Atmospheric pressure is composed of air covered with a thick layeratmosphereObjects in the atmosphere are subject to air moleculesto hitPressure generated.
It can also be considered that atmospheric pressure is the pressure exerted on objects in the atmosphere by the gravity of the atmosphere itself.
Aristotle's principle that there is no vacuum in nature has been used for centuries to illustrate the fact that if you want to empty a barrel of wine, you must open at the top and bottom at the same time.Because nature does not allow vacuum to be generated in the barrel, unless air enters the barrel from the top, the wine will not flow out from the bottom.For a long time, people have believed that this principle holds true in all occasions, including vacuum.However, in the 16th century, people found that if the vacuum pump used to pump water from the mine was placed 30 feet above the water level, the pump would not work, which led people to start thinking about whether the principle that there is no vacuum in the nature of the wine barrel has some restrictions on the pump.
In 1630, Giovanni Baliani of Genoa discovered a similar limitation on the siphon.When he tried to draw water from a reservoir more than 60 feet high, he found that the siphon could not work.When the siphon is fully filled with water, remove the plugs from both ends, as if a vacuum is created at the top of the siphon.
Around 1641, Gaspar Berti of Rome, after learning about these discoveries, tried to determine whether a vacuum could be created in a more scientific way.To this end, he designed a device consisting of a sphere with a tube about 40 feet longGlass containerComposition.Bertie attached the device vertically to the side of the tower. After closing the valve at the end of the pipe at the lower position, he injected a large amount of water from the opening at the top of the glass container at the higher position.When the tube and glass container are completely filled, seal the opening at the top of the container, and open the valve at the end. Water immediately gushes out from the bottom of the tube, forming a vacuum in the glass container.This experiment has made it clear for many years that if there is no vacuum in nature, how can it be allowed to create a vacuum in a glass container?Moreover, when the test is repeated, why does the water always drop to the same position in the pipe?
Being Galileo's assistantEvangelista TorricelliIn Florence, I learned the results of Bertie's experiment.Galileo believed that the power of vacuum supported the water in the pipe. After Galileo's death in 1642, Torricelli put forward his hypothesis that the atmospheric pressure supported the water in the pipe.He reasoned that we live at the bottom of the atmosphere and ocean, and the air pressure supports the water at the bottom of the pipe to reach a certain height.He believes that when the weight of the remaining water in the pipe is equal to the weight of the upper air pushed downward, the balance point is reached.From this hypothesis, Torricelli drew several inferences: one is that atmospheric pressure can support a column of mercury about 29 inches high in a sealed top tube.This conclusion comes from the fact that atmospheric pressure can support a 33 foot high water column, and the density of mercury is 13.6 times that of water.Thus, 33/13.6 × 12 inches=29 inches.The second corollary is that such a tube filled with mercury can be used to measure changes in atmospheric pressure.It was this inference that finally earned Torricelli the honor of proposing the barometer theory.Finally, Torricelli concluded that if such a device was placed in a place with thin air, such as a mountain top, the height of the mercury column would drop.
In 1647, French philosopherBlaise PascalUndertake the verification of the third inference of Torricelli.After learning the news of Torricelli's barometer test, Pascal made a barometer himself, and he firmly believed in the correctness of Torricelli's hypothesis.In order to prove the correctness of this theory, that is, contrary to the vacuum principle, he asked his brother-in-law Parry to send a barometer to the top of Dom, one of the highest mountains in Auvergne (region), France.A year later, Parry agreed to the request. He assembled two barometers in the cloister at the foot of the mountain and began the test.After observing that the two mercury columns rose to the same height, Parry dismantled the next barometer and asked a monk to monitor the mercury height of the other barometer all day.Then, Parry took the removed barometer and set off for the mountain with a group of witnesses.As soon as he reached the top of the mountain, he fitted the barometer.Everyone was surprised to find that the mercury surface at this time was more than 3 inches lower than that at the foot of the mountain.In the second test, he removed the barometer and reinstalled it at five different points on the top of the mountain. The results were identical each time.
