Jökull - 01.12.1959, Síða 3

Á R S R I T
JÖKLARANNSÓKNAFÉLAGS ÍSLANDS
9. ÁR REYKJAVÍK 1959
Micro Meteoroiogy over Dirt Coned. Ice
The abundant volcanic ash and tu££ in Ice-
land provide a clirt layer, discontinuous and o£
variable thickness but very widespread over
nrost of the glacier tongues. To assess the effect
of a dirt layer on tlie meteorological elements
responsible for ablation, experiments were made
on the snout of the Breidamerkurjökull as part
of the field work of the Durham University Ex-
ploration Society in 1951. A first analysis of
the work on rock debris and ice was published
in „Jökull", 1953, Vol. 3, pp. 23-51.
The surface of glaciers in Iceland receive heat
by: — radiation from sun and sky; forced con-
vection caused by the wind blowing air, above
0°C, over the glacier surface; condensation of
water vapour from the air, if tlie vapour pres-
sure increases with height. The heat is dissipat-
ed at tlie glacier surface by: — outgoing long
wave radiation; melting; evaporation, if the
water vapour pressure in the air decreases with
height.
Incoming and reflected radiation were mea-
sured by an Eppley Pile. Wind speed was mea-
sured by a sensitive anemometer at heights of
20, 50, 100, 200 cm., above the ice surface. Dry
and wet bulb temperatures were measured at
lieights of 3, 8, 30, 100, 140, 200 cm., using
aspirated thermocouples, shielded from direct
radiation. Ablation was measured at not less
than four stakes drilled into the glacier surface.
Observations were made over three areas: I —
comparatively clean ice; II — dirt-strewn ice
with dirt cones 30 cm mean height (range 10
to 200 cm); and III — with dirt cones 10 cm
mean heiglit (range 5 to 100 cm). The thick-
ness of the dirt layer was greatest near the
tops of the larger cones, but decreased rapidly
near the base of the cones. One cone had 20 cm
thickness of dirt on top, but 0.5 to 2.5 was
more typical of the cones on which ablation
was measured. Tliese areas were selected as the
rnost uniform in surface character, but they
were not extensive as desired and had an in-
creasingly varied surface up-wincl. All three sites
were approximately I km, from the edge of the
glacier ancl 100 metres west of the Esjufjöll
moraine.
Albedo, the proportion of the incoming radi-
ation which is reflected, averaged 14% (range
4—20) over the clean ice and 4% (range 2—8)
over the dirt covered ice. These low figures
indicate how the ice was really not very clean
but they can be considered representative of
rnany of the glacier tongues in Iceland.
Outgoing radiation has been calculated by
many authorities and various expressions deriv-
ed relating radiation with surface temperature,
vapour pressure and cloud amount (Lister Sc
Taylor, 1960). A mean value of outgoing radi-
ation, with zero and with full cloud, has been
LANDSBúKASAFN
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ÍSLANDS