air pressure can be measured in inches of mercury, millimeters of mercury, or hectopascals, depending on the location and scale of the instrument. To find out if the air pressure is rising or falling, you need to calibrate the barometer properly. After purchasing a barometer, it must be adjusted before it can be used to measure air pressure accurately.
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Part 1 of 3: Setting the Barometer
Step 1. Buy a barometer
There are three types of barometers available in the market. If you have an antique barometer, it is most likely mercury or aneroid. Aneroid or electronic barometers are easier to find. Before buying a barometer, check its usage height. Not all barometers work well at high altitudes. So, if you live high above sea level, buy a barometer that can be used specifically at that altitude. The following is a brief description of each type of barometer:
- Mercury: The mercury barometer, sometimes called the rod barometer, was the first barometer to be invented. This barometer uses an open tube system with a number of liquid mercury that rises and falls following changes in pressure. This barometer only works at altitudes up to 300 meters.
- Aneroid: Aneroid barometer does not use any liquid. This barometer uses a small box made of beryllium and copper that expands or contracts as pressure changes. This movement causes the mechanical needle to move pointing to the air pressure value.
- Electronics: Electronic barometers are a bit more complicated to understand, but they use sensors and strain gauges that cause a change in stress that can be changed to display the pressure value to the user.
Step 2. Know the local barometric pressure reading
If you are using an aneroid barometer, you will need to calibrate it according to your location. Listen to the local weather forecast to find out the current barometric pressure in your location. Make sure the reading is correct for your location. Even a few kilometers apart can affect the barometer reading.
- Setting the barometer according to the location will take into account the pressure difference caused by the altitude of the location.
- The factory setting for the aneroid barometer is sea level, but if you don't live at sea level, you'll need to calibrate it.
Step 3. Set the indicator needle on your barometer
Look for the small adjusting screw on the back of the barometer. With a small screwdriver, turn the adjusting screw to move the needle to the current air pressure at your location. Observe the face and stop turning the screwdriver when the needle points to the correct reading.
- If using a mercury barometer, you must use a conversion factor for your reading.
- The digital barometer has a sensor that calibrates the altitude automatically.
Step 4. Hang the barometer in a suitable place
The barometer will function equally well both indoors and outdoors. The pressure will be the same wherever the barometer is attached. Avoid locations that experience frequent changes in temperature, such as near bathrooms or heating machines.
- Tightly sealed and air-conditioned rooms are less affected by air pressure. If possible, avoid these rooms.
- Avoid locations exposed to direct sunlight as changes in temperature may affect the readings.
- Hang the barometer away from drafts, such as near a door or window. The air pressure in places like this is too volatile.
Step 5. Check occasionally to make sure your barometer is working properly
If you doubt that the reading is inaccurate, check the barometer with this easy trick. With the barometer fixed to the wall, slowly slide the bottom side to the side until it forms a 45-degree angle.
- If you use a bar barometer, the liquid mercury will rise to the top of the tube and produce a "tick" sound that can be heard as well as felt. The tube will be filled with mercury.
- If you are using an aneroid barometer, the indicator hand will rotate clockwise.
- If the barometer does not pass this test, you will need to have it repaired by a professional and reset the barometer before you can rely on its accuracy. However, most barometers can be used for years without having to be repaired.
Part 2 of 3: Using the Barometer
Step 1. Set the manual needle to the current reading
Turn the center barometer knob so that the arrow is directly above the indicator arrow (this is the current barometric pressure at your location). The adjusting needle can be identified by the presence of a notched arrow around its center.
- The control needle serves as a reference that allows you to easily know whether the air pressure is steady, rising, or falling.
- Remember, this needle is only found on the aneroid barometer. If you are using an electronic barometer, simply check the reading.
- If you are using a mercury barometer, you will need to correct the altitude when it is above sea level.
Step 2. Correct the altitude if you are using a bar barometer
To accurately measure barometric pressure using a bar barometer, you will need to correct the altitude of the location using a conversion graph. Look at the barometer with your eyes straight and note the number shown at the top of the mercury column. This pressure is in millimeters of mercury (mmHg).
- Find the elevation of your location and use the graph to find the relevant correction factor. Add a correction factor to the barometer reading. The results must match the readings from the local BMKG.
- If you are at an altitude above 300 meters, the bar barometer will not work properly.
Step 3. Check the barometer one hour later
Forecasting the weather using a barometer is done by observing changes in air pressure. Observe the barometer reading every few hours to see if the air pressure is changing or stable.
- If using an aneroid or mercury barometer, gently tap the surface of the barometer to release the pressure changes stored in the mechanism. Record the reading after the needle or mercury stops moving.
- Move the adjusting needle if the air pressure changes. That way, if you check it at another time, you'll see in which direction the air pressure changes.
Step 4. Record the change in air pressure
Keep a journal of all the readings taken with the barometer. Make a simple graph of changes in pressure during the day to help forecast the weather. Is the air pressure rising? Down? Stable? All this information is important for forecasting the weather.
- Do not expect big changes in the movement of the indicator needle. Daily changes typically range between 0.02 and 0.10 in one inch of the barometric scale. Larger changes are possible in winter and depend on location and altitude.
- Take periodic readings (every few hours) and graph them into your chart.
Part 3 of 3: Forecasting the Weather
Step 1. Predict rain if the air pressure drops
In general, when the air pressure drops, the weather rotates in the direction of the storm and rain indicators. The starting point of the reading is also important in forecasting. A higher reading indicates better weather even if the pressure is lower.
- If the reading is above 30.2 inches of mercury and dropping rapidly, this is a sign of cloudy, but warmer weather.
- If the reading is between 29.8 and 30.2 inches of mercury and it's falling fast, it's likely raining soon.
- If the reading is below 29.8 inches of mercury and it's falling slowly, it's likely raining; if it descends quickly, it is a sign that a storm is coming.
Step 2. Expect better weather if air pressure rises
As air pressure rises, the weather tends to improve as the high-pressure system moves past your location.
- A reading above 30.2 inches of rising mercury indicates that the weather will continue to improve.
- A reading between 29.8 and 30.2 inches of rising mercury indicates that the weather has not changed regardless of the conditions.
- A reading below 29.8 inches of rising mercury is a sign that the weather is sunny, but cooler.
Step 3. Expect sunny weather if the air pressure is stable
Stable air pressure indicates a long period of sunny weather and indicates that you will experience equally sunny weather. If the weather is clear and the air pressure is stable, look forward to even brighter weather! Higher pressure indicates warmer weather, while lower pressure indicates cooler weather.
- Powerful high-pressure systems air pressure about 30.4 inches of mercury. Any value above 30 is considered high pressure.
- A typical low-pressure system has about 29.5 inches of mercury. Any value below 29.9 is considered low pressure.
