 |
|
|
|
Amateur meteorologists love barometers:
the instrument that makes us all expert weather forecasters.
For with a barometer you can not only measure the current
air pressure, but also see which direction the needle is going,
and therefore have a fair chance of predicting the weather
to come
Put simply, when the pressure is falling, unsettled weather
is usually on the way, with low pressure systems bringing
cloud, wind and rain. Conversely, when the pressure is rising,
you can expect a high pressure system, which usually bring
clear, cloudless skies and settled weather.
Instruments to measure atmospheric pressure were first invented
in the 17th century, with scientists such as Galileo and Descartes
contributing to their development. |
| |
|
Barometers
We live at the bottom of a great ocean of air.
We can't see it but we can feel it every time the wind blows.
This air presses on us from all directions and we walk around
with a great weight sitting on our heads. We have, however,
evolved on this planet with this atmosphere and this gravity
so we don't notice the pressure on a day to day basis. If
we take off in an aeroplane, however, the changing air pressure
makes our ears "pop".
Many early scientists had seen the effects of this air pressure
but it was Toricelli (1608-1647) [pictured left] who
invented the first barometer - an instrument for measuring
air pressure - in 1643. |
 |
 |
He inverted a glass tube, closed at one end
and full of mercury, into a reservoir of mercury.
He saw that the column of mercury in the tube dropped a little,
leaving a vacuum above it in the tube. He then noticed that
the level of the mercury rose and fell regularly every day
and had rises and falls over longer periods as well.
Toricelli did not call his invention a barometer. Boyle
did that in 1665. The word comes from the Greek baros meaning
"weight".
Mercury is the favoured liquid for a barometer as it is so
heavy. The barometer tube only needs to be a metre long. This
makes it a convenient size for everyday use. Any liquid can
be used in a barometer if you have a tube long enough. Pascal,
the French scientist, made one with a tube 14 metres long.
|
He used red wine as his liquid. Water barometers have often been
made and they are popular in schools. Water is 13 times less dense
than mercury so you will need a tube about 13 metres long. A barometer
this size, however, is not practical.
Mercury in glass barometers are handsome instruments and very accurate
but they are difficult to transport. Mercury can be very nasty in
contact with other metals. Aluminium is particularly vulnerable.
A broken barometer on an aeroplane can have devastating consequences
which is why very strict safety procedures have to be followed if
moving a barometer by air.
The most common type of barometer is the aneroid barometer. Many
people have one in the home. The barometer in the picture with this
column is an aneroid barometer. These barometers do not use liquid
in a tube but a metal box from which some of the air has been extracted.
As the air pressure rises, the box is compressed; as the pressure
falls, the box expands. A pointer is attached to the box such that
it swings right and left as the box contracts and expands. A calibrated
card behind the pointer lets us read the pressure.
Setting a barometer
Many people have them and if they are to be any use, they must be
set correctly. The air pressure decreases with altitude so if we
measure the pressure at Lyneham and Snowdonia at the same time,
Snowdonia will have a lower pressure than Lyneham. We could set
our barometers to the correct local pressure and this is called
Station Level Pressure (SLP). It cannot, however, be compared with
the weather maps on TV or in the newspapers as the pressures on
those are all Mean Sea Level Pressures (MSLP). The Met Office reads
the barometers at SLP and then makes a correction to bring the reading
to MSLP. You can bypass the correction by setting your barometer
to MSLP. You can then make direct comparisons with the reports on
TV or in the papers.
Go to the BBC
Weather website link for Lyneham. Check the pressure given and
set your barometer to that. There is a screw in the back of the
barometer that allows you to set the pointer. Don't worry about
the pointer controlled from the front at this stage. Let the barometer
settle down and the next day check the pressure again just after
the hour when the new observation is posted. Reset your barometer
- it will need only a fine adjustment. Do this once more and you
will be ready. Check the setting every two months or so to keep
it accurate.
Reading the Barometer
The barometer should be read at the same time every day. When you
read it, move the pointer control on the front until the needle
sits over the barometer pointer. When you look at the barometer
the next day you will be able to see if the pressure has risen or
fallen. 9am is the traditional time for reading barometers but pick
a time that suits you. There are tides in the atmosphere and the
pressure falls from about 9am to about 3pm, rises to 9pm, falls
to 3am and then rises again to 9am. Toricelli noticed this with
his barometer. The movement is called the "diurnal variation
of pressure" and this is why the barometer must be read at
the same time every day.
The units we use to measure air pressure are hectoPascals (hPa).
This is the same figure as the earlier millibars (mb). Even earlier,
and still in the USA, the pressure was given by the height of the
mercury column in inches of mercury. The liquid must be specified
as it could just as easily be a lot more inches of red wine.
Can we forecast the weather with a barometer?
Many aneroid barometers have words like "fair", "change",
"rain", "stormy" on the front. Are these accurate
forecasts? No they are not. Most of the barometers we see get their
traditional faces from northern Europe where there are wide variations
in pressure. Rapidly falling pressures can indicate the approach
of a trough or a front or, on the coast, the development of a low.
You should look at the weather map to find out what is happening.
Any of these can bring bad weather so the barometer can be useful
there. High pressure, 1020hpa or higher, should bring fine days
with light winds. Most of the time, though, in our area, the barometer
needle will point to "change". That is just because there
is little variation in pressure from day to day.
Use the barometer well and you will find it a useful addition to
your knowledge and understanding of the weather.
|