Weighing the sky
How Does a Barometer Work?
A barometer weighs the column of air above it, and that weight changes before the weather does.
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Step 1 of 5
Air has weight, and you are under all of it.
The air above one square metre of ground weighs about ten tonnes. You do not notice because it presses equally from every direction.
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Step 2 of 5
The instrument feels that press.
Inside is a sealed metal box with the air pumped out. When the outside pressure rises the box squeezes; when it falls the box springs out.
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Step 3 of 5
A linkage turns that squeeze into a needle.
The box's movement is a fraction of a millimetre, so levers and a chain multiply it until it swings a pointer right across the dial.
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Step 4 of 5
Falling pressure means rising air.
Low pressure is where air is rising. Rising air cools, makes cloud and rain, so a dropping needle is a genuine warning of unsettled weather.
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Step 5 of 5
The speed of the fall matters most.
A slow drift down means a change tomorrow. A drop of several units in three hours means a storm is arriving, and soon.
Aircraft altimeters are barometers too, which is why pilots reset them to the local pressure before landing.
The short version
A barometer measures the weight of the air above it, and a falling reading means rising air, cloud and rain.
Try it yourself
Stretch a balloon skin over a jar and glue a straw to it as a pointer. The straw tips as the pressure outside changes.
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