ratio of 10:1. But this generalization can be tricky. If the weather system is super-cold,
such as an Arctic air mass over Canada and the northern United States, the below-
freezing temperatures might create more than 10 inches of snow per inch of rain.
Meteorologists often call this the “fluff factor,” because the snow seems “fluffier” due
to the fact that there is more air between the snow crystals at much colder tempera-
tures. In fact, in very cold air the snow-to-liquid equivalent ratio can be 15, 20, or even
404 30 to 1. (For more about weather and math, see “Math in the Natural Sciences.”)
What do the numbers on an aneroid barometer signify?
T
he non-liquid aneroid barometer measures air pressure. As the atmospheric
pressure changes because of storm systems (or lack of storms), the instru-
ment records the changes. (Because atmospheric pressure changes with dis-
tance above or below sea level, a barometer can also be used to measure alti-
tude.) The aneroid barometer contains a small capsule that acts like a bellows,
but with the air removed. When the air pressure increases, the sides of the cap-
sule are pushed in and the connected needle rises (moves clockwise). When the
air pressure decreases or falls, the capsule’s sides puff out and the needle moves
in a counterclockwise direction.
The numbers on a common barometer range from about 26 to 31, with divi-
sions of 10 or more in between each number. A needle (actually a hand similar to
a clock hand) points to the numbers on the barometer and moves in response to
the changing air pressure. These numbers are based on the principle that atmos-
pheric pressure supports 30 inches (76.2 centimeters) of liquid mercury in a
tube with one end sealed; and this information is based on the mercury barome-
ter, the first type ever made.
How is an aneroid barometer read? Well, a falling hand on a barometer indi-
cates a low pressure system is on the way with poor weather (usually a storm
with snow or rain); a steady barometer means there will be no changes with the
ongoing pressure system; and a rising barometer means high pressure and fair
weather. An even higher reading, around 31, means an extremely dry atmos-
phere. The timing of the barometric change is also telling: A change of a degree
either way in a few hours means that the weather will change quickly; a slow
change of 0.3 or so a day indicates weather arriving in 12 to 24 hours. A quick
rise in the barometer also often indicates high winds and unsettled weather.
All barometers work because of our weather systems: Changes in air pres-
sure are caused by differences in air temperature. And this, in turn, creates the
wind and weather patterns that carry the high and low pressure systems around
the Earth’s lower atmosphere.