Download AEN-46 Use of Home-Type Aneroid Barometers

departmentof agriculturalengineering
t]se of
Home,Type
Aneroid Barometers
by Tom Priddy and Mike Thompson
universityof kentucky. collegeof agriculture. cooperative
extensionservice
agriculture . home economics . 4h . development
pressurechange(pressurefalling, rising,or steady)tells us
Many peoplehaveweatherinstrumentsin their home
more about the possiblechangeswe can expect in the
humidity and
to measuretemperature,
or placeof business
atmosphericpressurel.This publicationdealswith under- weather.
The major causeof pressurechangeis due to changes
standingatmosphericpressureand the proper procedures
in the total massof air over a point. Thesechanges
for readingyour home-typeaneroidbarometer.
occur
becauseof movementsof high and low pressure
systemsin
the atmosphere.
WHAT IS ATMOSPHERICPRESSURE
AND WHY DOESIT CHANGE?
oxygen,
Air is really a mixture of gases-hydrogen,
water vapor, etc. The percentageof water vapor in this
mixture of gas is not constant.One might think that as
of
in air, the pressure
the amount of water vapor increases
However,the oppositeis true. As the amount
air increases.
This is
air pressuredecreases.
of water vapor increases,
becausethe water vapormoleculeis lighterrelativeto the
in water vaporis one reason
other gas molecules.Changes
changes.
why air pressure
Air pressureis the weight of all the air abovethe
earth's surfacepushingdown. Dry air tends to pile up,
usuallytalk about
TV weathermen
creatinghigherpressure.
(fair weather)and troughsof lower
domesof high pressure
dependson how
pressure
(wet weather).Sinceair pressure
much air we have above us, the higherwe go up into the
atmosphere,the lessair aboveus and the les air pressure
with
changes
pushingdown. This explainswhy air pressure
altitude(seeTable l).
Temperaturechangesare another reason for air
pressurechanges.If we were to weighequal containersof
warm and cold air. we would find that warm air hasless
weightthan the samesizecontainerof cold air. Cold air has
geater density. Unequal heating of the earth's surface
causesbelts and pocketsof warm and cold air all over the
earth.As thesemasses
of air moveoverus,we canobservea
implies
A changein pressure
changein barometricpressure.
a changein the weather.In addition,the tendencyof the
lAtmospheric pressure,
air presswe and barometric pre$ure
are interchangeable.
'ooll.rirli|c
l.f
E\tension
uork,
r)f (looperativc
Exiension
acts of NIay B and
Sefvice, University
1000
E
800
L
600
E
400
V
200
Sea
Level
29.00
29.25
29.50
29.75
(in Hg)
Pressure
Table l. Presure-heightrelationship
Elevation
(ftl
Pressure
(in Hs)
Pressure
(mb Hg)
SeaLevel
200
400
600
800
1000
5000
50000
29.92
29.72
29.52
29.32
29.12
28.92
24.92
3.44
1013.2
1006.4
999.6
992.8
986.0
979.2
843.1
116.6
WARM AND COLD FRONTS
Whenyou seea TV weathermap,locatedon the map
the
letters "H" (for high pressure)and "L" (for low
are
June 30,1914, in cooperation with the U.S. Department of Agricultu|c.
oI Kentucky College oi Agriculture.
AEI!J-,,6
pressure)and usually warm and cold fronts. A trough (or
depression)of low pressureis often associatedwith a cold
front, along which a mass of relatiraly cold air advances
like a wedp and pushesagainsta recedingmassof warmer,
more moist air. In this event, the wind will shift, and
pressure will fall as the front approaches and rise as it
moyesaway. Very often the advanceof the cooler air which
attends the passageof a cold front brings shower-type
precipitationand a drop in temprature.
When a massof relatiwly warm air advancesagainsta
recedingwedgeof cold air, it constitutesa warm front. The
passageof a warm front is generally accompanied by a
temperature rise, and by certain asociated changesin air
pressure,wind and cloudiness.Very frequently cloudiness
and precipitation, often of a steady nature, precedethe
movementof a front of this type over a givenlocation.
POUNDS, INCHES OR MILUBARS
How will you measureair pressure?In pounds? In
millibars?It doesn't really matter. Standardair pressureat
sea level (sealeral is the referencepoint) can be described
as 14.7 pounds per square inch (PSI), 29.92 inches of
mercury (Hg), or 1013.2 millbars.Poundsper squareinch
are easiestto 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 baronreter,refer to inches of
mercury. Millibars (mb) is a unit of pressureused by
weather bureausall over the world. Approximately 34 mb
equalsan inch of mercury. Sincea small changein pressure
can trigger a big change in weather, millibars have the
advantageof nrasuring thesechanps more accurately.
rises, the pointer indicates the amount of pressure chanp
(seeFigure 2).
i;iHi"r
I
II
I
Figurel.
