A barometer is a device capable of measuring air pressure, information that can then be used to predict the weather conditions for the next 12/24 hours. The air pressure is measured in hectopascals or in millibars, based on the area of residence and the measurement scale adopted by the instrument used to take the reading. To understand if the atmospheric pressure is increasing or decreasing, the barometer must be correctly calibrated. When purchasing such a device, in order to measure pressure properly, it must first be calibrated and configured carefully.
Steps
Part 1 of 3: Calibrate a Barometer
Step 1. Purchase a barometer
There are three types of barometers on the market. If you have an antique barometer, most likely it is a mercury or aneroid barometer. Today the most common types of barometers on the market are electronic or aneroid ones. Before purchasing your measuring instrument, check at what altitude it can be used. This is necessary because not all barometers work properly at high altitudes, if you live in the high mountains, it is therefore important to buy one that has been designed to measure atmospheric pressure at great altitudes. Below you will find a brief description of each type of barometer:
- Mercury: this type of barometer derives from the famous Torricelli tube, which was the first device to be invented to measure atmospheric pressure. This instrument is characterized by a Czech-bottomed tube filled with mercury, the open side of which is immersed in a tray also filled with the same chemical element. A sort of system of communicating vessels is thus created in which the mercury level varies as the atmospheric pressure varies. This type of instrument only works properly at altitudes above 305 meters.
- Aneroid: The operation of this type of barometer does not rely on any liquid. What is exploited is in fact a Bourdon aneroid, that is a small vessel built with beryllium and copper that expands or contracts in relation to changes in atmospheric pressure. The motion generated by this variation is transmitted to the indicator of the instrument through a system of levers and gears which displays the reading of the atmospheric pressure on a special graduated scale.
- Electronic: understanding the operation of this type of barometers is a little more difficult since they use sensors and strain gauges that cause a variation in the voltage which is then converted to be conveniently viewed and read by the user on the display.
Step 2. Get an accurate measurement of the atmospheric pressure present in the area where you are
If you have purchased an aneroid barometer, you need to calibrate it based on where you live. Use a local weather report to find the current atmospheric pressure value. Make sure your reading is correct for where you are, as even a few kilometers apart can change a barometer reading.
- The calibration of a barometer takes into account the difference in pressure due to the altitude of the point where it will be installed.
- In the case of an aneroid barometer, the manufacturer's settings are relative to a reference altitude of 0 meters, i.e. sea level. The instrument will then have to be recalibrated based on the altitude at which it will work.
Step 3. Correctly position the pointer of your barometer
Locate the small adjustment screw on the back of the instrument; then, with a small screwdriver, slowly rotate it to make the indicator pointer indicate the current atmospheric pressure in the area you are in. As you turn the adjustment screw, watch the barometer indicator to stop when the hand has reached the desired value.
- If you have purchased, or have, a mercury barometer, you must use a converter to be able to take the reading.
- Digital barometers are equipped with a special sensor for automatic altitude calibration.
Step 4. Hang the barometer in a spot that allows for easy reading
Hanging it indoors or outdoors makes no difference. The atmospheric pressure reading is not affected by where the barometer is hung, however you must be careful not to place it in a place that has a strong temperature range, for example near a bathroom or a heat source (such as a radiator).
- Well-insulated and air-conditioned environments are not affected by variations in atmospheric pressure, if possible, it is therefore better to avoid placing the barometer there.
- Do not place the meter in a spot that is directly exposed to sunlight as changes in temperature change the pressure measurement.
- Place the barometer away from highly ventilated areas, such as doors or windows. Atmospheric pressure is highly variable in the vicinity of these areas.
Step 5. Check the barometer periodically to make sure it is working properly
If you suspect that the meter measurement is inaccurate, you can run a test using a simple trick. While the barometer is hanging on the wall, lift the bottom side up to a 45 ° angle to the wall.
- In a mercury barometer, the mercury should rise to the top of the tube producing a well audible sound effect. The barometer tube should fill completely with mercury.
- If you are using an aneroid barometer, the indicator hand should rotate clockwise.
- If your barometer does not pass this simple control test, it is very likely that you will need to have the support of a professional to restore it to normal operation before you can use it normally. However, most barometers can function properly for years, without the need for any maintenance.
