How Do I Read A Barometer?
How Can I Tell If it's
How Can I Calibrate It To
Match The Weather Service?
contains instructions for handling, diagnosing, and altering mercury
barometers. The reader does so at his or her own risk. We will
accept no responsibility for damage to anyone's barometer resulting
from the processes described below. If you don't feel comfortable
following these instructions, then consider the well being of the
barometer and just don't handle it. Barometers are delicate and
tricky to work with. Please note that this is general barometer
information that does not apply, in some cases, to the units we sell
that are aimed at the racer.
A barometer is a device for
predicting weather changes, rather than one for giving you an
instant readout of current conditions. A change in the level of the
mercury indicates the passage of a high or low pressure front over
your area, and a corresponding change in weather.Falling
atmospheric pressure precedes stormy or unsettled conditions, and a
rise indicates the approach of settled weather. Weather indications
are written on the face of your barometer to guide you.
In the absence of violent weather conditions, the atmospheric
pressure over the large land mass of the United States seldom varies
more than half an inch above or below the 30 inch mark (at sea
level). Movement of the mercury may take several hours or a day, and
may be in very small increments. Also, your barometer can indicate
FAIR even in a rain storm, because the narrow and fast-moving low
pressure front that brought the rain into your area passed so
Because a barometer's mercury level moves very slowly and by minute
amounts, barometers all have a "set" indicator. This will
be a hand or pointer, usually made of brass, and operated manually. You
put this marker in place next to the top of the mercury on a stick
barometer, or over the black indicator hand of a dial barometer, and
it marks the height that the mercury was at in its tube when you
last checked. This marker makes the direction of change easy to
NOTE: Your banjo barometer may need a small physical jolt before the
main hand will settle into its final position. This is because the
shaft connecting the brass pulley in back to the main indicator hand
on the dial binds slightly in its tube. Just tap firmly on the wood
case a couple of times, and the hand will drop into its proper
reading. Stick barometers obviously do not require this.
SURE THE BAROMETER IS WORKING
The height of the column of
mercury inside its glass tube is affected by many things that can
cause the barometer to give "inaccurate" readings, but
before you can determine what your barometer is trying to tell you,
you will have to know if the barometer is working at all or if the
mercury system has a fault, and precisely what sort of a reading you
are comparing your barometer to.
1. Is the barometer actually working?
There are two forms of mercury barometer; stick (you read the
mercury height by looking directly at the top of the column inside
the glass tube, and compare it to a scale of inches printed or
engraved beside the column), and dial, also known as wheel or banjo
(you read from a hand pointing to numbers on a dial, corresponding
to the height of the mercury inside the glass tube in the rear of
is a quick test for each type to tell you if you have a functioning
mercury system. With the barometer hanging on its hook on the wall,
slide the bottom sideways in an arc, to about a 45 degree angle. Do
this gently, don't slap it sideways or jerk it suddenly.
On a stick barometer, the mercury should rise smartly to the top of
the glass tube and hit the end with a "tick" sharp enough
to be heard and possibly felt through the wood of the case. The
vacuum space should fill totally with mercury, with no air pocket at
the very tip. On a dial barometer, the indicator hand (the longer,
black one) should swing clockwise around the dial, past the end of
the indicated scale. If either type of barometer fails this test,
the mercury system will need to be put back in working order by a
competent restorer before there is any point in going further.
The most common problems that prevent either stick or dial
barometers from working may be pockets of air, interspersed in the
column of mercury (and probably in the vacuum space at the top as
well); some mercury may have been lost from spillage; or the mercury
itself has become so dirty and contaminated that it can no longer
function. Air often shows up as gaps or bubbles somewhere in the
length of the column, and just one small one can stop the mercury
from moving up and down.
In general, any of these problems means that the glass tube of
mercury needs to be emptied, cleaned and dried, and refilled with
clean mercury before there is any chance of the barometer working
again. This is a job for a specialist restorer (nitric acid is used
to clean the inner bore of the glass) and there is no point in
wasting your money just putting in more mercury.
Dial barometers may also have had the fiddly string linkage between
the float in the mercury reservoir and the indicator hand come off
the pulley, and this is a fault that you can correct yourself with
some patience and a modicum of mechanical aptitude.
By the way, has the mercury system been opened up to the atmosphere
so that it can work? Lots of barometer owners seem to have left the
cork stopper in the open end of the mercury tube, or have failed to
unscrew the closing screw at the bottom end of a stick
2. With what
reading are you comparing your barometer?
