Measuring Barometric Pressure

Evangelista Torricelli invented the Barometer in 1643 when through his experiments he proved the existence of atmospheric pressure. Records of barometric pressure go far back to 1692 for London. Atmospheric pressure is greatest at the earth’s surface and diminishes with altitude.

Fitzroy Storm Glass

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Admiral Robert Fitzroy was Captain of HMS Beagle, commanded Charles Darwin’s expedition and devised a simple instrument that could indicate approaching weather. It consists of a sealed tube within which is water, preferably distilled, along with ethanol, potassium nitrate, ammonium chloride and camphor. These ingredients provide a set of crystals that normally reside at the bottom of the tube but which will respond to changing air pressure and temperature. The composition of the liquid will change under differing weather conditions and the description of these is included for the observer to use. These instruments were widely used at the time by mariners and at numerous ports.

Mercury Barometer

The original mercury barometer consisted of a glass tube about 3 feet high with one end open and the other end sealed. The tube was filled with mercury, which was used being a very stable fluid and heavier than water. The glass tube sat upside down in a container, called the reservoir, which also contained mercury. As atmospheric pressure rose it pressed down on the liquid in the open dish of mercury that in turn pushed the column of mercury up the open end of the tube resting in the reservoir. As pressure dropped, the process was reversed.

Many old mercury barometers still exist, which use this principle but in a modified way, not using an open dish of mercury but a sealed unit which allows the instrument to hang on a wall. This is especially important, as we now know that mercury is toxic.

Aneroid Barometer

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Many homes will have hanging on the wall a simple, inexpensive aneroid barometer. This instrument provides a useful and fairly accurate reading of atmospheric pressure. The advantage of a barometer is that you don’t need to go outside to read it and it is always working. Besides the actual reading on a daily basis one of the most useful observations is to note whether the pressure is rising or falling and what is more important, the rate of change. If the pressure is observed to rise steadily it indicates a spell of improving weather over a short period of time. If the pressure begins to build steadily over a longer time period, the improved weather is likely to last for longer. This principle is also relevant when the pressure begins to fall. The aneroid barometer consists of a small round, metal box within which is a partial vacuum. Any increase or decrease in the air pressure will cause the box to move in or out. Attached to this box is a set of levers that are connected to a hand that moves over the outer dial indicating the actual pressure. There is an additional hand on the face of the dial that can be moved using a small, connected knob on the outer surface of the barometer, this can be rotated and set daily to that reading and the difference noted after 24 hours. Scribed around the dial is a set of words such as“Rain”, “Change” and “Fair”. These words should not be taken too literally.

It might be sufficient just to note the actual daily reading and the direction of change in the air pressure. However, for those wishing to have accurate data that can be compared with broadcast values, a barometer will need to be adjusted for its height above sea level. This adjustment will provide a standard value of air pressure at sea level that can be compared to any position locally, nationally and internationally.

Setting a barometer can be carried out by noting a reading, on the hour, for several hours. Next, using the internet, visit a couple of official sites nearest to you, such as the Meteorological Stations (www.metoffice.gov.uk), for their values at these hours. When comparing your readings to the official values you can then determine what adjustment needs to be made to your aneroid barometer by turning a small screw at the back of the instrument. I suggest that over the following couple of days you cross check your new values using the above method.

Electronic stations

There are a variety of budget stations that utilize wireless transmission of much weather related data to a display monitor, several of which have the facility to measure barometric pressure, but not all. They are quite accurate to 1mb and make an economic instrument that combines a number of features.

Advanced Weather Stations

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These top of the range pieces of equipment display many items of weather related data including barometric pressure and are very accurate. The data is displayed on a monitor that is powered by mains electricity but with battery back up to cover any power outage. Inputting the relevant data when setting up the station, including the height above sea level, will enable the electronics to set the correct barometric pressure for your station. Some weather stations have the facility to purchase software that allows all data to be downloaded to a computer. Using this data the station can prepare graphs that are then displayed on the computer from which it is possible to choose a display on a one hourly period to a daily basis, month or year.

Barograph

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A barograph is a large aneroid barometer which displays a trace of the atmospheric pressure over time. It consists of a stack of circular metal boxes, again containing a vacuum, whose movement in and out due to changes in atmospheric pressure, is transmitted using a set of levers that are connected to an arm at the end of which is a pen nib. This nib is loaded with ink and draws a trace on a strip of specially prepared gridded paper, which has been wound around a drum that revolves once every week and driven by a clockwork mechanism. The charts must be renewed every week and the clockwork mechanism then rewound. The ink trace provides an immediate visual indication of the air pressure but due to the size of the paper it is not possible to read to an accuracy of less than 2 or 3mb.

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