Re: How does a barometer work ?

There are two rather different things your question might mean:

1. How is the barometer designed as an instrument to give you a measurement of the air pressure?

   OR

2. How do you use barometer readings (air pressure readings) to find out about the weather?

Here is an answer for the first question:

There are two main types of barometer that are used. The mercury barometer is seldom seen these days: mercury is expensive, its toxicity is taken much more seriously than it once was, and the instrument itself is large and unwieldy. It uses a long glass tube with a sealed end in a reservoir of mercury. Mercury is very dense, and the air pressure is only strong enough to push the mercury up about 76 cm (30 inches). The exact height of the mercury column in the sealed glass tube is a measure of the air pressure -- indeed air pressure is often measured in units of "millimetres of mercury" or "torr", which is the same thing.

The smaller type of barometer that you usually see has inside it a hollow metal disk with a thin flexible top surface. This top surface is connected to a wire and spring. When the air pressure increases, it squashes in the top of the disk a bit, which stretches the spring, and moves the gauge needle.