Halfway down the mountain, Parry assembled the barometer again.He found that the mercury surface was about between the top and bottom of the mountain.Finally, as soon as he returned to the monastery, he asked the monk about his barometer observation.The friars reported that the mercury surface had not changed since they left that morning.So Pascal announced the test result to the world's intellectual circles, thus successfully overthrowing the nonexistence of naturevacuumPrinciple.[1]
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Due to the effect of gravity, near the surface of the earth, the earth is attractive, the density of air molecules is high, and the frequency of impact on the surface of objects is high, so the resulting atmospheric pressure is large.Far from the earth's surface, the earth has little attraction,atmosphereThe density of molecules is low, and the frequency of hitting the surface of objects is low, so the resulting atmospheric pressure is small.Therefore, the atmospheric pressure at different heights on the earth is different, and the higher the location, the smaller the atmospheric pressure.In addition, the temperature and humidity of the air also affect the atmospheric pressure.
In physics, the latitude is set at 45 degrees sea level (i.eZero altitude）The perennial average atmospheric pressure on is specified as 1Standard atmospheric pressure（atm）。This standard atmospheric pressure is a certain value.The value is 1 standard atmospheric pressure=760 mmHg=1.0133 Project atmospheric pressure=1.0133 × 10fivePa=0.10133MPa[2]
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The generation of atmospheric pressure is the result of the earth's gravity. Because of the earth's gravity, the atmosphere is "sucked" to the earth, thus generating pressure. The atmospheric pressure near the ground is the largest.The atmospheric pressure in meteorological science refers to the weight of atmospheric column per unit area（Atmospheric pressure）, that is, the pressure exerted by the atmospheric column on the unit area.
There are two units of air pressure: millimeter and millibar. The height of mercury column is used to express the height of air pressure, and millimeter (mm) is used.For example, the air pressure is 760mm, which means the atmosphere at that timepressureIt is equal to the pressure generated by the mercury column at the height of 760mm.The other isweather forecastMillibar (mb) often heard on the radio.It is a unit of air pressure expressed by the pressure of the atmospheric column on the unit area.1 mbar=1000 dynes/cm2 (1 bar=1000 mbar).Therefore, 1 millibar means that 1000 dynes force is exerted on an area of 1 square centimeter.When the pressure is 760 mmHg, it is equivalent to 1013.25 mbar, which is called a standard atmospheric pressure.[2]
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Atmospheric pressure is closely related to altitude, that is, atmospheric pressure decreases with altitude.Near 1000 hPa of offshore sea level, the air pressure drops by 1 hPa for every 10 m height rise;Near 500hPa (5500m), the air pressure drops 1hPa for every 20m height rise;Near 200hPa (12000m), the air pressure drops 1hPa for every 30m of height rise;It is used in aviation to determine the altitude of an aircraft.The altitude indicator on the aircraft is based onAneroid barometerThe barometric altitude of is converted into altitude and used as the scale of altimeter.the international civil aviation organization(ICAO) Assume that the air pressure and temperature in dry air and mean sea level are 1013.25hPa and 15 ℃ respectively, and the temperature decline rate of about 11km below the tropopause decreases by 6.5 ℃ per kilometer with altitude, etcStandard atmospheric conditionsAs the reference datum of altimeter, the atmosphere in this state is called ICAOStandard atmosphere（ICAO Standard Atmosphere）。
The pressure varies with the altitude of the atmosphere.The higher the altitude, the smaller the atmospheric pressure;The greater the altitude difference between the two places, the greater the pressure difference.
The weight of the atmospheric column is also affected by the density change. The greater the density of the air, that is, the more air mass per unit volume, the greater the atmospheric pressure generated.
Since the mass of the atmosphere is more dense near the ground, and more sparse toward the upper air, the change value of pressure with height is also larger closer to the ground.For example, in the lower layer, for every 100 meters of rise, the pressure will drop by about 10 millibars;At an altitude of 5 to 6 kilometers, the air pressure decreases by about 7 mbar every 100 meters;At the height of 9 to 10 kilometers, the pressure will only decrease by about 5 millibars every 100 meters.
The air pressure changes all the time.Under normal circumstances, the pressure rises in the morning and falls in the afternoon;The pressure is the highest in winter and the lowest in summer.But sometimes, for example, under the influence of a cold wave, the pressure will rise quickly, but the cold air will slowly decrease after passing the pressure.[3]
