International Polar Year 2007–2008 - WMO

Fig. 3.5-3. Servicing
the AWS at Butler
Island (U.K./U.S.A.).
(Photo: Jon Shanklin)
400
IPY 20 07–20 08
the measurements and can also monitor precipitation
falling from clouds (Fig. 3.5-4).
Measuring the amount of precipitation is difficult.
The snow is generally dry and what falls into a
standard rain gauge just as easily blows out again.
Equally, precipitation that has fallen elsewhere or at a
previous time can be blown around by the wind and
into the gauge. A simple technique is to measure the
depth of freshly fallen snow and assume that in the
long term there is a balance between transported
and falling snow. Specially designed snow gauges
may provide a solution, but gauges that work well
in temperate regions where snow falls do not cope
well with Antarctic precipitation and further design
studies are needed (Fig. 3.5-5). Electronic precipitation
detectors using scintillation in an infrared beam are
now being deployed in Antarctica and combination
of the outputs of two detectors at different heights
may provide the necessary discrimination between
precipitation and transport.
Upper atmosphere
The Antarctic atmosphere is very clear as there
are few sources of pollution. On a fine day it is
possible to see mountains well over 100 km away. In
these conditions, estimating distances can be very
deceptive. Objects may appear to be close by, when
in fact it would take many hours of travel to reach
them. Automatic instruments, which use infra-red
scintillation and scattering to measure near-surface
visibility, are becoming more common, however,
some have difficulty in discriminating variation in
visibility above 20km. Higher in the atmosphere,
the stratospheric aerosol load, largely originating
from volcanoes, is measured using sun tracking
pyrheliometers or photometers. In IGY, the primary
instrument was the Angstrom pyrheliometer, a manual
device, which even in skilled hands, took ten to fifteen
minutes to complete an observation. By IPY a number
of stations had installed automatic sun-photometers,
either as part of an international network or standalone.
These use measurements through a series of
filters to calculate the amount of obscuring material in
the solar beam (Fig. 3.5-6).