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From: Jarmo Moilanen (jarmom_at_hidden_email_address.net)
Date: 10/04/1999

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Hi Jacob, Brian and other list members,

Parry arc is not extremely rare haloform but I won't say that it is commmon. Here in Finland one can see Parry arcs (there is four subforms) about half of dozen times in a good year. I don't have first hand information about Parry in the mid latitudes but at least in the Neatherlands it seems to be more common than here in Finland (I live in latitude 65N). It is proven that at the South Pole Parry-orientation of ice
crystals is quite common and Parry arc is a common visitor in those diamond
dust displays at least in summer time.

Observers should notice also that there is one odd radius parhelion which can be easily misidentified as a suncave Parry arc. This parhelion is 23 degree upper parhelion which is only about one degree above suncave Parry arc and it's curvature is similar with it. When sun is about 45-55 degree and you can see separate arc above 22deg halo you are probably looking for 23deg parhelion. When you can see suncave Parry arc without any sign of sunvex Parry and the sun is less than 10-13 degree you are probably looking
for 23deg parhelion. When sun elevation is something else you have to look what other halos you can see or you have to measure arc from photos to be sure which one the suncave arc is. However, there is some features in 23deg
parhelion which may tell that halo arc is more likely 23deg parhelion than suncave Parry but it needs some experiences about both haloforms. 23deg upper parhelion seems to be one of the most common haloforms caused by pyramidal ice crystals and it is about as common as Parry arc.

There is more problems when we noticed that 23deg upper parhelion can be also easily misidentify as a 22deg upper tangent arc in high sun elevations. If you see a solitare suncave arc above sun when sun is high and you can not see any sign of 22deg lower tangent arc during a long observation period it is possible that you are looking for 23deg parhelion.
In most cases identification can not be sure without any accurate measurements (e.g. take a photo and measure distance of the arc from sun). If there is other halos they may help you but you have to remember that 23 parhelion is often observed with 23deg halo which can be easily recognized incorrectly as a normal 22deg halo.

In Greenler's book you can see one of the most common type of odd radius displays in the plate 2-23. There is right 18deg parhlion with 18deg halo, 23deg halo and 23deg upper parhelion (that bright arc in the top of the larger halo which is not a 22deg halo but 23deg halo).

Jarmo Moilanen
section leader of FHON (= Finnish Halo Observing Network) halot_at_ursa.fi (e-mail address for current section leader of FHON)

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