What Is Air Pressure, And What Links it To the History of the Barometer?
Most Of Us Know That The Barometer Measures Air Pressure, But Which Was Thought Of First?
What is Air Pressure?
Air pressure is the effect of the weight of air surrounding the earth, expressed as so much weight per unit of area - 14.7 poundsper square inch for example.
It is a measure of the weight of a column of air on the earth's surface, or on us for that matter. And that's pretty important, because that great mass of air contains the oxygen we breathe, that keeps us alive.
It's a pretty subtle sort of thing too. If we have 14.7 lbs pressing down on every square inch of us, why don't we feel weighed down? Three reasons - first it's equally distributedall over us, it's much less than the force of gravity, whichwe do feel, and we also don't feel crushed because the pressureinside our lungs is the same as the air outside, which we breathed in.
Not so obvious, and that's probably why it took so long to discover.
So let's consider what led to the discovery of air pressure, before checking out some air pressure facts and figures.
Air Pressure and the Early History of the Barometer
The development of the concept of air pressure is a classicexample of how scientific thought and experiment can haveresults that go far beyond the original problem.
It came more or less by accident, and that noted
astronomer and physicist Galileo was behind it all.
In the early 1600s, engineers found that no matter what they
did, their piston type pumps would not lift water more than
about 30 feet in one cycle.
Galileo was consulted, and though he didn't solve the problem
he realised that a partial vacuum was created during the action
of the pump
Galileo's studies were followed up by Gasparo Berti in 1640.
He filled a glass topped lead pipe over 35 feet long (10.7m)
and attached it to the side of his house. After submerging
the base of the pipe in an open container of water, he
opened a valve. Water flowed down into the container, leaving
a vacuum at the top. The column of water that remained in the
pipe was a little over 30 feet high, a little more than the
engineers managed in their less than perfect pumps.
That was fine, but the experiment was a little unwieldy -
neighbours were talking.
In 1643, Evangelista Torricelli, reputedly following up a
suggestion from the now dead Galileo, tried the same
experiment with mercury, a liquid 13.59 times heavier
than water. Again his aim was to create a vacuum, but with
mercury he only needed a tube greater than 30 inches (76cm) high.
He was successful, as was my science teacher when he repeated
the procedure in front of a group of appreciative 12 year olds
not quite so many years ago.
Torricelli deduced that something must be pushing the mercury
and water up their respective tubes. He found that that the
height of the liquid column varied in exactly the same ratio
as the differences in density between mercury and water. It
was also apparent that the diameter of the tube had no effect
on the height reached by the mercury.
This outside force could only be the weight of a mass of air
acting on the open basin at the bottom of the tube - in other
words air pressure.
How Can We Demonstrate Air Pressure?
A convincing demonstration of this was also part of my first
science lesson. Here's how it goes.
Take a small rectangular tin or can which can be sealed by a
screw-on top. Put a small amount of water in the bottom and
heat until the water is boiling and steam can be seen coming
out of the opening. Take the tin away from theheat, and with
great care, and preferably a pair of gloves, screw on the lid.
As the tin cools down, it will gradually buckle and collapse,
making some very satisfactory noises as it slowly folds in on
itself. Very impressive for new science students, but what
has happened here?
Once the water started boiling, liquid water turned into water
vapor, which is invisible until it cools down and we can see
steam. The water vapor drives all the air out of the tin, so
that when the lid is put on, the tin contains nothing but
water vapor. As the tin cools, the vapor turns back to liquid
water, slowly creating a partial vacuum, and a much lower
pressure inside the tin. Before the water was boiled, the air
in the tin was at the same pressure as the air outside, but
now it has gone. There is nothing to act against the pressure
of the outside air, and the tin slowly collapses.
This experiment doesn't work as well with stronger cylindrical
tins, and must never be attempted with a glass container.
Air Pressure Standards - A Short Deviation
Just so you have something to hang your hat on, the standard
air pressure at sea level, accepted by all the world's
meteorological organizations, is 1013mb at a temperature of
15°C. Although I give both metric and imperial values for most
things in these articles, the metric system is standard throughout
science, including meteorology.
Equivalent values are 14.7 lbs/sq in, 29.92 inches or 76cm or
760mm of mercury, or 101.3 hectopascals, at a temperature of
59°F. It's best not to mix the two systems, so use °F with
pounds/square inch and inches of mercury, and °C with everything
Some of you may be wondering why the metric system uses millibars
rather than, say, grams/sq cm. Good question, but the answer is
confused by different concepts of weight, mass and pressure, and
is far too complex to cover here. However we can say that a
millibar is equivalent to the pressure exerted by a kiligram on
a square centimetre, and that it is a word honored by a long
history and familiar to all meteorologists.
Let's move on quickly.
Air pressure decreases with height at about 1" of mercury per
1000', or by about 38 millibars over the same height. Metrically,
this transforms to about 1 millibar for every 8.2 metre change in
This simple conversion only applies over the first 3000 feet or
1000m above sea level, after which pressure continues to decrease
at less consistent rate.
This decrease in air pressure with height is very significant to
us. It explains why climbers need oxygen at altitude - the
pressure at the top of Mount Everest is about one hundredth
that at sea level.
Planes fly at that height too - 30,000 feet or more - and their
strong construction, thick small windows and circular cross
section are all designed to maintain a breathable atmosphere
for travellers, at a much greater air pressure than that outside.
That's also why oxygen becomes immediately available if the
plane loses inside air pressure.
So after that small diversion into the numbers of air pressure
Back to Torricelli, and The History of the Barometer
So how did we come to know these facts about air pressure?
Over time, Torricelli observed that the level of mercury in the
tube varied a little from day to day, suggesting that air
pressure varied, and it wasn't such a big step to relate this
to changes in the weather.
And so the barometer was born, although it wasn't until later
that it was given that name.
It also wasn't immediately apparent why air pressure changes were
Probably since the first time mankind learnt to grow food, a the
major concern has been the weather,and how to predict it. It
probably didn't matter so much in the earliest settled areas
of the Middle East, where crops were irrigated from major
rivers such as the Nile and Tigris-Euphrates, but in
more remote areas, and on the coast where fishing and the
beginnings of trade were important, the prediction of future
weather changes was very important.
Our ancestors, wherever they were , were not totally without
resources. Astronomy developed early in most societies and was
important in predicting seasonal changes, and you can bet
that the weather typical of each season was well remembered.
Among these were seasonal winds, and the weather they brought
with them. We know that weathervanes were invented early - the
first one known was at the Tower of the Winds in Athens around
400 BC. But apart from watching the clouds and looking out for
changes in wind direction, it was around 2000 years before the
next major advance in weather recording and forecasting.
And that was the growing realization that changes in air
pressure accompanied or foretold changes in weather. Most of
us are familiar with the highs and lows that are talked about
by TV weather people and shown on the weather maps. These are
areas of relatively high and low pressure, and it is their
movement that brings our ever changing weather.
Just how significant changes in air pressure became more obvious
as the usefulness of the barometer became apparent.
And what of the answer to the question at the top of the page?
Well, it seems that the principle of the mercury barometer came
before the concept of air pressure was developed. But the naming,
and the development of the barometer as an instrument to measure
air pressure, had to wait another 20 or so years, as we shall see
in the next article on the History of the Barometer.
And just in case you were wondering, here is a page on How To Use A Barometer. It's not as easy as you might think to use a home barometer for weather forecasting
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