The Aneroid Barometer

What a barometer does

A barometer is an instrument for measuring the weight (or as it is more frequently expressed, the "pressure") of the Air above it. It is well known that air has weight, but the earths atmosphere gets thinner (lighter) with increased height. It follows that as we ascend from sea level and low lying parts of the earth's surface, the weight of the air will grow less, that is the pressure will fall. An ordinary clock face type barometer placed in your car will give you a rough estimate of altitude as you drive through the country. If we use rough figures, we can say that the reading decreases by one inch, (approximately 34 millibars,) for each thousand feet of ascent. For example, if the barometer reads 30 inches, (1016mb) at the sea coast and you drive up a mountain road 1,200 feet high (365 meters) the hand should move down the scale to about 28.8 inches (975mb).

A barometers main use, however, is not to measure altitude, but to measure the actual changes in the weight of air in a particular place. High and low pressure systems in the atmosphere move around the earth's surface, and the movements shown on the face of a stationary barometer attached to the wall of your home, for instance, indicate the changes of pressure as they occur directly above you. These changes, when they are considered together with wind, temperature, moisture and cloud indications, can be a great help in forecasting the weather.

How it works

The aneroid barometer is operated by a metal cell containing only a very small amount of air, or a series of such cells joined together. Increased air pressure causes the sides of the cell or cells to come closer together. Once side is fixed to the base of the instrument while the other is connected by a series of levers and linkages to a rotating or Reading Hand that moves over a scale on the face of the instrument. This "reading hand" is usually black or red.
So that we can compare readings made at sea level with those made at more elevated places, where the pressure of the air is much lower, we must establish a uniform standard. Therefore all barometers must be reduced to mean sea level, or in other words to the pressure each barometer would record if it were at sea level directly below the place where it is situated. Your barometer, too, must be set to show sea level pressure if it is to give readings which can be compared with official barometric reports and used as a weather indicator.

Who invented it?

In 1643 and Italian mathematician named Evangelista Torricelli discovered the principle of the barometer by using a long glass tube closed at one end, which he put upside down in an open container holding liquid. He found that the pressure of the air bearing down on the liquid forced it up the tube, and the measurement of the various lengths along the column of liquid was therefore a means of expressing changes of air pressure. In order to have a tube of manageable length, the heaviest of all liquids, mercury, was finally chosen.

In 1646 Otto von Guericke established an approximate level of 34 feet using water and Pascal not to be outdone established a level of 64 feet using wine (it is not recorded whether red or white).

Mercury tube barometers are costly and require extreme care in handling. The Aneroid Barometer (aneroid = without fluid) was developed as a more practical mechanical means of measuring atmospheric pressure and is the most commonly used today.

Use of the barometer in forecasting

Many of you will know from the weather charts displayed on television or published in the newspapers that highs and lows move in general form west to east, especially in more southern latitudes. Bad weather is often associated with the lows, though moist onshore winds can cause rain in coastal areas even if the pressure is high. In other words, the actual reading of the barometer does not give unmistakable information concerning the weather to come.

Your barometer will show whether pressure is rising or falling, that is, whether a high or low pressure system is approaching, or perhaps developing, in intensity.

A word of caution! Owing to a daily atmospheric tide effect, the pressure will normally fall by about one-tenth to one-twentieth of an inch between 9 am and 3 pm and will rise by nearly a similar amount between 3 pm and 9 pm, even if weather systems are stationary. A smaller rise and fall occurs during the night and early morning. These daily ("diurnal") changes must be allowed for before you can really say whether the "glass" is rising or falling due to weather systems. The best way to avoid difficulty is to observe changes over 24 hour periods, using your movable pointer or set hand. In other words, check your barometer at the same time each day.

Weather processes in general are so complex and varied, that even the professional forecaster cannot expect to be completely accurate on all occasions. This, despite the fact that observations of pressure, temperature, humidity, winds clouds and weather are taken over a wide network several times a day in the upper atmosphere as well as at the earth's surface. Local topography further complicates matters.

Pressure Conversion Chart

Temperature Conversion Chart