Use of Home-Type Aneroid Barometers
K. Tom Priddy
Extension Ag. Meteorologist
Many people have weather instruments in their home or place of
business to measure temperature, humidity and atmospheric
pressure. This publication deals with understanding atmospheric
pressure and the proper procedures for reading your home-type
WHAT IS ATMOSPHERIC PRESSURE AND WHY DOES IT CHANGE?
Air is really a mixture of gases: hydrogen, oxygen, water
vapor, etc. The percentage of water vapor in this mixture of gas
is not constant. One might think that as the amount of water
vapor increases in the air, the pressure of air increases.
However, the opposite is true. As the amount of water vapor
increases, air pressure decreases. This is because the water
vapor molecule is lighter relative to the other gas molecules.
Changes in water vapor is one reason why air pressure changes.
Air pressure is the weight of all the air above the earth's
surface pushing down. Dry air tends to pile up, creating higher
pressure. TV weathermen usually talk about domes of high pressure
(fair weather) and troughs of lower pressure (wet weather).
Since air pressure depends on how much air we have about us, the
higher we go up into the atmosphere, the less air above us and
the less air pressure pushing down. This explains why air
pressure changes with altitude.
Temperature changes are another reason for air pressure
changes. If we were to weigh equal containers of warm and cold
air, we would find that warm air has less weight than the same
size container of cold air. Cold air has greater density.
Unequal heating of the earth's surface causes belts and pockets
of warm and cold air all over the earth. As these masses of air
move over us, we can observe a change in barometric pressure. A
change in pressure implies a change in the weather. In addition,
the tendency of the pressure change (pressure falling, rising, or
steady) tells us more about the possible changes we can expect in
The major cause of pressure change is due to changes in the
total mass of air over a point at the surface. These changes
occur because of movement of air mass systems in the atmosphere.
WARM AND COLD FRONTS
When you see a TV weather map, located on the map are the
letters "H" (for high pressure) and "L" (for low pressure) and
usually warm and cold fronts. A trough (or depression) of low
pressure is often associated with a cold front, along which a mass
of relatively cold air advances like a wedge and pushes against a
receding mass of warmer, more moist air. In this event, the wind
will shift, and pressure will fall as the front approaches and
rise as it move away. Very often the advance of the cooler air
which attends the passage of a cold front brings shower-type
precipitation and a drop in temperature.
When a mass of relatively warm air advances against a
receding wedge of cold air, it constitutes a warm front. The
passage of a warm front is generally accompanied by a temperature
rise, and by certain associated changes in air pressure, wind and
cloudiness. Very frequently cloudiness and precipitation, often
of a steady nature, precede the movement of a front of this type
over a given location.
POUNDS, INCHES OR MILLIBARS
How will you measure air pressure? In pounds? In
millibars? It doesn't really matter. Standard air pressure at
sea level (sea level is a reference point) can be described as
14.7 pounds per square inch (psi), 29.92 inches of mercury (Hg),
or 1013.2 millibars. Pounds per square inch are easiest to
understand. It refers to the actual weight of the air on an
object. Most barometers, however, are marked in inches of Hg and
millibars, and this can be confusing. Inches, even in the
aneroid barometer, refer to inches of mercury. Millibars (mb) is
a unit of pressure used by weather bureaus all over the world.
Approximately 34 mb equals an inch of mercury. Since a small
change in pressure can trigger a big change in weather, millibars
have the advantage of measuring these changes more accurately.
TYPE OF BAROMETERS
In terms of atmospheric pressure, there are two major types
of instruments used: mercurial and aneroid barometers, with
the aneroid type being the less expensive of the two and usually
found in the home. The mercurial barometer consists of a
vertical hollow glass tube, closed at the top, sitting in a pool
of mercury. With a vacuum in the glass tube, a column of mercury
will rise above the pool to a height equal to atmospheric
pressure in inches of mercury. As greater atmospheric pressure
is exerted on the pool of mercury, the column will rise higher in
the tube. At standard sea level pressure, the mercury is pushed
up into the tube about 30 inches.
The aneroid barometer measures air pressure without the use
of liquid. It consists of a small metal container with some of
the air removed. When the air pressure changes, the sides of the
container move to indicate the change. On most aneroid
barometers, a lever mounted on the end of the container is
attached to a pointer. As the lever dips and rises, the pointer
indicates the amount of pressure change.
SETTING YOUR ANEROID BAROMETER
One of the biggest problems after purchasing a barometer is
knowing how to set your barometer to get proper pressure
readings. The first thing you must do is call your local weather
office, airport, radio or television station to find out the
current barometric pressure reduced to sea level. This should be
done during a high pressure period, because there is less change
in pressure across the state. On the back of your aneroid
barometer there should be a screw to make this adjustment.
Making small clockwise/counterclockwise turns of the screw should
increase/decrease the pointer of the barometer to the proper sea-
A barometer can only measure atmospheric pressure. It does
not indicate the character of the weather that exists at any
place or time, nor does it, by itself, forecast the weather.
However the old saying, "As the barometer tends, so goes the
weather," has merit. As the barometer increases, atmosphere
pressure increases; implying improving weather conditions. As
the barometer decreases, atmosphere pressure decreases; implying
deteriorating weather conditions.
Barometric readings, taken at suitable intervals each day and
considered in conjunction with observations of clouds,
temperature and winds, can be useful in making accurate short
term weather predictions.