There are many sources of readings of
barometric pressure. The most common these days is The Weather
Channel on cable TV, or the evening weather news. Be sure the TV
station or other source is relatively close to you; just a few miles
can make a big difference. Also, a nearby airport control tower, or
pilots' data service, will have the current atmospheric pressure;
pilots of small planes use it to set their altimeters, which are
actually barometers. Airport readings are sometimes given as
"station" readings, which are uncorrected for elevation.
Be sure to ask if you use this source, then ask for the corrected
reading, or adjust it yourself with the table at the end of this
As you become accustomed to using a barometer, you'll learn to trust
the weather predictions on the register plate or the dial, and
forget the actual numbers altogether.
Atmospheric pressure readings given out to the
public by the U.S. Weather Service, and repeated by most television
weather broadcasts, are mathematically corrected for elevation above
sea level, so that weather reporting across the continent can use
standard nomenclature and also so that the weather stations don't
have to have custom built barometers for their readings. The
standard elevation used is sea level, even if it is reported in
Denver, Colorado, a mile above the sea. This is done by taking an
actual reading, then adding a factor to it to correct for the known
elevation. You will be doing the same thing, after you read the next
READING MATCH THE WEATHER SERVICE
If you are lucky enough to live
on the ocean front, or on a boat in the ocean, you know your elevation
is zero, or sea level (lakes don't count; the Great Lakes for
instance, are about 650 feet above sea level). The rest of us will
need to find our elevation above sea level. One of the best sources is
the local city or county engineer's office. The resident engineer
needs to know elevations all over his area for such things as water
services. Just call and ask. The office may be able to give it to you
right down to the block where you live. With that information in hand,
you can apply it as follows:
Stick barometers: There is no physical alteration you can make to
the barometer to correct its reading. Not to any of them, even the
ones with adjustable cistern capacities. They are all made to operate
at sea level, period. At elevations up to 1,000 feet, just use the
table below to make corrections to the reading on your stick barometer
and enjoy it for the lovely antique it is.
At elevations above about 1,000 feet, the common household variety
just flat won't work. Short of cutting a section out of the case and
shortening the glass tube as well, you can't make them read in synch
with the corrected broadcasts. Many owners mistakenly use the closing
screw at the bottom of the case to push the mercury up to the
"right" reading, a futile exercise. Doing this just limits
the capacity of the barometer's mercury system to accommodate high
changes in volume.
You should be aware that many stick barometers are constructed with
built in errors in the relationship between the surface of the mercury
in the cistern and the placement of the scale on the case. Fitzroys
are notorious for this; I've worked on dozens and I've yet to find an
accurate one. You can check yours with a tape measure. Measure from
any inches number on the register plate to the center of the cistern.
Twenty-nine inches on the scale should be placed 29 inches from the
center of the cistern.
Dial barometers: With the more-or-less standard 35 inch long
mercury tube that is generally used in most of these, there is
latitude for some change to adjust your reading up to elevations of a
maximum of 1,000 feet. With the barometer hanging on the wall, remove
the bezel that holds the glass over the dial. Two or three screws
around the perimeter are usually standard. Take the barometer off the
wall, keeping it upright, and open the back door.
Grasp the pulley in the back between thumb and forefinger with one
hand, and with the other turn the indicator hand on its shaft to the
right reading. Put the barometer back in its normal position on the
wall and check it. A few adjustments may be required to get the right
setting of the hand on the shaft. If you don't feel comfortable doing
this, don't try it. There is considerable potential for messing up the
workings of the instrument.
If you reside above 1,000 feet
and want your dial barometer to be accurate, a shorter mercury tube
can be fabricated and installed. We've done several and they have all
worked, up to 6,800 feet in one case. It isn't particularly expensive,
and nothing is damaged by installing a shorter tube in the case. The
chances of the tube you have now of being original, or even over 100
years old, are slim to none. Old glass corroded away in contact with
mercury, and the tubes were replaced wholesale in order to keep the
barometers working. Yours has probably been done several times, so
replacement has no effect on the antique value of the instrument as a
In conclusion, a scientifically
precise reading from your barometer doesn't really matter, in the
grander scheme of things. As long as the mercury level drops when a
storm is coming and rises for fair weather, that is really enough of
an indicator to make it a good predicting device. That's all it did
one or two hundred years ago, and it was good enough then.