Air is all around you. It presses against your body all the time. That’s why we call it air pressure. Usually, you don’t notice air pressure because it presses against you evenly in all directions.

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Anyone can measure air pressure using a tool called a barometer. When we use a barometer to measure the air pressure, the resulting number is called the barometric pressure.

Barometers work because air has weight. The air that surrounds us begins at the surface of the Earth and goes up to the top of the sky. All that air, the entire atmosphere, weighs about 5 million billion tons! We don’t get crushed because all that weight is distributed evenly over the entire surface of the Earth. The average force you feel is about 15 pounds on every square inch of your body. Because air is fluid, that force is delivered evenly over your entire body, not just on the top of your head.

We can measure how much the air presses down on us with a simple but clever device. It’s called a U-tube barometer and it works much like a teeter-totter. A teeter-totter can stay balanced as long as there is equal weight on both sides. If the weights are different, the teeter-totter tips to favor the heavy side. The same is true for a U-tube barometer. Look at the tube at left. It contains some water.

With both ends open to the air, the water levels in each side are the same. This is because the air pressure over each side is the same. The water is balanced between the downward forces on each side, just like a teeter-totter.

Blow some air into the left side. The water levels change because you have made the air press down more than the air on the right side. In other words, you have increased the air pressure on the left side. The water moves until it balances the downward forces on both sides.

The loss of water on the left side reduces the downward force from water weight on the left side. Rising water on the right side adds to the downward force from water weight on the right side. This balances the force from the air pressure you added on the left side.

On the other hand, when you suck air from the left side, the water levels change in a different way. By removing air from the left side, you have made the air press down less than the air on the right side. Again, the water moves until it balances the downward forces on both sides.

If you use a strong vacuum pump to remove all the air from the left side, the air pressure there will be zero. The only downward force on the left side would be from a tall column of water on that side. The force on the other side that balances all that water weight is the downward force from the normal air pressure on the right side. When this is done, the water level on the left side rises to a height of about 34 feet. This is not very practical, so U-tube barometers don’t actually use water as the liquid.

Mercury is used instead, because it is a very dense liquid metal. Mercury is about 14 times more dense than water. Because mercury is so heavy, normal air pressure pushes it to a height of only about 30 inches, or 760 millimeters.

Meteorologists measure air pressure by counting the inches of mercury on a barometer. Other scientists who work with gas pressures prefer to use millimeters. One millimeter of mercury is known as a “Torr.” The Torr unit is named in honor of Evangelista Torricelli, who invented the mercury barometer in 1643.

Torricelli filled a long glass tube with mercury and inverted the tube into a dish. He observed that some of the mercury did not flow out and that the space above the mercury in the tube was a vacuum.

Torricelli was the first person to create and sustain a vacuum. He concluded that the day to day changes in the height of the mercury were caused by changes in the atmospheric pressure.

At sea level, the barometric pressure is high because sea level is the lowest place you can go. More air above you creates more pressure. In the high mountains or in an airplane, the barometric pressure is less. Because you are higher in the atmosphere, there is less air on top of you, so the pressure it creates is less.

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In any given location, when the barometric pressure drops, you can expect a storm. When it increases, you can expect sunny weather.