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# What does it mean when a barometer is rising or falling?

### How Lasers Work

“Barometers are used to predict the weather.iStockPhoto

­A barometer measures air pressure: A "rising" barometer indicates increas­ing air pressure; a "falling" barometer indicates decreasing air pressure.

In space, there is a nearly complete vacuum so the air pressure is zero. On Earth, because there are many miles of air molecules stacked up and exerting pressure due to the force of gravity, the pressure is about 14.7 pounds per square inch at sea level (see this page for a good explanation of atmospheric pressure).

­The interesting thing about air pressure is that it is different at different points on the planet and it changes over time. Why might that be? If you’ve read How Hot Air Balloons Work, you know that hot air is less dense (lighter) than cooler air. Therefore, on any given day you would expect the air over a desert to­ have a lower pressure than the air over an ice cap. And that would be true. These same sort of pressure differences occur all over the planet for various reasons.

These pressure differences have a big effect on the weather, so if you know the current air pressure at your house, as well as the pressure trend, you are able to predict certain things about the weather. As a very loose rule, a high-pressure area will be clear, and a low-pressure area will be cloudy and rainy.

Originally Published: Apr 1, 2009

### Barometer

##### How does a barometer work?

Simply put, a barometer acts like a balance that ‘balances’ the weight of the atmosphere (or air around you) against the weight of a mercury column. If the air pressure is high, the mercury will rise. At low air pressure, the mercury goes down.

##### What is a barometer used for?

We use barometers mostly to measure atmospheric pressure. Changes in air pressure help meteorologists predict changes in the weather. For instance, if the barometer shows low atmospheric pressure, it indicates the impending arrival of a low-pressure system.

##### What do barometer readings mean?

A high reading on a barometer often indicates high pressure, which meteorologists interpret as good weather. A low reading on the barometer is often a precursor to bad weather patterns.

##### Can barometric pressure affect human beings?

The barometric pressure is a measure of how much force the air around you is applying against your body. A change in barometric pressure can trigger a migraine, for example.

##### Who invented the barometer?

The barometer was first invented in 1643 by Evangelista Torricelli to measure air pressure. Torricelli would use a tube of water to conduct his earliest barometric experiments. More advanced variations of barometers have been invented since then.

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• How a barometer measures air pressure
• Measuring Pressure

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