# Air properties

You can't catch or see the air, but we know it exists. Through its properties, it is possible to prove its existence.

Air is matter and occupies the entire space of the environment in which there is no other matter. For example, in a half-water bottle, air occupies the other (upper) half of this bottle.

The air has mass. On Earth, everything that has mass also has weight, that is, it is attracted by terrestrial gravity, which is the force that pulls everything to its center.

The air is compressible. It then presents compressibility. It is the property that air has to shrink when compressed. We can demonstrate this property by experimenting with the syringe. When plugging its hole it is difficult to push the plunger all the way. But we can see that the air inside the syringe decreases in volume, proving its compressibility.

The air has elasticity. When we plug the hole in the syringe and then release the plunger, we notice that the plunger tends to return to the starting position. Then the air returns to its initial volume and thus the elasticity of the air is proven. That is, elasticity is the property that air has to return to its initial volume when compressing.

The air expands. It has the property of expandability. When a volatile substance (which turns into gas) comes into contact with air, we smell it. This is because this substance expands and mixes with atmospheric air in a larger volume.

Air expandability is the property that air has to increase in volume, taking up all available space.

The air exerts pressure. The atmospheric air mass exerts pressure on the earth's surface, which is the atmospheric pressure. In general, we do not feel the effects of atmospheric pressure because atmospheric air enters our bodies. From the lungs it passes into the blood and other body fluids, exerting a pressure equal to the atmospheric pressure from the inside out.

## Historical Experiences

In the seventeenth century, two historical experiments were conducted on the effects of atmospheric pressure: the Magdeburg Hemisphere and the Torricelli Experiment.

The mayor of the German city of Magdeburg, Otto von Guericke, conducted a public experiment to prove that atmospheric pressure exists. He had two hemispheres of copper each half a meter in diameter. He joined the two copper hemispheres into a hollow sphere and, with a bomb, drew almost all the air from within.

Before taking air out, the hemispheres were easily separated because the pressure was the same, inside and out. But when the air was reduced, the pressure inside was less than the externally acting atmospheric pressure. This pressure difference brought the two hemispheres together so that it took 16 horses (eight on each side) to separate them.

Still in this century, the Italian physicist Torricelli built a barometer, which is a device capable of measuring atmospheric pressure. He took a tube about 1m long, closed at one end. Filled it with mercury (Hg, liquid and dense metal). He fingered the other end and inverted the tube, dipping it into a container that also contained mercury.

Removing his finger, he noted that the metal did not completely come down from the tube because atmospheric pressure exerted on the surface of the mercury contained in the container did not allow all of the mercury to escape from the tube. The experiment was conducted at sea level, so it was agreed:

1atm = 76cm Hg = 760mmg Hg

## Pressure and altitude

A person who is at sea level (on the beach, for example) has a greater amount of air above him than a person who is 800m above sea level.

So the higher the altitude, the lower the atmospheric pressure exerted on it. And the lower the altitude, the higher the atmospheric pressure. The same device that serves to measure atmospheric pressure is used to measure altitude. O barometer, so it is also used as an altimeter.

## Winds

Wind is the air in motion. A sun-heated layer of air expands, becoming less dense and rising. A layer of cold air will take its place. This cold air is also heated and rising.

Thus, air currents are formed, which constitute the winds. In warmer regions (less dense air), atmospheric pressure is lower than in colder regions (denser air). Therefore, the wind always goes from high pressure to low pressure regions.

Wind speeds vary according to the difference in pressure between two regions and the distance between them. Depending on the speed, the wind is given a different name: breeze, trade winds, cyclones and hurricanes.

The breeze is a light and pleasant wind. It can be maritime or terrestrial. The sea breeze occurs during the day and moves from sea to land. The land breeze occurs at night and moves from land to sea.

The trade wind is mild and persistent. It acts in the lower layers of the atmosphere, over large regions, from regions of high pressure near the poles, moving towards equatorial regions. It favors maritime navigation. They can be northeast (northern hemisphere) and southeast (southern hemisphere).

Cyclones or typhoons have speeds above 100 km / h. Hurricane is also a cyclone because it reaches speeds exceeding 300km / hour. They have a rotating movement that forms spiral air currents (swirls).

Winds can be used for navigation (sailing boat) and windmills (as a source of electricity).