How to Adjust a Chelsea Barometer

The inner scale of this barometer is in millibars, and weather icons clearly indicate that as pressure drops, the weather gets stormy.

Barometers measure atmospheric pressure, the force of the column of air pushing down on you, either in units of millibars (mb) or inches of mercury (inHg). Tracking changes in atmospheric pressure is a good way to predict changes in weather, as when pressure generally falls rapidly before a storm. Chelsea Clock manufactures and markets combination clock-barometers, and they calibrate them at the factory for atmospheric pressure at sea level. If your altitude isn't close to sea level, you will have to adjust your barometer in order to obtain an accurate reading.

Instructions

1. Determine your present local barometric pressure, either by calling your local weather bureau or checking online (see Resources).

2. Convert from mb to inHg, if you get the pressure in mb, because Chelsea Clocks use inches of mercury. Divide the mb value by 33.86 to get inHg. For example, 1,004 mb divided by 33.86 yields 29.65 inHg.

3. Adjust the barometer's black indicator hand to the correct barometric pressure by turning the small screw on the back of the barometer case. From now on, your Chelsea barometer will read accurately.

Tips & Warnings

• Chelsea Clock recommends that a professional set your barometer if your altitude is much more than 5,000 feet above sea level.

References

• Chelsea Clock: Chelsea Clock Product Manual
• The Weather Channel: Glossary - Inches of Mercury

Resources

• National Weather Service: Current Denver-Boulder, CO Pressure Readings
• Weather For You: Latest Reported Conditions - Current Pressure