Barometers are used in weather forecasting and it uses a mechanical tool to track the changes in atmospheric pressure over a period of time. It is an important tool used in today’s smartphones. It uses atmospheric pressure data to make accurate elevation readings. It ranges from 51 to 68 cm and scale is between 25" to 32". The barometric pressure ranges from 954 to 1073 mBar.

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Detailed Description for Barometer

Barometer, device used to measure atmospheric pressure. Because atmospheric pressure changes with distance above or below sea level, a barometer can also be used to measure altitude. There are two main types of barometers: mercury and aneroid.

In the mercury barometer, atmospheric pressure balances a column of mercury, the height of which can be precisely measured. To increase their accuracy, mercury barometers are often corrected for ambient temperature and the local value of gravity. Common pressure units include pounds per square inch; dynes per square centimetre; newtons per square metre (the SI unit called the pascal); inches, centimetres, or millimetres of mercury; and millibars (1 millibar equals 1,000 dynes per square centimetre, 100 pascals, or 0.75 millimetre of mercury). Atmospheric pressure at sea level is about 14.7 pounds per square inch, equivalent to 30 inches (760 millimetres) of mercury, 1,013.2 millibars, or 101,320 pascals.



Water-based barometers

Goethe's device

The concept that decreasing atmospheric pressure predicts stormy weather, postulated by Lucien Vidi, provides the theoretical basis for a weather prediction device called a "storm glass" or a "Goethe barometer" (named for Johann Wolfgang Von Goethe, the renowned German writer and polymathwho developed a simple but effective weather ball barometer using the principles developed by Torricelli). The French name, le baromètre Liègeois, is used by some English speakers.[9] This name reflects the origins of many early weather glasses - the glass blowers of LiègeBelgium.[9]

Mercury barometers

mercury barometer has a glass tube with a height of at least 84 cm, closed at one end, with an open mercury-filled reservoir at the base. The weight of the mercury creates a vacuum in the top of the tube known as Torricellian vacuum. Mercury in the tube adjusts until the weight of the mercury column balances the atmospheric force exerted on the reservoir. High atmospheric pressure places more force on the reservoir, forcing mercury higher in the column. Low pressure allows the mercury to drop to a lower level in the column by lowering the force placed on the reservoir. Since higher temperature levels around the instrument will reduce the density of the mercury, the scale for reading the height of the mercury is adjusted to compensate for this effect.

Schematic drawing of a simple mercury barometer with verticalmercury column and reservoir at base

Torricelli documented that the height of the mercury in a barometer changed slightly each day and concluded that this was due to the changing pressure in the atmosphere.[1] He wrote: "We live submerged at the bottom of an ocean of elementary air, which is known by incontestable experiments to have weight".

The mercury barometer's design gives rise to the expression of atmospheric pressure in inches or millimeters or feet (torr): the pressure is quoted as the level of the mercury's height in the vertical column. Typically, atmospheric pressure is measured between 26.5 and 31.5 inches of Hg. One atmosphere (1 atm) is equivalent to 29.92 inches of mercury.

Design changes to make the instrument more sensitive, simpler to read, and easier to transport resulted in variations such as the basin, siphon, wheel, cistern, Fortin, multiple folded, stereometric, and balance barometers. Fitzroy barometers combine the standard mercury barometer with a thermometer, as well as a guide of how to interpret pressure changes. Fortin barometers use a variable displacement mercury cistern, usually constructed with a thumbscrew pressing on a leather diaphragm bottom. This compensates for displacement of mercury in the column with varying pressure. To use a Fortin barometer, the level of mercury is set to the zero level before the pressure is read on the column. Some models also employ a valve for closing the cistern, enabling the mercury column to be forced to the top of the column for transport. This prevents water-hammer damage to the column in transit.

On June 5, 2007, a European Union directive was enacted to restrict the sale of mercury, thus effectively ending the production of new mercury barometers in Europe.

Vacuum pump oil barometer

Using vacuum pump oil as the working fluid in a barometer has led to the creation of the new "World's Tallest Barometer" in February 2013. The barometer at Portland State University (PSU) uses doubly distilled vacuum pump oil and has a nominal height of ~12.4 m for the oil column height; expected excursions are in the range of ±0.4 m over the course of a year. Vacuum pump oil has very low vapor pressure and it is available in a range of densities; the lowest density vacuum oil was chosen for the PSU barometer to maximize the oil column height.[13]



 Barometric pressure and the pressure tendency have been used in weather forecasting since the late 19th century.When used in combination with wind observations, reasonably accurate short-term forecasts can be made. Simultaneous barometric readings from across a network of weather stations allow maps of air pressure to be produced, which were the first form of the modern weather map when created in the 19th century. Isobars, lines of equal pressure, when drawn on such a map, give a contour map showing areas of high and low pressure.Localized high atmospheric pressure acts as a barrier to approaching weather systems, diverting their course. Atmospheric lift caused by low-level wind convergence into the surface low brings clouds and sometimes precipitation.The larger the change in pressure, especially if more than 3.5 hPa, the greater the change in weather that can be expected. If the pressure drop is rapid, a low pressure system is approaching, and there is a greater chance of rain. Rapid pressure rises, such as in the wake of a cold front, are associated with improving weather conditions, such as clearing skies.


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