Torricelli, a mathematician and colleague of Galileo, invented the barometer in 1643. Scientists and meteorologists still use barometers today. A barometer measures air pressure, which indicates weather patterns. When used in conjunction with measurements such as temperature and altitude, you can make quite accurate weather predictions. The two main types of barometer, mercury and aneroid, read the same. One uses mercury, the other uses non-liquid cells. Most likely, you have the latter at home because of its light weight and ease of use. They have minimal accuracy differences.
Note if the numbers on the barometer are in hectopascal units, millimetres or inches. Ranges look like this: Hectopascal, hPa, would range from approximately 950 to 1050, millimetres from 730 to 790 and inches from 25 to 35. Mercury barometers may say Hg, the chemical symbol for mercury. Some barometers have a second ring denoting hPa, or millibars, in addition to inches or millimetres.
Tap on the glass. This ensures the mechanism is not sticking.
Check the placement of the needles. Round barometers have two needles, like hands on a clock. The largest needle shows the reading of air pressure. The second needle, or "set pointer," turns manually, usually from a knob at the centre of the glass face. Mercury barometers may have visible liquid, similar to a thermometer. In this case, a sliding bar can line up with the reading to give the "set point."
Compare the placement of the main needle to the set pointer. If the main needle has moved up the scale from the pointer needle, it means a rise in pressure. Down the scale means a drop.
Note how much the pressure has changed. Rises in pressure, especially of at least 7 hPa, 0.24 inches or 5mm mean the approach of high-pressure systems. Likewise drops in pressure relate to low-pressure systems. As a rule, drops in pressure denote rain; extreme drops indicate stormy weather. High pressure means dry, but not necessarily warm, weather.
Set the pointer hand behind the main hand. Then, wait 24 hours before making the next reading.
Air pressure drops and rises frequently in small amounts throughout the day. Check the barometer reading at the same time each day. Do not count on barometric readings that say "rainy," "sunny," etc. The rapid changes say far more. Plus, readings depend on location. For example, according to the Bureau of Meteorology in Australia, "The pressure may well never fall to the values shown for stormy or rain for most places within Australia."
If you have a new barometer, it must first be set.