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After going trough all the Finnish elliptical halo observations it seems
evident, that elliptical halos are connected to middle level clouds,
especially Altocumulus. There are 17 reliable reports, and in almost all
of
them there is description of Altocumulus showing up in the sky same time
with
the elliptical halo. Sometimes Stratocumulus is reported and in one case
Altostratos.
The elliptical haloes are not seen in the Ac itself, but in white "mess"
cloud, that seems to be connected with the Ac. The "mess" might be a virga
of
Ac or something. It look a little bit like Cs, but the short existence of
elliptical haloes (out of 16 observations five were 1 minute or less)
indicates clearly the lower level origin of the elliptical halo cloud.
Our best elliptical halo display on 12.4.1993 (with three ellipses
simultaneously in the sky) was accompanied by beautiful, well formed Ac
lenticularis clouds, which were also photographed by Jukka Ruoskanen.
Those
kind of clouds are rather rare in flat Southern Finland. Was it just
accident,
that spectacular elliptical halo display appeared during spectacular Ac
lenticularis?
Here is a tip for people living near mountains: Orographic Altocumulus
lenticularis is often formed near mountains. If some white "mess" spreads
around the Ac cloud, try to look for elliptical halos. Sunglasses are
necessary to see these very close to sun located halos.
Marko Riikonen
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