Understanding how
a Barometer works...
Your barometer is
just as accurate a weather forecaster as the TV
meteorologist you watch with the news. Under most
conditions it forecasts weather for the 12 to 24
hours ahead. Its mission is to measure air or atmospheric
pressure and indicate its rising or falling.
The pressure is
shown on your barometer's dial, usually expressed
in "inches" referring to "inches
of mercury" (inch Hg). The early barometers
measured the pressure by the rise or fall of a column
of mercury. This mercury barometer was invented
back in 1643 by a pupil of Galileo named Evangelista
Toricelli.
Meteorologists use
"millibars" in charting atmospheric pressure
and your barometer has a second scale or ring which
reads in millibars (mb), or "hectopascals"
(hPa). The latter is used in some countries as a
unit of pressure. Standard air pressure at standard
elevation (sea level) at 15°C and 45° latitude is
1013 hPa or 29.92 inches of mercury.
Most dial type barometers
employ an air pressure sensor with a limited range
of sensitivity. Generally the working altitude range
covers only about 3000 to 4000 feet. So be sure
to choose a barometer that has a sensor range factory
set for use at your elevation (e.g. 0-3000 ft.,
3000-7000 ft. or 7000 ft and above). Masterline
Group has wide selection of barometers factory set
for use in altitudes above 3,000 feet.
> Click here to view
our altimeter / barometer products.
Your barometer's
reading may not exactly coincide with a broadcast
due to difference in location, time of reading and
other factors.
Note that when using
a weather barometer (usually with a scale range between
28 and 31 inches Hg) at a location above sea level,
the reading must be corrected back to sea level. This
is automatically accomplished when you initially match
your barometer's reading to that reported by local
TV or radio weather forecast. These reported readings
have already been "corrected" to sea level,
thus eliminating any pressure differences due to elevation.
The chart below illustrates how to interpret your
barometer's inches of mercury reading. This provides
a reasonably accurate forecast for the next 12 to
24 hours.
Barometric Reading
Forecast Over 30.20"
> Rising or steady - Continued fair
> Slowing falling - Fair
> Rapidly falling - Cloudy, Warmer
29.80" to 30.20"
> Rising or steady - Same as present
> Slowing falling - Little change
> Rapidly falling - Precipitation likely
Under 29.80"
> Rising or steady - Clearing, cooler
> Slowing falling - Precipitation
> Rapid falling- Storm
NOTE:
The words, "stormy",
"rain", "change", "fair",
"dry", etc. are traditional carry overs
from the days when weather forecasting was less
of a science, and should not be used in predicting
weather changes.
Do not expect large changes in the position of the
indicating hand. Normal daily changes may be in
a range of .02 to .10 of an inch on the barometer
scale.
Changes in pressure readings will be of greater
magnitude in winter months than in summer and will
also depend on location and altitude.
