A barometer is a scientific instrument used in meteorology
to measure atmospheric pressure. Pressure tendency can forecast short term
changes in the weather. Numerous measurements of air pressure are used within
surface weather analysis to help find surface troughs, high pressure systems,
and frontal boundaries.
Evangelista Torricelli is universally credited with inventing
the barometer in 1643.
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.
In 1646, Blaise Pascal along with Pierre Petit, had repeated
and perfected Torricelli's experiment and went even further to test the
mechanical theory. If, as suspected by mechanical philosophers like Torricelli
and Pascal, air had lateral weight, the weight of the air would be less at
higher altitudes. Therefore, Pascal wrote to his brother-in-law, Florin Perier,
who lived near a mountain called the Puy de Dome, asking him to perform a
crucial experiment. Perier was to take a barometer up the Puy de Dome and make
measurements along the way of the height of the column of mercury. He was then
to compare it to measurements taken at the foot of the mountain to see if those
measurements taken higher up were in fact smaller. In September 1648, Perier
carefully and meticulously carried out the experiment, and found that Pascal's
predictions had been correct. The mercury barometer stood lower the higher one
went
Aneroid Barometers
An aneroid barometer, invented in 1843 by French scientist
Lucien Vidie uses a small, flexible metal box called an aneroid cell (capsule),
which is made from an alloy of beryllium and copper. 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.
Changes in atmospheric pressure are one of the most commonly used ways
to forecast changes in the weather because weather patterns are carried around
in regions of high and low pressure. Weather maps use lines of equal pressure
called isobars to indicate areas of equal pressure.
A slowly rising atmospheric
pressure, over a week or two, typically indicates settled weather that will
last a long time. A sudden drop in atmospheric pressure over a few hours often
forecasts an approaching storm, which will not last long, with heavy rain and
strong winds.
By carefully watching the pressure on a barometer, you can forecast
local weather using these simple guidelines:
·
Decreasing barometric pressure indicates storms,
rain and windy weather.
·
Rising barometric pressure indicates good, dry,
and colder weather.
·
Slow, regular and moderate falls in pressure
suggest a low pressure area is passing in a nearby region. Marked changes in
the weather where you are located are unlikely.
·
Small rapid decreases in pressure indicate a
nearby change in weather. They are usually followed by brief spells of wind and
showers.
·
A quick drop in pressure over a short time indicates
a storm is likely in 5 to 6 hours.
·
Large, slow and sustained decreasing pressure
forecasts a long period of poor weather. The weather will be more pronounced if
the pressure started rising before it began to drop.
·
A rapid rise in pressure, during fair weather
and average, or above average pressure, indicates a low pressure cell is
approaching. The pressure will soon decrease forecasting poorer weather.
·
Quickly rising pressure, when the pressure is
low, indicates a short period of fair weather is likely.
·
A large, slow and sustained rise in pressure
forecasts a long period of good weather is on its way.
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