Determining Altitude By Air Pressure
2014-15
The weight of air (atmospheric pressure) is measured with an instrument called a barometer. The standard
pressure at sea level is 1013.2 millibars or 760 millimeters (29.92 inches) of mercury. Since surface elevations
vary, barometric readings are usually adjusted to indicate what the pressure would be if the barometer were
located at sea level.
There are two types of barometers: aneroid and mercury. The former is the type we have in class. It has a metal
box from which most of the air is removed. The top of the box is a thin metal disk that bends under pressure. The
pressure causes a spring, in the box, to move. The spring is attached to a needle that indicates the amount of
pressure exerted by the air. The face of the aneroid barometer is inscribed with words such as fair, change, rain,
and stormy. Notice that “fair weather” corresponds with high pressure and “rain” is associated with low pressure.
To predict weather, it is important to know how the pressure has changed over a few hours. Most aneroid
barometers have a dial that you can move (shown in black) to coincide with the current reading. Later, when you
check the instrument, you can see if the pressure has gone up or down.
The latter is more accurate, but much less practical. It is constructed out of a long (more than 3 feet) glass tube
that is closed at one end. The tube is filled with mercury and the open end is placed upright in a dish of mercury
(When the open end is submersed in mercury a vacuum is created.). The mercury moves up and down the tube
depending on the outside air pressure.
Determining Altitude By Air Pressure
2014-15
QUESTIONS:
1. Does air weigh anything, if so how much ? Give an example to demonstrate this.
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2. What happens to the pressure of the air as you go up in the atmosphere ? ____________________________
3. What happens to the pressure of the air as you go deeper into a mine ? _____________________________
4. Why does air pressure change with a change in altitude ? ________________________________________
5. What would happen to a balloon you blew up if you took it to the top of Mt. Everest, into space, or deep into a
mine ? Explain.
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6. Would breaking the stick become easier, harder or remain the same if you did it on top of a mountain ? Explain
your answer.
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Use the aneroid barometer shown above, to answer the following:
8. What is the barometric pressure now ? ________ mb
_______ mm
9. What was the barometric pressure when you set the black dial ? ________ mb
_______ mm
10. Does this change in pressure suggest the weather will become fair or stormy ? _______________________