Atmospheric pressure is the force with which we are affected by the surrounding air, i.e. the atmosphere. The article will present experiments during which we will make sure that air pressure really exists. We will find out who measured it for the first time, what occurs when the atmospheric pressure is unevenly distributed, and much more.

## Manifestations of atmospheric pressure

If the air presses on everything around, then it weighs something. Is this really true, why then does it seem weightless to us? Let's conduct experiments that show that atmospheric pressure actually exists.

Fill the syringe with water to the middle, and then pull the piston up. The water will follow the piston. The reason for this is atmospheric pressure, but when people did not yet know about its existence, they said that nature simply does not tolerate emptiness. We now know that when the piston rises, an area is createdreduced pressure, and the atmosphere squeezes water into the syringe.

## Experience with a plastic card and a jar

Fill a glass jar to the top with water, cover the top with a piece of plastic, for example, a card. Let's turn the jar over and see that the card is holding and not falling. The force of water pressure is compensated by the pressure force of the atmosphere. Nothing presses on the water from above, but the atmosphere presses from below, as a result, the card is held. If air enters between the plastic and the jar, the card will fall off and the water will pour out.

## Torricelli device

The Italian scientist Torricelli measured atmospheric pressure for the first time. He did this with the so-called mercury barometer. First, Torricelli filled a glass tube with mercury to the top, took a large bowl of mercury, turned the tube over, plunged it into the bowl and opened the lower end. Mercury began to descend, but did not come out completely, but descended to a certain height.

It turned out that this level is 760 mm. Therefore, the pressure of the atmosphere is able to hold a mercury column of 760 mm. If the pressure rises, then it can hold a column of greater height, if it decreases, less. If so, then its size can be judged by the height of the pillar. Therefore, in practice, the pressure of the atmosphere and gases is often measured precisely in millimeters of mercury. Let's establish a relationship between millimeters of mercury and the usual units of pascal.

## How millimeters of mercury and pascals are related

Atmospheric pressure raises mercury by 760 mm. It means thata column of mercury 760 mm high presses with a force equal to the normal level of atmospheric pressure. 1 mm Hg is the pressure produced by a 1 mm high column of mercury. Imagine that the height of the mercury column is 1 mm. Calculate the hydrostatic pressure corresponding to this altitude.

P=1 mmHg Hydrostatic pressure is calculated by the formula: ρgh. ρ is the density of mercury, g is the acceleration due to gravity, h is the height of the liquid column. ρ=13, 6103 kg/m3, g=9, 8 N/kg, h=110 -3 m. Substitute these data into the formula. After the conversion, 13.69.8=133.3 N/m2 will remain. N/m2 - this is Pascal (Pa). If we convert atmospheric pressure to hectopascals, then 1 mm Hg. Art. corresponds to 1.333 hPa.

## Hg and weather

Torricelli watched the readings of the mercury barometer for a long time. He noticed an interesting thing. When the column of mercury drops, that is, when the atmospheric pressure becomes low, after a while bad weather sets in. When the mercury column rises, after some time bad weather is replaced by good weather. That is, the measurement of atmospheric pressure allows you to make a weather forecast.

Now meteorological services around the clock, every 3 hours, measure atmospheric pressure. Jules Verne's book The Fifteen-Year-Old Captain describes the observation of the barometer and the weather. The protagonist of the book discovered that if the mercury column falls quickly, then the weather deteriorates sharply, but not for long, if the mercury level decreases slowly, over several days, thenthe weather will deteriorate gradually, but last for a long time.

## What happens when the atmospheric pressure is unevenly distributed

Let's consider a synoptic map. It contains the values ​​of atmospheric pressure in different areas, cities, countries, continents. The direction of movement of air masses is indicated by arrows. Why does the wind blow? Atmospheric pressure is greater in some places and less in others. From where it is larger, the wind blows to where it is smaller. We see it in the direction of the arrows on the map.

If you look at the whole planet, you can see that it is different in different parts. Areas of high pressure are marked in purple, where the wind arrows are swirling and moving clockwise. This area of ​​high pressure is called an anticyclone. It usually has clear weather.

But Spain and Portugal. Here we observe two most powerful anticyclones. The twisting of air currents is connected with the rotation of the globe.

And here are two powerful areas of low atmospheric pressure - only 965 hectopascals. This is a cyclone, the air in it rotates counterclockwise.

Thus, you can observe the distribution of atmospheric pressure in different places on our planet. Nowadays, meteorologists accurately predict weather changes that occur when atmospheric pressure is unevenly distributed.

## Pressure at and above sea level

Suppose the barometer shows a pressure of 1006 hPa. But iflook at the synoptic map of a given area, city, it may turn out that the atmospheric pressure is different there. Why is this happening? The fact is that synoptic maps show the values ​​​​of atmospheric pressure at sea level. We can be at a certain height above sea level, so the pressure that the barometer shows in the room is less than at sea level.

## Altimeter

How to measure the height of your location? There are special instruments similar to a barometer, but their scale is graduated not in units of pressure, but in units of height. Tourists and pilots have such devices. They are called altimeters or parametric altimeters. When the pilot is on the ground, he sets the altimeter to zero, because his height above the ground is zero. If necessary, he sets the arrow to the height above sea level, depending on whether it is important for him to know at what height the airfield is above sea level, or not. In the case of long-haul flights, this can be useful, especially if the airfield is in the mountains. Then, looking at the altimeter needle, the pilot determines the altitude.

## Why does atmospheric pressure increase with altitude

After we learned that when the atmospheric pressure is unevenly distributed, wind occurs, let's figure out why the pressure decreases with increasing altitude. Air has weight, so it is attracted to the earth, exerts pressure on it. If we place a barometer in a certain layer of the atmosphere, then it will be pressed by that layer of the atmosphere,which is above. It should be noted that the atmosphere has no clear boundaries.

If we place a barometer at sea level, the pressure will be equal to the sum of the pressure in this layer of air and the pressures in the overlying layers of the atmosphere. That is, as the altitude increases, the pressure decreases. The question arises: is it possible to calculate the atmospheric pressure according to the formula Р=ρgh? No, because the value of air density is not constant in different layers of the atmosphere. At the bottom, the air is under more pressure, so it is denser, and at the top, it is less dense.