LOIV PRE6SUFE
HIGH PhESIURE
TYPES OF BAROMETERS
In terms of atnnspheric pressure,there are two major
types of instruments used: mercurial and aneroid type
barometers,with the aneroid type being the lessexpensive
of the two and usually found in the home. The mercurial
baronrter consistsof a vertical hollow glasstube, closedat
the top, sitting in a pool of mercury.With a vacuumin the
glasstube, a column of nrercurywill rise abovethe pool to
a height equal to atmospheric pressurein inches of
nrercury.As greateratrtosphericpressureis exertedon the
pool of rnercury,the column will risehigherin the tube. At
standardsea level pressure,the mercury is pushedup into
the tube about 30 inches(seeFigure | ).
The aneroid barometer nreasuresair pressurewithout
the use of liquid. It consistsof a small metal container with
some of the air removed.When the air pressurechanges,the
sidesof the container rnve 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
Figure2.
SETTING YOUR ANEROID BAROMETER
One of the biggest problems after installing a
barometer is knowing how to set your barometerto get
proper pressurereadings.The first thing you must do is call
your local weatheroffice, airport, radio or televisionstation
to find out the current barometric pressurereducedto sea
level. This should be done during a high pressureperiod,
becausethere is lesschangein pressureacrossthe state.On
the back ofyour aneroidbarometerthere shouldbe a screw
to makethis adjustment.
EATHER PREDICTIONS
A barometercan only measureatmosphericpressure.
It doesnot indicatethe characterof the weatherthat exists
AEN4l6
pressure tendency, and wind direction are known.
Remember,wind direction is the direction from which the
wind is blowing. A north wind comes from the North, a
south wind comesfrom the Sbuth.
at any place or time, nor does it, by itself, forecastthe
\rreather. However, barometric readings,taken at suitable
intervals each day and consideredin conjunction with
observationsof clouds,temperatureand winds, can be useful in making short term weatherpredictions.
BAROMETRIC WEATHER CHART
Table 2 is a Barometric Weather Chart which may be
used in weather forecastingwhen the barometricpressure,
Teblc 2.-Berometric wcather chart for wcather forecarting
(Berornctcr rceding murt bc adjurtcd to sca level prcrurc)
Earomtic
pcrsurc actbn
I
;;*---
tr
S W qudrent
NI
S E quedrrnt
ry
N E qrndrrnt
=-=-=-Jr==-.==-'
Contd.fair 24 hr
lower temp.
I
Contd.fair l2 hr
30.20
or
Steady
highet
Contd.fair 48 hr
Littlc temp.chg.
Contd.fair l2 hr
Uttlc temp.chg.
6
Rainin 244E hr
Contd.fair
lower temp.
8
30.20
Falling
or
higher
Contd.fair 24 hr
Slowly risingtcmp.
9
Fair 6.12hr
Risingtcmp.
l0
Rainwithin 12 hr
Risingtemp.
Windincreasing
ll
Rainin 24.4Ehr
30.20
to
Rising
29.80
Fair 48 hr
lower temp.
Fair 48 hr
lower temp.
Fair
Clear
Cooler
30.20
Contd.rainy
or
Rising
higher
A
Wind Dircction
N Wqur&rnt
to
Falling
29.80
;;----or
:i:::
29.80
or
lower
Rising
_ __
Steady
29.80
or
Falling
lower
EXPLANATION:
J
4
t2
4
Fair l2 hr
Little temp.chg.
6
Rainin l2-24 hr
7
E
Fair l2-24hr
Little temp.chg.
9
Rain probable
Rainin 6-12hr
fusingtemp.
Windincreasing
lt
Rainwithin 12 hr
Clearing,few hr
lower temp.
I
Clearingby 6 hr
Clearing
Clearing,
Cooler
4
Tlueatening,
Lnwer temp.
Contd. stormv
5
30.20
Clear
Cool
3
I
Steady
29.80
to
)
Fair
)
Changing
9
l0
)
3
Contd.rain
or no change
Rainy,clearing
n l2-24hr.
8
Severestorm
imminent,
Clearingby 24 hr
ll
Heavyrain,
Scrtre N E gde
12
6
Increasing
rain,
Cleuingby l2 hr
l0
No changc
To find the weather prediction look at the right of the barometric pressure and action
into the square beneath the wind direction. Example:
C7 means "Continued rain or no
change" because the square with this prediction was found from the barometric pressure
"29.80 or lower", "Steady", and wind from the "southeast quadrant".
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telated matterc should be directed to Equal OpportunityOllice, College ol Agriculturc,University ol Kentucky Foom S-105,Agticultural Science Building-Notth,
Lexinglon,Kentucky 40546.
5M-+t0