Edited by Maluniu, Teresa, Miri125, Jordan and 1 other
A barometer measures air pressure and can forecast the weather within a 12 to 24 hour period. The pressure is measured by inches of mercury, which indicated by a gauge on the instrument. In order to tell whether inches of mercury are rising or falling, you must the calibrate the barometer correctly. When you purchase a barometer, it must be adjusted before you can use it to accurately measure atmospheric pressure.
1Listen to a local weather forecast to find the current barometer pressure for your location. Make sure the reading is correct for your location. Even a few miles can affect a barometer reading.Ad
2Set the black indicating hand. The indicating hand is the thinner of the two hands.
- Locate the adjusting screw on the back of your barometer.
- With a small screwdriver, turn the adjusting screw to move the hand to your location's sea level.
- Watch the face of the dial and stop turning the screwdriver when the hand reaches the appropriate reading.
- Some barometers are already factory set to a certain sea level; if it happens to be yours, you may not have to do this step.
3Adjust the set hand, or pointer. Turn the barometer's center knob to set it to the current barometric pressure for your location. You can identify the set hand by its notched arrow about halfway down the hand.
- The set hand will serve as a reference that allows you to easily tell if the pressure is steady, rising or falling. You shouldn't have to re-set it unless you suspect inaccurate readings.
4Use your barometer to forecast the weather.
- A barometric reading over 30.2 that is rising, steady or slowly falling indicates fair weather. A rapidly falling barometer indicates cloudy or warmer weather.
- If the reading is 29.8 to 30.2 and is rising or steady, expect the same weather you are currently experiencing. If the barometer is slowly falling, expect little change. If it's falling rapidly, expect precipitation.
- If the barometer reading is under 29.8 and is rising or steady, expect cool and clear weather. If the barometer is falling slowly, expect some precipitation. A rapidly falling barometer could indicate stormy weather.
- Daily changes in your barometer should only range from 0.02 to 0.1 psi, or pound per square inch of pressure. Greater variation in barometer readings occur in winter and also depend on your location's altitude.
5Check occasionally to be sure that your barometer works properly or whenever you suspect inaccurate readings. With the barometer hanging on the wall, slowly slide the bottom of it sideways to a 45-degree angle.
- If you have a stick barometer, the mercury should rise to the top of the tube and produce an audible "tick" sound that you may also feel. The tube should fill with mercury.
- If you have a dial barometer, the indicator hand should rotate clockwise around the dial.
- If your barometer fails this test, you may need to have it professionally serviced and re-set before you can count on its accuracy. However, most barometers can last years with no servicing.
- It doesn't matter whether you hang a barometer inside or outside. The pressure will be the same, regardless.
- Barometers are sensitive to altitude and have a specific altitude range in which they can function. Make sure your barometer will work at the altitude where you live.