When you want to know what the weather's doing, you check the barometer. It tells you whether the air pressure is rising or falling, which tells you whether you're in for good weather or bad over the next day or so. It's not much good for long term predictions, but it's an excellent tool for figuring out whether the weather will hold for the rest of the day.Pressure and Weather
The basic rule to follow is that high pressure is good, low pressure is bad. Rising pressure means the weather is getting better, falling pressure means it's getting worse. Lower pressure means air is rising, and rising air gets colder so more water condenses out producing precipitation. You also get winds rushing in from high pressure areas to low pressure areas. Higher pressure means air is descending. As it descends it gets warmer and can absorb more moisture so the skies clear up.Mercury Barometer
The original barometers, most of which you would class as scientific collectables today, relied on a tube sitting in a pool of mercury. Air pressing down on the mercury pushed it up into the vacuum in the tube. The higher the pressure, the more the mercury rose. Unfortunately not only is mercury toxic, but you need a lot of it for a mercury barometer as standard atmospheric pressure creates a column seventy-five centimetres tall.Aneroid Barometer
The mercury barometer was followed by the aneroid barometer. These barometers rely on an evacuated metal capsule attached to a spring mechanism. Air pressure causes the capsule to flex, which drives the needle on the barometer. Higher pressure makes it flex more; lower pressure makes it flex less. They are very common in the home.Digital Barometers
The third kind, which are very common in both smartphones and digital weather meters rely on a simple digital transducer. These barometers have no moving parts and often come with a thermometer and wind speed meter.