Know what that is the atmospheric pressure? Well, it is the power per area of determined extension that the air that forms the atmosphere on the plane of the earth performs. Its value at sea level is 101.325 Pa If you want to know more about the atmospheric pressure and its characteristics, we invite you to read this article.
Atmospheric pressure
As we have indicated before, the atmospheric pressure It is a pressure or push that the air that is in the atmosphere exerts on the terrestrial extension and is measured by means of an instrument called an aneroid barometer. In any of these instruments it can be seen that the greatest amount of pressure that is marked on them will coincide with the English word fair.
With this term fair several expressions are designated that mean stable weather or without changes, anticyclonic, good weather or without clouds. But the existence of less than fair pressure means that there will be harsh climate changes, with relatively strong, cyclonic winds, which may be accompanied by probably very intense rains.
The intermediate zone of an aneroid barometer is the one that will be marked when there is an intermediate atmospheric pressure, and that zone of the meter is called the zone of change or change, in English, and in it a modification of the meteorological conditions can be indicated, of the Types of weather rainy or windy, or from cloudy weather to good weather or vice versa.
La atmospheric pressure at one point, it will coincide in number with the weight that a column without movement of air will have in a straight portion that will propagate from any point on the earth to the final upper demarcation of the atmosphere.
Because the density of air decreases with increasing height, weight cannot be calculated unless the difference in air density versus altitude or pressure can be determined. . Because of this, it is not easy to calculate the exact atmospheric pressure over a specific point on the earth's surface.
Another point to keep in mind is that both temperature and air pressure change constantly, in measurements of time and space, which makes the calculation more difficult. If it is possible to take a measurement of the atmospheric pressure at a certain point, but not many conclusions can be drawn from it.
But if several measurements are made, a modification of the pressure can be observed throughout the time periods in which they were made and thus useful data can be obtained, which can be combined with other meteorological reports, such as the Temperature and Humidity, the wind or the atmospheric temperature, which allows calculating averages that will give us a very close idea of the atmospheric weather of the place where the measurement was made, allowing short-term forecasts to be made.
Variations of the Atmospheric pressure
The atmospheric pressure at a pre-established point observes variations that are linked to meteorological modifications. On the other hand, it must be taken into account that at an established point, the atmospheric pressure decreases with height, as we have indicated before. Atmospheric pressure decreases at a rate of 1 mmHg or Torr for every 10 meters of altitude in areas that are close to sea level.
What is usually done is to use some tools, which are called altimeters, which are simple aneroid barometers that have been calibrated in height, but these tools are not very precise.
Atmospheric pressure also changes with latitude. In fact, lower atmospheric pressure is experienced if the point is at sea level at an equatorial latitude. The reason for this is the bulge of the equator on Earth. The lithosphere is exaggerated at the planet's equator, but the hydrosphere is even more bulging, so that the coastal areas of the equatorial expanse are a few kilometers further from the center of the Earth than in temperate areas and, in particular, in the polar areas.
And, because of a lower density, the atmosphere will be much wider in the equatorial zone of the earth than the hydrosphere, therefore its thickness is wider than it is in the temperate and polar areas. Thus, the equatorial area is the area in which low atmospheric pressures continuously predominate due to the dynamics caused by the Earth's rotation.
Also for this reason, the atmospheric temperature is lower in temperate zones, at a rate of one degree for every 154 meters of altitude on average, while in the zone between the tropics this rate applies every 180 meters of altitude.
The normal atmospheric pressure translates into 1 atmosphere, it has been conceptualized as the atmospheric pressure calculated at sea level, which was specifically adopted in 101 325 Pa or 760 Torr. However, in 1982, the IUPAC suggested that, in In case the physical characteristics of an element are to be obtained, the normalized pressure must be defined exactly as 100 kPa or ≈750,062 Torr.
Taking into account that it is an integer, the suggested change brings with it another practical advantage, since 100 kPa equates to a height of about 112 meters, which is somewhat close to the average 194 meters at which most people live. world population.
How is atmospheric pressure measured??
Atmospheric pressure is measured, as we have already indicated, by means of a tool called a barometer, which was invented in 1643 by the physicist and mathematician Evangelista Torricelli.
Most common types of Barometers
The most commonly used barometers are the following:
- The mercury barometer. It is a glass tube 850 mm high, obstructed in its upper section and open in its lower section. This tube must be filled with mercury and is placed on top of an open container that is also filled with mercury. If we are at the same level as the sea, the mercury mark inside the tube should drop to a height of about 760 mm, leaving an empty space in its upper section.
- The aneroid barometer does not use mercury and is the one used in navigation. Its structure is a metal box, called a vidi capsule, in which a vacuum has been partially created. The function of the box is to contract or decrease according to the pressure exerted on it, communicating its movements to a needle, which is the one that will indicate the measurement of atmospheric pressure on a graduated surface.
The precision with which a barometer measures is not what really matters in order to arrive at a prediction of the Weather Elements, what matters is the oscillation of the pressure that is observed during the passage of time. To calculate this oscillation of pressure in relation to time, a device called a barograph is used. The barograph can measure pressure and also shows fluctuations by displaying a graph over a period of time.
Atmospheric Pressure Units and equivalences
The units of atmospheric pressure and their equivalent measurements are as follows:
- Baria: is the pressure exerted by the force of a DINA per square centimeter.
- Bar: one bar is the equivalent of 1.000.000 bars.
- millibars: is a unit of pressure equal to one thousandth of a bar and equal to 1000 bar.
- hectopascal: A hectopascal has exactly the same value as a millibar, and the two units are often used interchangeably.
