How do Barometers Work?

Ever since the barometer was invented in 1643 by Italian physicist Evangelista Torricelli, it has played a critical role in weather forecasting. This scientific instrument remains popular even in the 21st century. But how, exactly, does it work? What does a barometer measure, and what do these measurements mean? Chelsea Clock has the answers to these questions for sailors and weather fanatics alike.

What is a Barometer?

A barometer measures atmospheric pressure and its short-term changes. The two most common types of barometers are the mercury and the aneroid barometer. The classic mercury barometer consists of a glass tube with a sealed top end and an open bottom sitting in a mercury-filled reservoir. The mercury level in the tube goes up or down as the air pressure does.

In recent years, the aneroid barometer has become a common (and much safer) alternative. They have a flexible metal box inside that is vacuum-pumped so changes in pressure cause the metal to expand or shrink. This movement triggers mechanical levers that move a needle on the face dial. French scientist Lucien Vidi invented the aneroid barometer in 1844. All Chelsea Clock barometers are aneroid barometers.

How to Read a Barometer

The two factors in reading a barometer are the current atmospheric pressure and what direction it is moving. Here are the most common air pressure values and how to interpret them:

  • High pressure (over 30.20 inHg) that’s steady, rising or slowly falling means fair weather.
  • High pressure that’s rapidly falling means warmer but cloudy conditions.
  • Normal pressure (29.80-30.20 inHg) that’s rising or steady means no short-term changes in conditions.
  • Normal pressure that’s falling slowly means small weather changes.
  • Normal but rapidly falling pressure means likely rain (or snow in colder conditions).
  • Low pressure (under 29.80 inHg) that’s steady or rising means cooler weather and clearing skies.
  • Low pressure that’s slowly falling means rain is likely coming.
  • Low pressure falling fast means a storm is on its way.

The master craftsmen at Chelsea Clock have designed a variety of handmade barometers for use in ships, homes or offices. By using this guide, you’ll be able to tell what weather is coming at land or at sea. Visit our collection to find a barometer for yourself or as a luxury gift.