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The barometer is an instrument used in meteorology to measure atmospheric pressure; It is one of the essential instruments in weather stations. Variations in atmospheric pressure are associated with weather conditions, so their measurement makes it possible to predict the weather. Other parameters that help predict the weather are temperature, humidity, and the direction and speed of the winds.
How does a barometer work to measure atmospheric pressure?
But what is atmospheric pressure ? The atmospheric pressure in a certain place on Earth is the weight of the air column of the atmosphere on a certain surface; that is, it is the weight of all the molecules that make up the air in the atmosphere that are on a certain surface. The atmospheric pressure measured with a barometer is also often called barometric pressure.
Although there is a wide variety of barometers that measure atmospheric pressure, basically two types are used in meteorology: the mercury barometer and the aneroid barometer.
the mercury barometer
The classical mercury barometer, also called a Torricelli tube, consists of a 100-centimeter long glass tube with one end open and the other sealed. This tube is filled with mercury and placed upside down in an open mercury trough. The weight of the column of mercury offsets the pressure of the atmosphere at the open end, so the height of the column will be a measurement of atmospheric pressure. A reference value of atmospheric pressure, measured at sea level at a temperature of 15°C, corresponds to a height of the mercury column of 760 millimeters (760 mmHg).
the aneroid barometer
The aneroid barometer consists of a small, flexible, sealed metal case. Small changes in air pressure outside the box cause the walls of the box to deform; this is due to the fact that the pressure inside the box does not change. The deformation of the walls of the box activates a mechanical device that translates into the movement of a needle on a graduated scale. In this way, the movement of the needle represents the variations of atmospheric pressure.
Mercury barometers are large in volume, brittle, and contain a toxic heavy metal, so they are used less frequently. Meanwhile, aneroid barometers are accurate and compact than mercury barometers, and are generally more suitable for many uses.
In what units is the pressure of a barometer expressed?
Atmospheres, millibars, millimeters of mercury, and pascals are the most common units used to measure barometric pressure. Read more about it below.
- Millimeters of mercury (mmHg) are the most common units associated with the operation of mercury barometers. They record the height of the mercury column in millimeters.
- Millibars (mb) are the units of atmospheric pressure used by meteorologists. It is one thousandth of a bar, which is a unit that measures the pressure of gases.
- Pascals (Pa) are the units of pressure in the SI international system; 1 mb is equivalent to 100 Pa.
- Atmospheres (Atm) are the atmospheric pressure at sea level at a temperature of 15°C .
The following equivalences allow to convert the values of atmospheric pressure between the different units:
1 Atm = 760 mm Hg = 101325 Pa = 1013.25 mb
The barometer and weather prediction
The reading of the atmospheric pressure in a barometer usually gives information about the weather situation, particularly about changes that may occur or if it will remain in the same condition. Let’s look at some examples on how to interpret barometric readings.
The increase in barometric pressure is associated with a dry, cool and pleasant climate; In general, the increase in barometric pressure leads to better weather. Conversely, when the barometric pressure drops, the weather worsens. If the descent is sudden there is a chance of a storm. When the barometric pressure remains constant there will be no change in the weather.
high atmospheric pressure
A situation of high atmospheric pressure can be considered when the barometric pressure values are above 767 mmHg, equivalent to 102268.9 Pa or 1022.689 mb. Pressure variations from high values are associated with the continuity of good weather conditions if the pressure does not change or if it drops slowly; if the descent is rapid, the weather will be cloudy and warmer.
normal atmospheric pressure
Normal atmospheric pressure values are in the range between 757 and 767 mmHg, which is equivalent to 100914.4 to 102268.9 Pa or 1009.144 to 1022.689 mb. The continuity of these barometric pressure values or the increase from these values is associated with the continuity of weather conditions. A slow descent means little change; while, if the descent is rapid, it is likely that it will rain or snow if the temperature is low enough.
low atmospheric pressure
Low atmospheric pressure corresponds to barometric pressure values less than 757 mmHg, equivalent to 100914.4 or 1009.144 mb. If the pressure increases from these values or remains constant, the weather will be clear and cooler. If the pressure drops slowly it is possible that it will rain, but if the drop is fast a storm is coming.
isobaric curves
Weather maps, which indicate the distribution of weather conditions in a region, often have isobar curves drawn. Isobaric curves are lines joining places of equal pressure; for example, an isobar will be the line that joins all the points that have a barometric pressure of 768 mmHg, and below the isobar corresponding to 770 mmHg can be drawn. All sites that lie between the two isobars will have pressures between these two values. The barometric pressure distribution helps predict changes in the climate of that region.
Sources
- Dominguez, Hector. Our atmosphere: how to understand climate changes . First edition. LD Books, 2004. ISBN 9707320524.
- Proper use of the barometer . Time.com. Available at https://www.tiempo.com/ram/150/adecuado-uso-del-barometro/ .
