are checked and emptied every 24 hours, except during storms when they can be
checked more often.
A typical rain gauge has a funnel that empties into a test-tube-like container
that is calibrated in centimeters and/or inches. The funnel increases the amount
of rain that the device can “catch” (that is, it increases the aperture). When the
liquid passes into the cylindrical portion of the device, the increments are
expanded, making it possible to read rainfall amounts accurate to a small frac-
tion of a centimeter or inch (±0.025 cm or ±0.01 in is typical).
The readings of a rain gauge can be influenced by various factors. High wind
reduces the amount of rain that falls into the funnel, because the rain tends to
blow across it, rather than down in. Special rain gauges are needed to accurately
measure the amount of rain that falls in windstorms, particularly tropical
cyclones. If a rain gauge is not emptied often enough, some of the water evapo-
rates, producing an artificially low reading. During a significant storm, a rain
gauge will overflow if it is not emptied often enough.
SNOWFALL MEASUREMENT
For measurement of snow accumulation, two figures are determined: the actual
amount of snow as it has fallen on a flat surface (absent drifting or blowing), and
the amount of liquid that remains when the snow is melted. The ideal surface for
measuring snow depth during and after a snowstorm is an outdoor tabletop. This
minimizes settling that can occur on warm ground when snow melts partially as
it falls. Simply place a ruler or yardstick down into the snow, making sure it is
oriented vertically, and making sure that the zero point on the ruler or yardstick
is right at the end of the stick.
To measure the melted snow equivalent, a variety of techniques can be used.
The simplest method is to collect a cylindrical sample of the snow from a flat
surface, melt it down, and then measure the depth of the resulting pool of water.
If this depth is too small to accurately measure, or if greater accuracy is desired,
the contents of the cylindrical collecting container can be poured into a rain
gauge, and the reading from the gauge multiplied by the ratio of the square of
the diameter of the top of the rain gauge (that is, the funnel opening) to the
square of the diameter of the cylindrical snow collector. If d
c
is the diameter of
a cylindrical snow collector, d
r
is the diameter of the top of the rain gauge, and
s
r
is the reading of the rain gauge (in centimeters or inches) after the melted
snow has been poured into it, then the melted snow equivalent, s
m
(in centi-
meters or inches, respectively) is:
s
m
= s
r
d
r
2
/d
c
2
CHAPTER 3 Observation and Forecasting
86