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Just a few minutes ago I observed a rather unusual atmospheric phenomenon
outside my office window. It was around 0730 local time (1330 UTC) and
the skies were mostly clear with scattered cumulus and altocumulus clouds.
Winds are presently from the north at about 10 kph. Appearing in a layer
below the clouds was a series of dark, transparent trapezoids. The sides
of each one of the trapezoids 'pointed' back to a single cloud. The
angular width appeared to match the angular width of the corresponding
cloud. It appears these may be 'broken' crepuscular rays. I have
attached a MS Word diagram to help illustrate the effect, unfortunately I
don't have a camera in my office.
For the last few days the skies of Edmonton have been full of smoke from
nearby forest fires (mostly from a fire in the Slave Lake region to the
north of Edmonton). Just last Monday we set an all-time record for
reduced visibility due to smoke (600 m). Currently the visibility on the
ground is quite good (24 km at 0800 local time), but there appears to be
some smoke aloft. I checked the 1200 UTC sounding (0600 local time) and
there appears to be no strong nocturnal inversion layer possibly due to
the relatively high winds and cloud cover during the night.
What appears to be occurring is the smoke has been confined to a layer
below the clouds producing a large amount of scattering of the visible
light. The surface layer and layer between the smoke and the cloud base
appears to be more clear and therefore, would not scatter as much light.
The cloud would cast a shadow onto the smoke layer and thus prevent the
scattering of the sunlight and produce a darker region.
Any comments?
Russell (Russ) D. Sampson
Department of Earth and Atmospheric Sciences
1-26 Earth Sciences Building
University of Alberta
Edmonton, Alberta, Canada
T6G 2E3
Fax: (403)-492-7598
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