By convention of experts in the field, the expression of 760 mm was adopted as the measure of normal pressure, considering that this measure is taken at sea level, with a temperature of 0°C and a latitude of 45°. Then the expressed formula would be as follows: 760 mm = 1.013,2 mb = 1013,2 hPa = 1 atmosphere
Air movement and wind formation
This relationship will depend on the type of pressure that exists in the area in question:
- Low pressure zone: Because of sunlight, the earth's surface heats up and, by transfer, heats the air present. When air heats up, it spreads and, by reason of spread, has a smaller measure of cold air. The effect of this is that the hot air rises and creates a low pressure zone. At the same time, the warm air rising from the ground begins to cool as it rises in height. But that cooling air becomes dense and descends, creating a high pressure area.
- High: In a zone of high pressure, which is also called an anticyclone, the air masses descend extensively. As it descends, the air is heated by being close to the earth's surface. This heating has the effect that there can be no condensation and, consequently, no clouds are produced. Near the ground, air rushes out from the high in the direction of the depression.
The difference in pressure between an area of high pressure and an area of low pressure creates movement. In low pressure areas, when air rises, a vacuum is created that is filled by air coming down from the high pressure area and looking for a place to go. This movement causes air currents that always move from high pressure areas to low pressure areas and wind is formed.
The Isobars
Continuously, different pressure measurements are made simultaneously in different places on earth and with these measurements lines are drawn that join points that have the same measurements or pressure values. These lines are called isobar lines. For this reason, isobars are lines that join points of the same pressure at a certain moment.
When isobars are drawn and analyzed, it can be seen that there are areas of high and low pressure. These schemes or pressure systems are directly linked to the weather on the surface of the earth. Typically, high pressures produce pleasant weather and low pressures are associated with unstable weather and, occasionally, rain.
In an isobar chart, the smaller the circle of a low pressure area, the lower the pressure. Thus, as the isobars move further from the center, the air pressure will be greater. In a high pressure area, it will be the other way around. The higher the pressure at the center of the anticyclone, the further away from it the pressure will decrease.
- Storm: Low pressure areas are represented by the letter B on a map or diagram of isobars. They are also called a storm, depression or extra tropical cyclone. In low pressure areas, the pressure value decreases as it approaches the center.
In that area, the wind will turn to the left, counterclockwise, if we are in the northern hemisphere, with an angle that will be between about 25° to 35° from the isobar line towards the heart of depression If we are in the southern hemisphere it will turn to the right but with the same angle. A curious fact is that storms usually move from west to east.
- Anticyclone: High pressure areas are also called anticyclones and are designated by the letter A. In an anticyclone the pressure is greater than 1013 mb and increases as it gets closer to its center. The wind moves, in the northern hemisphere, counterclockwise and in the southern hemisphere, in the same direction as clockwise, always with a twist angle that is between 25° and 35° from the isobar line outward from the center of the anticyclone.
The anticyclones tend to remain stationary and act as shields to the passage of the fronts. An example of this is the anticyclone system of the Azores Islands, which remains immovable in the summer season, giving rise to summer sunshine in Spain and with little rain, which normally occurs more frequently in the north of the Iberian Peninsula. .
History of Atmospheric pressure
In ancient times, they had no idea that air had a weight. They thought that it was a body that, by its nature, had a tendency to rise, explaining this rise of the liquids in the pumps by the horror vacui, which means horror of the void, which is a tendency of nature.
At the time when some Italian gardeners insisted on wanting to raise water by sucking with a propeller pump, they realized that the height of 10,33 m could not be exceeded, which with the English measurement scale is about 34 feet. .
When consulting with Galileo Galilei, it was determined that the horror of nature in a vacuum was limited by a force equivalent to the weight of 10,33 m of water, which is the equivalent of 1 atm of pressure, and the name at that altitude of altezza limitatíssima.
As early as 1643, Torricelli took a meter-long glass tube and filled it with mercury. He held the clogged tube with one finger, turned it upside down, and lowered it into a vessel filled with mercury. When he removed his finger he was able to verify that the mercury descended until a column was formed whose height was 13,6 times smaller than that observed when carrying out the same experiment with water.
Since Torricelli knew that mercury is 13,6 times heavier than water, he was able to deduce that both columns of liquid were supporting the same weight, concluding that only air could induce that force.
After Torricelli's death, Pascal heard about his experiments through Father Mersenne, who exposed them in a publication. Although in principle he accepted the idea of the horror of the emptiness of nature, he did not take long with the Contributions by Blaise Pascal to change his mind upon realizing the results of the experiments he performed.
Using a curved tube and using it in such a way that the atmosphere could not influence the liquid, he was able to observe that the columns reached the same level. But, as soon as the action of the atmosphere was accessed at one of the extremes, the level changed.
These results led him to carry out a definitive experiment, which consisted of taking a barometer to different heights and checking if it was really the weight of the air that influenced the rise of the liquid in the tube. He then wrote to his brother-in-law Perier expressing a hypothesis according to which if the level of mercury is lower at the top of a mountain than below it, gravity and air pressure must be the cause of this movement.
In 1648, Perier, following the hypothesis of his brother-in-law, was encouraged to carry out the experiment, climbing to the summit of the Puy-de-Dôme. When comparing the measurements made at the top, at about 1000 meters, and at the base, the latter taken by Father Chastin, they found a difference of three and a half lines between the two. Thus, the theory of the horror of nature's emptiness was definitively discarded and it was found that the air had weight.
Certainly Pascal, Perier and Chastin are responsible for carrying out the experiment, but it was Descartes who, in a letter he wrote in 1638, twelve years before Torricelli's experiment, had already argued that air had weight, and compared it to a large wool quilt that covers the earth above the clouds and the weight of that wool should press on the mercury, preventing the mercurial column from descending.
However, the idea of atmospheric pressure it did not begin to spread until the demonstration of the year 1654, by the burgomaster and inventor Otto von Guericke.