68 SAILMAGAZINE.COM AUGUST 2007

FOR CENTURIES, THE BAROMETER

was the sailor’s most importantweather-forecasting tool. Not these days,though. Weather information has becomeso readily available via radio, VHF, the In-ternet, even your cell phone, that on manyboats the poor old barometer has beenrelegated to purely decorative duty. This isa shame, because the barometer is no lessuseful to the voyager today than it was 300years ago. When its measurements of at-mospheric pressure are used in tandem with

your observations of wind direction andstrength, it can be a reliable (though notinfallible) short-term weather forecaster, andit doesn’t depend on a power source.

Back in 1643, a contemporary and com-patriot of Galileo Galilei, Evangelista Tor-ricelli, invented the barometer whentrying to measure the weight of the at-mosphere. He filled a glass vial about ayard long with mercury and placed itopen-end-down into a reservoir of mer-cury. As some of the mercury dropped into

the reservoir, a vacuum was formed at thetop of the tube. Torricelli observed thatrising atmospheric pressure forced moremercury up from the reservoir into the tube,while declining pressure allowed themercury level to drop. He and otherphysicists soon worked out that these fluc-tuations could be linked to changes inweather conditions.

Not many of us have room in our sa-loons for a 30-inch glass tube packedwith a toxic element, so it’s just as well thatthe aneroid barometer was invented in 1843.An aneroid (Greek for “without liquid”)is a flexible metal canister enclosing apartial vacuum. Lower atmospheric pres-sure lets the canister expand; higher pres-sure makes it contract. Springs or leversattached to the aneroid drive the pointeron the face of the barometer that indicateschanging pressure.

Here in the U.S., the National WeatherService ignores metrics and still suppliesground-level pressure readings in inchesof mercury, but is in step with the rest ofthe world in reporting air pressure aloftonly in millibars (also known as hec-topascals) to avoid confusing airline pi-lots. All weather-service barometer readingsare corrected to sea level. There’s a goodreason for sailors to give inches the heave-ho and think in terms of millibars; that’sthe unit used on the weather contours inweather maps. It is unambiguous, and itavoids the use of decimal points, whichcan be lost in a weatherfax transmissionor on a computer screen.

It doesn’t really matter, though, whetherthe gradations on the face of your barom-eter are in inches or millibars or both; what

The venerable barometer still has a place on boardBY PETER NIELSEN

»

PH

OT

OS

CO

UR

TE

SY

OF

WE

EM

S A

ND

PL

AT

H

Sea-level air pressure is about 14.7 pounds per square inch,which is equivalent to a column of mercury 29.92 inches high. Foreach 1,000 feet above sea level, the pressure drops by about 1 inchof mercury, but this is academic for us sailors (unless you’re ona lake in the Rockies). Over the course of a year, your barometerreadings will range over about 1.5 inches, from a low of circa 29inches (983 millibars) to a high that could be as much as 30.5 inches(1035 mbar); however, pressure plunges to as low as 27 incheshave been recorded during extreme weather events. Averagesea-level pressure is 1013 mbar.

GAME OF INCHES BAROMETER LORE

“Quick rise after lowPortends a stronger blow”

“Long foretold, long lastQuick notice, soon past”

“When the wind backs and the weather glass fallsPrepare yourself for gales and squalls”

The barometerdoes not predictweather, so don’tput any faith in thewords on the face ofthe instrument. Theyhark back to the days whenweather forecasting was avery inexact science

Barometer 101

The innerworkings

are beautifulin their

simplicity, whichis why many

barometer makers makesure you can see them

The gradations on theinstrument face will be

in millibars (akahectopascals)and (in North

America) ininches

To accurately measure trends,you should set the movablepointer (lower of the two)in line with the pressureindicator each timeyou take a reading

