How to use a Barometer

A barometer, which is an instrument that is used to measure air pressure of the Earth’s atmosphere, can create a pressure reading by utilising air, water, mercury and an aneroid setup. The first attempts to measure atmospheric pressures occurred in the mid-seventeenth century. The changes in pressure are used to forecast short term changes in the weather, such approaching cold fronts, surface troughs and much more, such as an increase in barometric pressure that may be indicative of rejuvenating weather conditions. Recently the reliance on electronic barometers has increased, as these computerised instruments have the ability to store recorded pressure readings, and therefore calculate atmospheric trends over prolonged periods of time.

AIR BAROMETERS are made from a piston and some sort of cylinder; as the pressure of the atmosphere increases, the pressure inside the cylinder also increases. This is due to the fact that increase in pressure equals an increase in temperature. Thus the air molecules inside the cylinder move faster and hit the piston on a more regular basis. As this happens the increased pressure moves the piston out. By measuring the distance the piston has moved, the air pressure can be determined.

WATER BAROMETERS work on the principle of decreasing atmospheric pressures. Also called a Goethe barometer, they consist of a sealed glass capsule that is filled up with water, half way. A narrow spout is incorporated into the structure of the capsule, below the water plane. So when the air pressure increases, the water in the spout rises above the water level in the container; and when the air pressure decreases the the water within the spout drops below the water level in the capsule. Knowing the expansion of water allows to calculate the pressure difference.

MERCURY BAROMETERS are made up of a glass tube with a height of about 750 millimetres and are closed at one end. The open end of the tube has a filled mercury reservoir. This setup works on the principle of expansion. The mercury rises and adjusts due to an increase in pressure; this keeps happening, until the weight of the liquid mercury balances out into an equilibrium with the atmospheric pressure force that pushes down on the mercury reservoir. Temperature changes the density of the mercury, and because of this the scale needs to be adjusted in accordance with the metal’s expansion. This is measures in millimetres of mercury, or (mm of Hg).

ANEROID BAROMETERS utilise a small metal cell, which is called an aneroid cell. The box is constructed from a beryllium and copper alloy. The pressure inside the cell is almost at a vacuum; thus to support the box from collapsing, a powerful spring is installed. The expansion or compression of the spring works other multiple mechanisms, so that the smallest movements can be amplified and can be read on the screen of the cell.

Whether it is a professional, government funded organisation or a hobbyist that is conducting the investigation, the barometer has proven to be valuable, as it is pressure that mostly determines upcoming weather conditions, such as storms, cyclones, cold fronts, high pressure systems and winds, as these dynamic phenomena can only occur due to pressure changes. Therefore setting up a barometer correctly is important if accurate weather predictions are to be made. It is done in the following way:

A) Identify the type of barometer used. B) Read the instructions, that are included with your barometer. Then adjust the alteration screws, valves and knobs on the instrument to set the device at the correct local surface pressure reading, from a reliable source. C) Move the needles or cycle through the digital number displays, etc., so that the base pressure is the same as that of the surface pressure of the atmosphere at your location. D) Record the reading that is obtained from the barometer. Take a few readings for consistency and quality of the device. Then cross check with official weather report readings. This method will allow you to determine any errors in your barometer and to check for the instrument’s accuracy.

The best time to set up the barometer is in high pressure conditions. The systems are anticycloncic in the southern hemisphere and cyclonic in the northern hemisphere of the planet. This will usually be indicated by warm and dry conditions, with as little windsheer as possible.