Normal atmospheric pressure

Updated July 22, 2021 Author: Dmitry Ivanetscu
Hello, dear readers of the KtoNaNovenkogo.ru blog. In the weather forecast, among the well-known indicators (temperature, precipitation, humidity, wind speed, etc.), atmospheric pressure is also present.

It is expressed not in our usual units of measurement (atmosphere, bar, pascal), but in millimeters of mercury.

We will tell you below where this pressure comes from, how it is measured, what kind of “mercury column” is and how it affects human health.

Physical entity

Celestial bodies, including the Earth, attract the matter surrounding them, including the gas shell - the atmosphere (if there is one). The force of attraction (also called the gravitational field) depends on the mass of the body. The Earth is 80 times heavier than the Moon, so the Moon is a satellite of the Earth, and not vice versa.

The air attracted by the Earth presses on its surface, thus creating atmospheric pressure.

At sea level, atmospheric air pressure is slightly more than 1 kg/cm2. It is estimated that an air column weighing 13-15 tons , and about 150 kg on the palm.

But we do not feel this, since the pressure of the external environment is balanced by the pressure created by the air in the tissues of our body.

Stability or anticyclone

When the air is colder and sinks, the barometric pressure increases because there is more air at the surface and therefore exerts more force. This causes atmospheric stability or also called an anticyclone. Anticyclone situation It is characterized by the fact that it is a zone of calm without winds, since the coldest and heaviest air slowly descends in a circle. The air rotates almost imperceptibly clockwise in the northern hemisphere and counterclockwise in the southern hemisphere.

Anticyclone on the atmospheric pressure map

The standard is considered to be atmospheric pressure at sea level at a latitude of 45 degrees at a temperature of 0 degrees Celsius. It is 760 mmHg and is called normal.

If the deviation is higher, the pressure will be increased, if it is lower, the pressure will be decreased. If we use the international system of measurement (SI) accepted in physics, then normal pressure will correspond to a value of 101,325 pascals (Pa) .

Where absolute accuracy is not needed, the concept of standard pressure is used, i.e. 760 mmHg Art. = 100,000 Pa.

Geography

Why is atmospheric pressure low along the equator and high above the poles?
Near the equator, the air becomes very hot, expands and rises. Therefore, low pressure is formed. Around the poles, the air is heavy due to low temperatures. It goes down and the pressure gets high

What determines the level of atmospheric pressure?

unevenly across regions of the globe , as it depends on a number of interrelated factors.

Height – the higher the measurement point is above sea level, the less pressure it experiences from the atmosphere.
This is explained by a decrease in the weight of the air column: as you gain height, it becomes shorter, and the air in it becomes rarefied, and therefore lighter.
At an altitude of 5 km, atmospheric pressure is only half of normal, at 15 km it is 8 times less, at 20 km it is 18 times less.
Read also: Diagnosis and treatment of polycythemia in newborns
Temperature – as air heats up, the density of air decreases, and as it cools, it increases. During the day, the temperature changes several times, and with it the pressure:
in the morning and evening it increases, and in the afternoon and midnight it decreases.
In cold weather, the air is colder and denser than in hot weather, and therefore the pressure in winter will be greater than in summer.
The temperature at the surface of the earth can fluctuate between -600 C - +600 C, and the pressure can “jump” from 640 to 790 Hg.
Movement of continental and maritime air masses (monsoons, trade winds, typhoons, tsunamis, etc.), leading to the formation of warm and cold air fronts, as well as zones with low or high pressure (cyclones and anticyclones).
Geographical location - on Earth there are belts characterized by consistently high or low atmospheric pressure.
In the equatorial zone, due to high temperatures, the pressure is always low, and in the cold polar latitudes it is high. In the tropics, pressure is higher than in temperate zones.

Atmospheric pressure is subject to seasonal fluctuations, but this does not change the overall picture. Pressure indicators are generally stable, and areas of high and low pressure remain unchanged.

Pressure at altitude

You already know that the upper layers of the atmosphere exert pressure on the lower ones. This means that the air at the surface of the Earth is maximally compressed. However, the higher we rise above the Earth, the fewer layers of air become that compress the lower layers, and accordingly, the pressure decreases. It is precisely these pressure drops that we immediately feel on ourselves.

Remember: the higher the altitude, the lower the atmospheric pressure.

Instrument for measuring atmospheric pressure

Atmospheric pressure is measured by instruments called barometers . There are two types of them: mercury and metal (aneroids).

The first was invented in the 17th century by the Italian E. Toricelli. Its main element is a glass tube filled with mercury , the upper end of which is sealed, and the lower (open) end is immersed in a cup of mercury. The weight of the mercury column in the tube is balanced by the atmospheric air pressure acting on the surface of the cup.