» WEATHER FORECASTINGKnow-How

AUGUST 2007 SAILMAGAZINE.COM 69

does matter is how fast and in which di-rection the pointer moves. The lower theair pressure, the worse the weather in storefor you will be, and vice versa; the fasterthe pressure change, the more severe theweather change will be. Inthe 17th and 18th cen-turies, as mariners began tocarry first “storm glass” wa-ter barometers and thenmercury barometers onboard their ships, theyshared their scrupulousrecordings of pressure vari-ations and wind strength and direction,and a cause-and-effect consensus gradu-ally evolved that enabled sailors to pre-dict short-range weather changes withconsiderable accuracy. The table reproduced

on this page is a typical example.You can safely ignore the words on

the barometer face that indicate “fair,”“change,” or “rain.” They merely note typ-ical conditions attendant to that particu-

lar pressure range. In orderto get an idea of the com-ing weather, you need tokeep an eye on the trend—rising or falling pressure.Passagemakers should log thepressure reading hourly, forthe rate of change is a goodindicator of the severity—

or otherwise—of the weather to come. Ionce owned a boat that had a barograph,which recorded the pressure variations ona sheet of paper over the course of a week.Now it is increasingly common to see

electronic barometers on board boats,and many of these can store trend infor-mation. They use detection cells—a typeof capacitor whose resistance is affected bychanges in air pressure. They are alsoconsiderably cheaper than barographs,which cost from several hundred dollarsto north of a thousand. I wish I’d kept mine.

If you sail mainly around the coast, whereweather information is both readily avail-able and accurate, your barometer prob-ably resides forgotten. Offshore andbluewater sailors still have a healthy re-spect for the instrument; at sea, there isalways a place for the old ways. F

» WEATHER FORECASTING

BAROMETERREADING(INCHES)

BAROMETERACTION

RISING

STEADY

FALLING

RISING

STEADY

FALLING

RISING

STEADY

FALLING

CONT’D FAIR FOR 24 HOURSCONT’D FAIR FOR 12 HOURS FAIR WEATHER CLEAR AND COOL

Lower temperatures

CONT’D FAIR FOR 48 HOURS CONT’D FAIR FOR 12 HOURS RAIN WITHIN 24 TO 48 HOURS

CONTINUED FAIR No decided change in temperature No decided change in temperature Lower temperatures

CONT’D FAIR FOR 24 HOURS FAIR FOR 6 TO 12 HOURS RAIN WITHIN 12 HOURS RAIN WITHIN 24 TO 48 HOURS

Slowly rising temperatures Rising temperatures Wind increasing force…rising temp

FAIR FOR 48 HOURS FAIR FOR 48 HOURSFAIR CLEAR WITH COLDER WEATHER

Lower temperatures Lower temperatures

CONTINUED FAIR WEATHER FAIR FOR 12 HOURS RAIN WITHIN 12 TO 24 HOURS NO CHANGENo decided change in temperature

FAIR FOR 12 TO 24 HOURS RAIN IMMINENT

RAIN WITHIN 6 TO 12 HOURS RAIN WITHIN 12 HOURS

No decided change in temperature Wind increasing force…rising temp

CLEARING WITHIN A FEW HOURSCLEARING WITHIN 6 HOURS CLEARING WEATHER CLEARING AND COOLER

Lower temperatures

CONT’D THREATENING WEATHERCONT’D STORMY WEATHER CONT’D RAIN OR NO CHANGE

RAINY WEATHERLower temperatures Clearing in 12 to 24 hours

CHANGING WEATHERINCREASING RAIN SEVERE STORM IMMINENT HEAVY RAIN

Clearing within 12 hours Clearing within 24 hours Severe northeast gale, colder

30.20OR

HIGHER

30.20to

29.80

29.80OR

LOWER

WIND DIRECTION

A B C

NW QUADRANT SW QUADRANT SE QUADRANT NE QUADRANT

The barograph (above) records readings,allowing you to keep an eye on trends; the electronic barometer (left) does the same thing

This chart came with an old barometer I bought. It assumes an average windvelocity of 10mph and is only for use in North American waters. I compared it toactual weather forecasts last winter and found it was surprisingly accurate. Notethat diurnal variations of up to 3mbar are common; before you use this table youshould observe your barometer’s actions over a 24-hour period of stable weatherto account for them. This table would look good laminated and posted at the navstation: go to sailmagazine.com to download a PDF.

WEATHER CHART