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

aneroid barometer.


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 weather.

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.


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.


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.


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.


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-

level pressure.


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.