It is one of the basic sensors included in weather stations. While an array of barometer types exist, two main types are used in meteorology: the mercury barometer and the aneroid barometer. The classic mercury barometer is designed as a glass tube about 3 feet high with one end open and the other end sealed. The tube is filled with mercury.
This glass tube sits upside down in a container, called the reservoir, which also contains mercury. The mercury level in the glass tube falls, creating a vacuum at the top. The first barometer of this type was devised by Italian physicist and mathematician Evangelista Torricelli in The barometer works by balancing the weight of mercury in the glass tube against the atmospheric pressure, much like a set of scales.
Atmospheric pressure is basically the weight of air in the atmosphere above the reservoir, so the level of mercury continues to change until the weight of mercury in the glass tube is exactly equal to the weight of air above the reservoir. Once the two have stopped moving and are balanced, the pressure is recorded by "reading" the value at the mercury's height in the vertical column.
If the weight of mercury is less than the atmospheric pressure, the mercury level in the glass tube rises high pressure. In areas of high pressure, air is sinking toward the surface of the earth more quickly than it can flow out to surrounding areas. Since the number of air molecules above the surface increases, there are more molecules to exert a force on that surface. With an increased weight of air above the reservoir, the mercury level rises to a higher level.
If the weight of mercury is more than the atmospheric pressure, the mercury level falls low pressure. In areas of low pressure , air is rising away from the surface of the earth more quickly than it can be replaced by air flowing in from surrounding areas.
Since the number of air molecules above the area decreases, there are fewer molecules to exert a force on that surface. With a reduced weight of air above the reservoir, the mercury level drops to a lower level. We've already explored how mercury barometers work. One "con" of using them, however, is that they're not the safest things after all, mercury is a highly poisonous liquid metal. For example, if the barometer reads hectopascal at the sea coast and you drive along a mountain road metres high, the pointer should move down the scale to about hectopascals.
These changes, when they are considered together with wind, temperature, moisture and cloud indications, can be a great help in forecasting approaching weather.
About , an Italian mathematician named 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 in the container forced it up the tube, and the measurement of the various lengths of the column of liquid was, therefore, a means of expressing the changes in air pressure.
In order to have a tube of manageable length, the heaviest of all liquids, mercury, was later used. Today we have finely constructed mercurial barometers capable of giving very accurate readings.
They are costly and they need special care in handling. 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. The increased air pressure causes the sides of the cell or cells to come closer together.
One side is fixed to the base of the instrument while the other is connected by means of a system of levers and pulleys to a rotating pointer that moves over a scale on the face of the instrument.
This pointer is usually black. The aneroid barometer below consists of a closed sealed capsule with flexible sides. Any change in pressure alters the thickness of the capsule. When the atmospheric pressure increases, the cell gets compressed and the inward movement of the cell wall is transmitted to the pointer mechanically and it then registers a higher reading on the scale.
Levers magnify these changes, causing a pointer to move on a dial, or numbers to change on a digital read-out device.
To repeat what was said at the beginning, a barometer is an instrument which measures air pressure. The pressure may well never fall to the values shown for Stormy or Rain for most places within Australia. Twitter Facebook Pinterest Google Classroom. Encyclopedic Entry Vocabulary. A barometer is a scientific instrument used to measure atmospheric pressure , also called barometric pressure.
The atmosphere is the layers of air wrapped around the Earth. That air has a weight and presses against everything it touches as gravity pulls it to Earth.
Barometers measure this pressure. Atmospheric pressure is an indicator of weather. Changes in the atmosphere, including changes in air pressure, affect the weather. Meteorologist s use barometers to predict short-term changes in the weather. A rapid drop in atmospheric pressure means that a low-pressure system is arriving. Low-pressure systems are associated with cloud y, rain y, or wind y weather.
A rapid increase in atmospheric pressure pushes that cloudy and rainy weather out, clearing the skies and bringing in cool, dry air. A barometer measures atmospheric pressure in units of measurement called atmospheres or bars. An atmosphere atm is a unit of measurement equal to the average air pressure at sea level at a temperature of 15 degrees Celsius 59 degrees Fahrenheit. The number of atmospheres drops as altitude increases because the density of air is lower and exert s less pressure.
As altitude decrease s, the density of air increases, as does the number of atmospheres. Barometers have to be adjust ed for changes in altitude in order to make accurate atmospheric pressure readings.
The mercury barometer is the oldest type of barometer, invent ed by the Italian physicist Evangelista Torricelli in Torricelli conduct ed his first barometric experiments using a tube of water. Water is relatively light in weight, so a very tall tube with a large amount of water had to be used in order to compensate for the heavier weight of atmospheric pressure.
In order to keep his experiments more secretive, Torricelli deduce d that he could create a much smaller barometer using mercury , a silvery liquid that weighs 14 times as much as water. A mercury barometer has a glass tube that is closed at the top and open at the bottom. At the bottom of the tube is a pool of mercury.
The mercury sits in a circular, shallow dish surrounding the tube. The mercury in the tube will adjust itself to match the atmospheric pressure above the dish. As the pressure increases, it forces the mercury up the tube. The tube is marked with a series of measurements that track the number of atmospheres or bars. Observer s can tell what the air pressure is by looking at where the mercury stops in the barometer. In , the French scientist Lucien Vidi invented the aneroid barometer.
An aneroid barometer has a sealed metal chamber that expand s and contract s, depending on the atmospheric pressure around it. Mechanical tools measure how much the chamber expands or contracts. These measurements are align ed with atmospheres or bars.
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