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# Atmospheric Pressure

## The pressure exerted by gas particles in Earth’s atmosphere

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Atmospheric Pressure

#### What do storm reports inform people of?

The pressure in the atmosphere is an important factor in determining what the weather will be like. If the barometric pressure is high in an area, this will cause air to move to a region of lower pressure. The greater the difference in pressure between the two areas, the stronger the winds will develop. Under certain conditions, the winds can produce a tornado (a violent rotating column of air that reaches from a thunderstorm down to the ground).

### Atmospheric Pressure

Atmospheric pressure is the pressure exerted by gas particles in Earth’s atmosphere as those particles collide with objects. A barometer is an instrument used to measure atmospheric pressure. A traditional mercury barometer consists of an evacuated tube immersed in a container of mercury. Air molecules push down on the surface of the mercury. Because the inside of the tube is a vacuum, the mercury rises inside the tube. The height to which the mercury rises is dependent on the external air pressure.

(A) A barometer measures atmospheric pressure as the height of a column of mercury. (B) A modern aneroid barometer in the form of a dial is used by meteorologists to help them predict upcoming weather.

A more convenient barometer, called an aneroid barometer , measures pressure by the expansion and contraction of a small spring within an evacuated metal capsule.

#### Atmospheric Pressure and Altitude

At sea level, a mercury column will rise a distance of 760 mm. This atmospheric pressure is reported as 760 mmHg (millimeters of mercury). At higher altitudes, the atmospheric pressure is decreased and so the column of mercury will not rise as high. On the summit of Mt. Everest (elevation of 8848 m), the air pressure is 253 mmHg. Atmospheric pressure is slightly dependent on weather conditions. From the graph we can see the decrease in atmospheric pressure as the altitude increases. At sea level, the atmospheric pressure would be a little over 100 kPa (one atmosphere or 760 mm Hg). If we climb to the top of Mount Everest (the highest mountain in the world at 29,029 feet or 8848 meters), the atmospheric pressure will drop to slightly over 30 kPa (about 0.30 atmospheres or 228 mm Hg). This marked decrease in atmospheric pressure results in much lower levels of oxygen. Teams that climb this mountain must bring supplies of oxygen with them in order to breathe at these high altitudes.

Effect of altitude on atmospheric pressure.

#### Summary

• Atmospheric pressure is the pressure exerted by gas particles in Earth’s atmosphere as those particles collide with objects.
• A barometer measures atmospheric pressure.
• Atmospheric pressure decreases as the altitude increases.

#### Practice

1. Who invented the mercury barometer?
2. List two factors that will affect how closely your barometer will agree with the local weather station.

#### Review

1. Define atmospheric pressure.
2. What is an aneroid barometer?
3. How does the atmospheric pressure change as the altitude increases?