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Chris Luginbuhl: "Re: halos in Philippines" (Jul 1, 22:45):
>
> But actually the answer must be more complicated than this, I think:
first,
> does sufficient water vapor for ice-xtal formation more often reach into
> the polar upper atmosphere than into the tropical? Colder may not be
> relevant if there is not enough water to form clouds.
You are right about this, the key factors are the low temperature and
high vater vapour partial pressure. These conditions are mainly the
result of vertical motion in an atmosphere. In tropical atmosphere, most
of the significant vertical motions are associated to convective
disturbations such as thunder clouds, while in mid- and polar latitudes
forced ascend/descent by frontal processes play the most important role
(I take it that you know the basics). What I have seen here in Finland,
typical ice clouds (cirrus, cirrostatus) are usually much wider spread
and better formed when generated by frontal activity...
>
> Second, tropical conditions may more often result in extensive
> lower-atmosphere cloud development, which may then tend to obscure upper
> atmosphere halo-generating clouds.
... So one would think that northern latitudes would be more benefical
to Halo observations. However, one should note that typical midlatitude
weather system (cyclone) has a period of 3-5 days. One may see pretty
neat haloes when the system is arriving, but then the lower clouds tend
to block the view. Ice clouds formed from convective clouds
(cumulogenitus, cumulonimbogenitus), on the other hand, has no such
drawback, at least if they are not connected to a typhoon. Thus, it is
reasonable to conclude that one could actually see more halos in
tropical region, but perhaps not that well formed displays. This is the
conclusion Marko Riikonen made, if I remember right. Anyway, I want to
point out that our atmosphere is pretty complicated system and I
concentrated on only a few key factors. Thus, this should not be taken
as a general truth (especially because it is several years since I have
studied these phenomena).
>
> I am just speculating: I myself do not know whether haloes are more
frequent
> in higher latitudes or in lower. I do however think the question is
much
> more complicated than simply the temperature of the tropopause.
yeah.
> The accumulating data on halo occurrences that is being accumulated
might
> be helpful in this regard, though the important information about the
> visibility of upper clouds / obscuration by lower clouds is not easy to
> account for.
>
One should also consider what is the best way to measure the occurrence,
and when the measure has been chosen, one should remember what it is.
That is, one should not conclude that from more halo days necessarily
follows more halo displays.
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