Water-based barometers
The concept that decreasing atmospheric pressure predicts
stormy weather, postulated by Lucien Vidie, provides the theoretical basis for
a weather prediction device called a "storm glass" or a "Goethe
barometer" (named for Johann Wolfgang Von Goethe, the renowned German
writer and polymath who developed a simple but effective weather ball barometer
using the principles developed by Torricelli).
The weather ball barometer consists of a glass container
with a sealed body, half filled with water. A narrow spout connects to the body
below the water level and rises above the water level. The narrow spout is open
to the atmosphere. When the air pressure is lower than it was at the time the
body was sealed, the water level in the spout will rise above the water level
in the body; when the air pressure is higher, the water level in the spout will
drop below the water level in the body. A variation of this type of barometer
can be easily made at home.[8]
Mercury barometers
A mercury barometer has a glass tube with a height of at
least 84 cm, closed at one end, with an open mercury-filled reservoir at the
base. The weight of the mercury creates a vacuum in the top of the tube.
Mercury in the tube adjusts until the weight of the mercury column balances the
atmospheric force exerted on the reservoir. High atmospheric pressure places
more force on the reservoir, forcing mercury higher in the column. Low pressure
allows the mercury to drop to a lower level in the column by lowering the force
placed on the reservoir. Since higher temperature at the instrument will reduce
the density of the mercury, the scale for reading the height of the mercury is
adjusted to compensate for this effect.
Torricelli documented that the height of the mercury in a
barometer changed slightly each day and concluded that this was due to the
changing pressure in the atmosphere.[1] He wrote: "We live submerged at
the bottom of an ocean of elementary air, which is known by incontestable
experiments to have weight".
The mercury barometer's design gives rise to the expression
of atmospheric pressure in inches or millimeters (torr): the pressure is quoted
as the level of the mercury's height in the vertical column. 1 atmosphere is
equivalent to about 760 millimeters of mercury.
Design changes to make the instrument more sensitive,
simpler to read, and easier to transport resulted in variations such as the
basin, siphon, wheel, cistern, Fortin, multiple folded, stereometric, and
balance barometers. Fitzroy barometers combine the standard mercury barometer
with a thermometer, as well as a guide of how to interpret pressure changes.
Fortin barometers use a variable displacement mercury cistern, usually
constructed with a thumbscrew pressing on a leather diaphragm bottom. This
compensates for displacement of mercury in the column with varying pressure. To
use a Fortin barometer, the level of mercury is set to the zero level before
the pressure is read on the column. Some models also employ a valve for closing
the cistern, enabling the mercury column to be forced to the top of the column
for transport. This prevents water-hammer damage to the column in transit.
On June 5, 2007, a European Union directive was enacted to
restrict the sale of mercury, thus effectively ending the production of new
mercury barometers in Europe .
Aneroid barometers
See also: Barograph
Old aneroid barometer
Modern aneroid
barometer
An aneroid barometer, invented by the French 19th century
engineer and inventor Lucien Vidie, uses a small, flexible metal box called an
aneroid cell. This aneroid capsule (cell) is made from an alloy of beryllium
and copper.[9] The evacuated capsule (or usually more capsules) is prevented
from collapsing by a strong spring. Small changes in external air pressure
cause the cell to expand or contract. This expansion and contraction drives
mechanical levers such that the tiny movements of the capsule are amplified and
displayed on the face of the aneroid barometer. Many models include a manually
set needle which is used to mark the current measurement so a change can be
seen. In addition, the mechanism is made deliberately "stiff" so that
tapping the barometer reveals whether the pressure is rising or falling as the
pointer moves.
Barographs
A barograph, which records a graph of some atmospheric
pressure, uses an aneroid barometer mechanism to move a needle on a smoked foil
or to move a pen upon paper, both of which are attached to a drum moved by
clockwork.[10]
[edit]
More unusual barometers
There are many other more unusual types of barometer. From
variations on the storm barometer, such as the Collins Patent Table Barometer,
to more traditional looking designs such as Hooke's Otheometer and the Ross
Sympiesometer. Some, such as the Shark Oil barometer,[11] work only in a
certain temperature range, achieved in warmer climates
An unusual location of a barometer is its location in the
new Samsung Galaxy Nexus smartphone.[12]
[edit]
Applications
See also: Surface weather analysis and Weather forecasting
Digital graphing
barometer.
Barograph using five stacked aneroid barometer cells.
Using barometric pressure and the pressure tendency (the
change of pressure over time) has been used in weather forecasting since the
late 19th century.[13] When used in combination with wind observations,
reasonably accurate short-term forecasts can be made.[14] Simultaneous
barometric readings from across a network of weather stations allow maps of air
pressure to be produced, which were the first form of the modern weather map
when created in the 19th century. Isobars, lines of equal pressure, when drawn
on such a map, gives a contour map showing areas of high and low pressure.[15]
Localized high atmospheric pressure acts as a barrier to approaching weather
systems, diverting their course. Atmospheric lift caused by low-level wind
convergence into the surface low brings clouds and potentially
precipitation.[16] The larger the change in pressure, especially if more than
3.5 hPa, the larger the change in weather can be expected. If the pressure drop
is rapid, a low pressure system is approaching, and there is a greater chance
of rain . Rapid pressure rises, such as in the wake of a cold front, are
associated with improving weather conditions, such as clearing skies.[17]
Compensations
Temperature
The density of mercury will change with temperature, so a
reading must be adjusted for the temperature of the instrument. For this
purpose a mercury thermometer is usually mounted on the instrument. Temperature
compensation of an aneroid barometer is accomplished by including a bi-metal
element in the mechanical linkages. Aneroid barometers sold for domestic use
typically have no compensation.
[edit]
Altitude
As the air pressure will be decreased at altitudes above sea
level (and increased below sea level) the actual reading of the instrument will
be dependent upon its location. This pressure is then converted to an
equivalent sea-level pressure for purposes of reporting and for adjusting
aircraft altimeters (as aircraft may fly between regions of varying normalized
atmospheric pressure owing to the presence of weather systems). Aneroid
barometers have a mechanical adjustment for altitude that allows the equivalent
sea level pressure to be read directly and without further adjustment if the
instrument is not moved to a different altitude.
