Atmospheric pressure is the force with which the surrounding air acts on us, that is, 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 happens when there is an uneven distribution of atmospheric pressure, and much more.
Manifestations of atmospheric pressure
If the air presses on everything around, then it weighs something. Is it really true, why then does it seem to us weightless? We will carry out experiments that indicate that atmospheric pressure actually exists.
Fill the syringe with water to the middle, and then pull the piston up. Water will follow the piston. The reason for this is atmospheric pressure, but when people still did not know about its existence, they said that nature simply does not tolerate emptiness. Now we know that when the piston rises, a region of reduced pressure is created, 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 with a piece of plastic on top, for example, a card. Turn the jar over and see that the card holds and does not fall. The pressure of water is compensated by the pressure of the atmosphere. Nothing presses on top of the water, but the atmosphere presses on below, as a result, the card is held. If air enters between the plastic and the can, the card will disappear and the water will spill out.
Device torricelli
For the first time, the Italian scientist Torricelli measured atmospheric pressure. He did this with a so-called mercury barometer. First, Torricelli filled the 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 sink, but not all came out, but dropped to a certain height.
It turned out that this level is 760 mm. Therefore, atmospheric pressure can hold a column of mercury of 760 mm. If the pressure rises, then it can hold a column of a higher height, if it drops, it is less. If so, then its size can be judged by the height of the column. That is why in practice the pressure of the atmosphere and gases is often measured precisely in millimeters of mercury. Let us establish the connection between millimeters of mercury and the usual units of Pascal.
How millimeters of mercury are connected and pascal
Atmospheric pressure raises mercury by 760 mm. This means that a mercury column with a height of 760 mm presses with a force equal to the normal level of atmospheric pressure. 1 mmHg is the pressure produced by a 1 mm high mercury column. Imagine that the height of a column of mercury is 1 mm. We calculate the hydrostatic pressure corresponding to this height.
P = 1 mmHg Hydrostatic pressure is calculated by the formula: ρ * g * h. ρ is the density of mercury, g is the acceleration of gravity, h is the height of the liquid column. ρ = 13.6 * 10 3 kg / m 3 , g = 9.8 N / kg, h = 1 * 10 -3 m. Substitute these data into the formula. After the conversion, 13.6 * 9.8 = 133.3 N / m 2 will remain. N / m 2 - this is Pascal (Pa). If we translate atmospheric pressure into hectopascals, then 1 mmHg. Art. corresponds to 1.333 hPa.
Mercury column and weather
Torricelli watched 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 some time the weather comes. When the mercury column rises, after some time the weather gives way to good weather. That is, measuring atmospheric pressure allows you to make a weather forecast.
Now meteorological services around the clock, every 3 hours, measure atmospheric pressure. In the book of Jules Verne “Fifteen-year-old captain” describes the observation of the barometer and the weather. The protagonist of the book found that if the mercury column drops quickly, the weather deteriorates sharply, but not for long, if the mercury level decreases slowly over several days, then the weather will deteriorate gradually, and hold on for a long time.
What happens when there is an uneven distribution of atmospheric pressure
Consider a synoptic map. It displays the values of atmospheric pressure in different areas, cities, countries, continents. The direction of movement of the air masses is indicated by arrows. Why is the wind blowing? Atmospheric pressure in some places is greater, but less in others. From where it is larger, the wind blows to where it is smaller. We see this 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, in which the arrows of the wind are twisted and move clockwise. Such a high pressure region is called an anticyclone. It usually has clear weather.
And here is Spain and Portugal. Here we observe two powerful anticyclones. The swirling of air flows is associated 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, we can observe the distribution of atmospheric pressure in different places of our planet. Nowadays, meteorologists accurately predict the change in weather that occurs when there is an uneven distribution of atmospheric pressure.
Pressure at and above sea level
Suppose a barometer shows a pressure of 1006 hPa. But if you look at the synoptic map of a given area, city, it may turn out that the atmospheric pressure there is different. Why is this happening? The fact is that on weather maps the values of atmospheric pressure at sea level are shown. 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 not calibrated 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 needle to zero, because its height above the ground is zero. If necessary, he sets the arrow to a height above sea level, depending on whether it is important for him to know at what altitude the airfield is above sea level or not. In the case of long-distance flights, this can be useful, especially if the airfield is in the mountains. Then, looking at the arrow of the altimeter, the pilot determines the height.
Why does atmospheric pressure increase with height
After we learned that when the atmospheric pressure is unevenly distributed, wind arises, we will understand why the pressure decreases with increasing altitude. Air has weight, therefore, it is attracted to the earth, produces pressure on it. If we place the barometer in a certain layer of the atmosphere, then that layer of the atmosphere that is higher will press on it. It should be noted that the atmosphere has no clear boundaries.
If we place the barometer at sea level, then the pressure will be equal to the sum of the pressure in this layer of air and the pressures in the overlying atmosphere. That is, with increasing height, the pressure decreases. The question arises: is it possible to consider atmospheric pressure by the formula P = ρ * g * h? No, because the density of air is not constant in different layers of the atmosphere. Below, the air is under a lot of pressure, therefore it is more dense, and above it is less dense.