If you install a graduated scale next to the tube, you can track pressure fluctuations by changing the height of the mercury column. Mercury barometers are used at weather stations, as they give very accurate readings .

In addition, a recorder can be connected to this device, which will record pressure fluctuations over a certain period and record them on paper tape.

The aneroid barometer appeared later, in the middle of the 19th century. It was designed by the French engineer L. Vidi. The principle of operation of the device is quite simple: a membrane metal box from which air has been pumped out (i.e. there is a vacuum inside it) reacts sensitively to changes in pressure (as it increases, the box compresses, and when it decreases, it decompresses).

The movement of the membrane is transformed by a lever mechanism into a circular movement of the pointer along the dial, calibrated according to the readings of the mercury barometer. This device gives some error , but in most cases it is quite acceptable.

Cold fronts

A series of cold fronts are those in which a mass of cold air replaces hot air . They are strong and can cause atmospheric disturbances such as thunderstorms, rainstorms, tornadoes, high winds and short snowstorms before the passage of a cold front, followed by dry conditions as the front advances. Depending on the time of year and its geographic location, cold fronts can last from 5 to 7 days in a row.

Cold wind

Standard atmospheric pressure for St. Petersburg, Moscow

For a person, comfortable if it is in the range of 750-765 mm Hg, that is, in the normal area.

However, keep in mind that this is not a strict framework. People adapt over time to local geographic and climatic conditions, so a lower rate may be the norm for residents of mountainous areas.

On meteorological maps of Russia, isobar lines are drawn that conditionally divide the territory into zones with approximately equal atmospheric pressure.

The table below shows, as an example, data on normal atmospheric pressure and its permissible deviation from the norm for some cities.

City	Average annual pressure, mm Hg.	Permissible maximum, mm Hg.
Moscow	747-748	755
Saint Petersburg	753-755	762
Rostov-on-Don	740-741	748
Ekaterinburg	735-741	755

As you can see, the highest average annual pressure is in St. Petersburg. This is understandable: the central part of the city is located at only 1-5 m above sea level.

Meteor dependence what to do?

The movement of mercury by more than one division in 3 hours is a reason for stress in the strong body of a healthy person. Each of us feels such fluctuations in the form of headaches, drowsiness, and fatigue. More than a third of people suffer from weather dependence to varying degrees of severity. In the zone of high sensitivity are populations with diseases of the cardiovascular, nervous and respiratory systems, and elderly people. How to help yourself if a dangerous cyclone is approaching?

Notes[edit | edit code]

Sources[edit | edit code]

Footnotes[edit | edit code]

↑ The formula assumes the temperature is the same at all altitudes. In fact, the temperature of the atmosphere changes with altitude according to a rather complex law. Nevertheless, the formula gives good results, and it can be used at altitudes of up to 50-100 kilometers. Thus, it is not difficult to determine that at the altitude of Elbrus - about 5.6 km - the pressure will drop by approximately half, and at an altitude of 22 km (the record height for the rise of a stratospheric balloon with people), the pressure will drop to 50 mm Hg. Art.

Pressure stage[edit | edit code]

The height to which one must rise or fall for the pressure to change by 1 hPa (hectopascal) is called the “pressure level”. The pressure stage is convenient to use when solving problems that do not require high accuracy, for example, to estimate pressure from a known height difference. Assuming that the atmosphere does not experience significant vertical acceleration (that is, it is in a quasi-static state), from the basic law of statics we obtain that the pressure level is equal to:

History[edit | edit code]

Traditionally, it was believed that suction pumps work because “nature is afraid of a vacuum.” But the Dutchman Isaac Beckman, in the theses of his doctoral dissertation, defended in 1618, argued: “Water raised by suction is not attracted by the force of emptiness, but is driven into an empty place by overlapping air” (Aqua suctu sublata non attrahitur vi vacui, sed ab aere incumbentein locum vacuum impellitur).

In 1630, the Genoese physicist Baliani wrote a letter to Galileo about an unsuccessful attempt to construct a siphon to lift water up a hill approximately 21 meters high. In another letter to Galileo (dated October 24, 1630), Baliani suggested that the rise of water in the pipe was due to air pressure.

The presence of atmospheric pressure led people to confusion in 1638, when the Duke of Tuscany's idea to decorate the gardens of Florence with fountains failed - the water did not rise above 10.3 meters. The search for the reasons for this and experiments with a heavier substance - mercury, undertaken by Evangelista Torricelli, led to the fact that in 1643 he proved that air has weight [5]. Together with V. Viviani, Torricelli conducted the first experiment in measuring atmospheric pressure, inventing the first mercury barometer - a glass tube in which there is no air. In such a tube, mercury rises to a height of about 760 mm.