Geoscience Reference
In-Depth Information
Figure 12.6
Mean fractional
catch as a function of wind
speed for rainfall, and
snowfall when measured with
shielded and unshielded
gauges. (Adapted from from
Larson and Peck, 1974.)
gauge subsequently being blown out. Sometimes 'blow fences' are set up
around the gauge to protect the gauge from wind. These shielding fences
may perhaps be made from horizontal plastic strips mounted between
poles. Sheltering of this type can reduce the under-measurement of snow,
but does not eliminate it, see Fig. 12. 6.
(2)
Snow (and other types of frozen precipitation) takes time to melt, so precipi-
tation intensity measurements are difficult and sublimation of the precipita-
tion stored in the gauge prior to melting can add to under-measurement
problems. The only solution to this problem is to use some form of heating
in the gauge.
(3)
Snow can completely cover the gauge and the surrounding area in heavy
storms. The only solution is to mount a large gauge well above the ground
and accept greater wind-related errors because wind speed is greater farther
from the ground.
Because measuring snow with gauges is so difficult, observers are obliged to seek
alternative methods to measure snowfall. If the observer is present at the time of
measurement, one technique used is to push the inverted funnel from a conven-
tional gauge into the accumulated snow cover on the ground, then to remove the
funnel and snow it contains, melt the snow, and measure the water so formed.
Sometimes a
snow board
might be used if the snowpack is deep, this being a thin,
white, slightly rough board that is left on the snow pack after a storm to act as a
(new) reference level. The inverted funnel method described above might then be
used to measure the subsequent snow deposited.
One very common approach to measuring snowfall is simply to measure the
height of the snow and to assume a density for the snow pack. Often the snow is
assumed to have a density of 10% of that of water. A snow board might again be
used with this approach to allow measurement of increments in the snow pack.
More than 80% of Canadian meteorological stations use this approach. Another
simple approach is to deploy a container of known weight containing antifreeze
and subsequently to weigh the container after snow accumulation has occurred.