Part 2 of 3: Use a Barometer
Step 1. Manually position the pointer of the reference indicator on the current barometric pressure value
Turn the knob in the center of the barometer so that the arrow overlaps that of the indicator (the measurement obtained indicates the barometric pressure currently present in your location). The reference indicator is normally characterized by a different color and shape than the one that actually measures the atmospheric pressure (it is possible that it has a small arrow in the final half).
- The reference indicator is used to keep track of the value detected by the barometer allowing you to readily understand if the atmospheric pressure is rising, falling or stable.
- Remember that this reference hand is only present in barometers equipped with aneroid. If you have purchased an electronic instrument, you simply have to look at the display to be able to read the pressure value.
- If you have a mercury barometer and you are at an altitude above sea level, you need to correct the indicated measurement to that value.
Step 2. If you are using a mercury barometer, adjust the reading to the altitude you are at
To obtain an accurate measurement with this type of instrument, a conversion of the measured value must be performed to adapt it to the actual altitude. You can do this by using a suitable conversion table. Observe the barometer by holding it at eye level, then take note of the value indicated by the top of the mercury column. This is the atmospheric pressure expressed in millimeters of mercury (mmHg).
- Now locate the altitude value you are at, it is used to find the relative correction factor to use to calculate the correct pressure value. Add the correction factor to the value indicated by the barometer. The measurement obtained at the end of the calculation should correspond to that provided by the local meteorological service.
- When at an altitude below 305 meters, mercury barometers do not work properly.
Step 3. After an hour, recheck your meter measurement
Forecasting weather conditions using a barometer is based solely on changes in atmospheric pressure. To understand if the pressure is changing or staying constant, you need to take regular measurements, within hours of each other.
- If you are using an aneroid or mercury barometer, gently tap the front of the meter to release any pressure changes registered by the internal mechanism. Once the gauge needle or the mercury has stopped moving, make a note of the measurement.
- If the atmospheric pressure has changed, reposition the reference indicator to the new value, so that the next time you read it, you'll know right away in which direction it's moving.
Step 4. Draw a graph based on the changes in pressure
Keep a record of all measurements made by the barometer. To help with your weather forecast, draw a small graph using the daily changes in atmospheric pressure. Knowing if the pressure is increasing, decreasing or stable is essential to be able to predict a change in weather conditions.
- Don't expect large movements of the indicator hand. Normally, the daily variations in atmospheric pressure are between 0, 5 and 2, 5 mm on the measurement scale adopted by the instrument. In winter, large variations in pressure can occur depending on the location and altitude.
- Take blood pressure measurements regularly (every few hours), then use them to draw your own reference graph.
Part 3 of 3: Predict the Weather Conditions
Step 1. If the atmospheric pressure drops, it means rain is expected
Generally speaking, if the pressure drops, the weather is changing and thunderstorms and rain can be expected. In order to make reliable predictions, a lot of weight must be given to the initial blood pressure measurement. When starting from a very high value, you can still expect good weather even if subsequent measurements indicate a decline.
- If the measurement of a mercury barometer is above 1043 mbar with a tendency to decrease rapidly, this value indicates cloudy weather, but in a warmer climate.
- If the measurement is between 1,029 and 1,043 mbar, but is rapidly decreasing, the rain is most likely on its way.
- If the measurement is below 1,029 mbar and drops slowly, it means that it will most likely rain soon; if, on the other hand, the pressure drops rapidly, it means that a storm is imminent.
Step 2. As atmospheric pressure tends to rise, forecasts indicate an improvement in weather conditions
When the air pressure rises, it indicates that a high pressure system is moving towards your location, which means that good weather is coming.
- Continuously increasing atmospheric pressure measurements above 1043 mbar indicate stable good weather.
- If the measurement is between 1,029 and 1,043 mbar with a tendency to increase, it means that the weather conditions will remain stable.
- If the measurement is below 1,029 mbar but tends to increase, it means that the sky is clearing, but the climate will nevertheless be colder.
Step 3. Weather forecasts are more accurate when atmospheric pressure is stable
This condition indicates long periods of good weather with a tendency to remain stable. If the sky is clear and sunny and the atmospheric pressure is stable, you can expect many warm, bright days. High pressure indicates a generally hot climate, while low pressure indicates a generally cold climate.
- A strong high pressure system occurs when the atmospheric pressure reaches 1,050 mbar. Any reading above 1,036 mbar is considered high pressure.
- Normally there is a low pressure system when the atmospheric pressure value is about 1.019 mbar. Any reading below 1,033 mbar is considered low pressure.
