Barometers
Last updated: November 19, 2009.
Is it going to rain today? Is it going to stay fine? And just how can
you tell? One easy way is to measure the air pressure. If it's rising
and the pressure is high, chances are it'll be a fine day; if the
pressure is falling, it's more likely to be wet, windy, and dull.
Instruments that measure air pressure are called barometers and
people have been using them for weather forecasting and scientific
research for hundreds of years. Let's take a closer look at how
they work!
Photo: A traditional aneroid barometer dating from the mid-20th century. The needle turns clockwise when the pressure is rising (indicating fair or dry weather); it turns anticlockwise when the pressure is falling (indicating rain or stormy weather).
What is air pressure?
If you've ever been scuba diving,
you'll know just what pressure feels like. Dive
down beneath the surface of the sea and you'll soon feel the weight
of water pressing in on you. The deeper you
go, the more water there
is above you, the more it weighs, and the more pressure you feel. But
there's pressure pushing in on your body even if you never go in
the sea.
Look up at the sky and try to imagine the weight of the atmosphere:
the huge amount of gas surrounding our planet and
pulled to its surface by gravity. All
that gas might look like a vast, empty cloud of nothing, but it still
has weight. And it still presses down on your body. That's air
pressure. When you're under the sea, the weight of water pressing
in on your body makes it hard to breathe from your oxygen tank. Air
pressure never has this effect because our bodies are hollow and our
lungs are full of air, so the air presses equally on the inside and
outside of our body at the same time. That's why we don't feel
air pressure in the same way we feel water pressure.
Why air pressure changes from place to place
Air pressure varies all across our planet. It's highest at sea level
(where there's
the most amount of air pushing down) and gets lower the higher up you
go. Way up in the atmosphere, there's much less air—so there's
less oxygen to breathe. That's why mountain climbers often have to
use oxygen cylinders. It's also why airplanes have to have
pressurized cabins (internal passenger compartments, where the air is
kept at
higher pressure than it would normally be at that altitude) so people
can breathe comfortably.
Even in one place, the air pressure is constantly changing. That's
because Earth is
constantly spinning and moving round the Sun, so different parts are
being warmed up by different amounts. When the air cools and falls,
it increases the pressure nearer to the ground. Regions of high
pressure like this are linked with fine weather. The opposite happens
when the air warms and rises to create regions of low pressure and
wet weather.
How can we measure air pressure?
Imagine you're an inventor and your job is to create a machine that can
measure air
pressure. How are you going to do it? Think about air pressing down
on you and see if you can imagine building something that will
measure its pressure. See if you can sketch something now on a piece
of paper. Here's a clue. Imagine the air pressing down is contained
inside a giant, invisible tube pressing down on Earth's surface
next to your feet.
If you imagined something a bit like a pair of scales that can
measure
the weight of the air in the tube, congratulations! That's pretty
much the solution. A device that can measure air pressure (which we
call a barometer) works by measuring how much the air is
pressing down on it.
Photo: A combined barometer and altimeter (instrument for measuring height above sea level)
used for weather forecasting. Photo by Andy Dunaway courtesy of Defense Imagery.
How barometers work
Modern barometers are completely electronic and show
the pressure reading on an LCD display. The two traditional kinds of barometer are called Torricellian and aneroid (dial) barometers—and here's how they work.
Torricellian barometers
The simplest kind of barometer is a tall closed tube standing upside
down in a bath of mercury (a dense liquid metal at room
temperature) so the liquid rises partly up the tube a bit like it
does in a thermometer. We use mercury in barometers because it's
more convenient than using water. Water is less dense (less heavy, in effect) than
mercury so air pressure will lift a certain volume of water much higher up
a tube than the same volume of mercury. In other words, if you use water, you need a really
tall tube and your barometer will be so enormous as to be
impractical. But if you use mercury, you can get by with a much
smaller piece of equipment. A piece of apparatus like this is called
a Torricellian barometer for Italian mathematician Evangelista
Torricelli (1608–1647), a pupil of Galileo's, who invented the first
instrument of this kind in 1643.
Photo: A Torricellian barometer (sometimes called a mercury barometer) is an inverted (upside-down) glass tube standing in a bath of mercury. Air pressure pushes down on the surface of the mercury, making some rise up the tube. The greater the air pressure, the higher the mercury rises. You can read the pressure off a scale marked onto the glass.
At sea level, the atmosphere will push down on a pool of mercury and
make it rise up in a tube to a height of approximately 760mm (roughly 30in). We call this
air pressure one atmosphere (1 atm). Go up a mountain, and take your
Torricellian barometer with you, and you'll find the
pressure falls the higher you up go. The atmosphere no longer pushes down on
the mercury quite so much so it doesn't rise so far in the tube. Maybe it'll
rise to more like 65cm (25 in). The pressure on top of Mount Everest is slightly
less than a third of normal atmospheric pressure at sea level (roughly 0.3 atm).
Aneroid barometers
Torricellian barometers are useful and accurate, but mercury is
poisonous—and no-one really wants a great lake of mercury slopping around in their
home. That's why most people who own barometers have ones with
easy-to-read dials, which are called aneroid barometers.
Instead of having a pool of mercury that the atmosphere pushes down
on, they have a sealed, air-tight metal box inside. As the air
pressure rises or falls, the box either squashes inward a tiny bit or
flexes outward. A spring is cunningly attached to the box and, as the
box moves in and out in response to the changes in air pressure, the
spring expands or contracts and moves the pointer on the dial. The
dial is calibrated (marked with numbers) so you can read the air
pressure instantly.
Photo: An aneroid barometer in close-up. You can clearly see the spring that makes the pointer rise or fall as the pressure changes. The sealed metal box is immediately behind the spring.
Aneroid barometers measure the air pressure when you knock their glass
faces. When you first inspect them, the needle shows the pressure as it was when you
last looked at them—however long ago that might have been. Give the
glass a sharp tap and the needle will jump to a new position showing
the pressure as it is now. The way the needle moves is important. If
it moves clockwise, up the dial, the pressure is increasing so the
weather is likely to be getting hotter, drier, and finer; if the
needle turns counterclockwise, the pressure is decreasing and the
weather is likely to get cooler, wetter, and poorer.
Measuring pressure
There are lots of different units you can use for measuring pressure.
Historically, scientists described ordinary atmospheric pressure
as "one atmosphere" and said it was equivalent to "76cm (760mm) of
mercury," sometimes written 76cmHg or 760mmHg (because Hg is the chemical symbol for mercury). You
might also come across an old unit called the Torr: 1 Torr (named for
Torricelli) is also equal to 760mmHg or 1013.25 millibars (another
increasingly archaic unit). In modern SI units, one atmosphere is equal to 101,325 Pa (pascals) or
101.325 kilopascals (thousands of pascals or kPa). Pascals and
kilopascals are the preferred scientific units for measuring pressure now.
Further reading
Websites
Books
If you're interested in how pressure helps to determine our weather and how you can use pressure measurements in weather forecasting, these books are worth a look:
