May 21, 2002
To my friends
Frank and Dolores.
Since your cabin is located in an undulating and mountainous
terrain with great variety of water absorbing surfaces, hence variable weather conditions
and evaporation, I thought a barometer would be of a guiding help determining
your ventures into the hills and lakes
Frank and
Pemichangan Lake Trout April 26/02
Aneroid
Barometer.
Most barometers are of the aneroid type and function without
liquid. The aneroid barometer goes back
to 1843; it consists of a small metal box, almost totally evacuated of
air. One side is immovable, and the
opposite side is connected to a strong spring to keep the box from
collapsing. The movable side will expand
if the air pressure decreases and will compress if the air pressure
increases. A pointer indicates the
position of the movable side.
At sea level, atmospheric pressure is equal to about 15 lb per sq
in, or 29.92 in. of mercury. This is equivalent to 101.3 kilopascals, the
pressure unit meteorologists now use, besides millibars.
Standard sea-level pressure, 1,013.25 millibars, is equivalent to
the pressure exerted by a column of mercury 760 mm high
The atmospheric pressure on Earth at sea level = 1,000 millibars.
If you lived on Mars you would need a pressure suit at 7.5 millibars. Frank,
record your best fishing and hunting days in millibars including the moon phase
and become the fishing-forecasting guru on Pemichangan Lake. Recording inclement
weather and storms is a really interesting hobby. I am sure Dolores will also enjoy the study.
In a very interesting book by James Gleick
"CHAOS Making A New Science" deals with the phenomena of weather in
extraordinary detail and much scientific thought. Also this book is not really
about the weather. Nevertheless an enjoyable read about nonlinearity and
flowing geometries that sustain complex systems like the weather, the heart and
the mind. Dolores you will perhaps know of the book? Also it is a bit dated.
Frank, you need to calibrate the
barometer with the one at the Ottawa Airport control tower and note their
elevation. There is a small adjustment screw in the back inside the housing.
The weatherman will give you a conversion factor for your elevation. You can do
this over the phone. That is the way
I set up mine. The elevation of your lake is 168m. Your Cabin elevation is about 183m + or minus at
the main floor. (Between the first two contour lines.) I had thought that the lake
elevation was much higher in the Gatineau hills.
Your aneroid barometer should be checked from time to time against
a mercury barometer to verify calibration.
Enjoy your barometer from your
friend
Fred.
Some metrological science background.
Stratification and Static Stability
Temperature distribution is not alone in determining the state of
the atmosphere. Pressure and density
also are important. Atmospheric pressure, usually expressed in units called
millibars, is the force that the total mass of air in an imaginary vertical
column exerts on a given horizontal area of the Earth's surface.
Standard sea-level pressure, 1,013.25 millibars, is equivalent to
the pressure exerted by a column of mercury 760 mm high. If, like water, the atmosphere were
incompressible, pressure would decrease uniformly with height, and the
atmosphere, like the ocean, would have a definite upper limit. In reality the atmosphere is compressible;
that is, density (mass per unit volume) is proportional to pressure.
This relationship, called BOYLE'S LAW, implies that density
decreases with height in the atmosphere:
as height increases, less mass remains above a given point; therefore
less pressure is exerted. At sea level
the density of air is about 1 kg per cu m (8 oz per cu ft). Both pressure and
density decrease by about a factor of 10 for every 16 km (10 mi) increase in
altitude.
Density does not depend only on pressure; for a given pressure, it
is inversely proportional to temperature.
This relationship, known as Charles's law, implies that the depth of an
air column bounded by two constant-pressure surfaces will increase as the
temperature in the column increases.
Thus the vertical distance over which pressure decreases to half of its
surface value ranges from about 5,800 m (19,850 ft) in the tropics to 5,100 m
(16,575 ft) near the poles.
When an AIR MASS rises, it expands (because of the reduction in
pressure). In expanding, it must work against the pressure force exerted by the
surrounding air. According to the
principle of conservation of energy (the first law of thermodynamics), the work
done in expansion must be balanced by an equal reduction in the internal energy
of the air mass.
Since the internal energy is proportional to temperature, an
expanding air mass must cool.
Conversely, an air mass that is compressed must warm. Temperature
changes caused by compression or expansion of a gas in the absence of heat
exchange with the surroundings are called adiabatic changes.
The process of adiabatic cooling or heating is essential to an
understanding of vertical convection in the atmosphere. A mass of dry air rising adiabatically in
the atmosphere cools at a rate of about 10 deg C per km (17 deg F per mi). Since, on the average, the decrease of
temperature with height (the LAPSE RATE) in the troposphere is only 6.5 deg C
per km (11 deg F per mi), an adiabatic ally ascending air mass becomes cooler
and denser than its surroundings and tends to sink back toward its original
level.
Thus the mean vertical temperature structure is said to be
statically stable with respect to a dry adiabatic displacement. In regions such as DESERTS, where the air
near the ground is strongly heated, the lapse rate of temperature in the lower
troposphere may exceed 10 deg C per km (17 deg F per mi). In such situations an air mass displaced
vertically upward will become warmer than its surroundings and will be
accelerated farther upward. Such
statically unstable conditions result in vigorous mixing and upward heat
transport, which tends to reduce the lapse rate toward the average value
observed elsewhere. An in house
barometer can point out these phenomena.
There is a lot more to the weather, but let us leave that to the
weatherman who uses some very sophisticated instruments to forecast the
weather. Due to the weather's dynamic
geometry forecasting is at the very best of times sketchy and general. Weather
can in a very short time change from static to unstable in a fringe region of a
pattern. This change may not be detectable to a forecaster.
So the old saying, " of what you see is what you get" is
not too far out of line. I remember a
high-pressure system sitting in the Gulf of Alaska in 1993 for a very long time,
apparently immoveable, presenting the Pacific North coastal regions with a near
rainless dessert climate. Climatologists were short of answers why the
surrounding weather pattern was unable to move this system. A question arises,
Where does the seasonal average local climate come in, or is there one? The answer is obvious, there is non.
We are near the
end of May and only a few birds have arrived from the south to our central
Alberta region and ice is still on the lake with temperature much below AVERAGE.
Please remember that this little write up on the weather scratches only on
the matter in the most miniscule way. Consider that the many satellites
send multi gigabytes of data on a continuous basis to earthbound monitors and
weather maps are updated every 30 minutes.
But the time it takes to update the map the weather has changed and the new map is only hindsight. Much like the stock market forecast.
