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   Is a circumhorizon arc "rare"?

How often you see the arc depends on where you are. In the USA you might see a circumhorizon arc (CHA) five or more times each summer. In middle latitude Europe you will be lucky to see a CHA once.   In northern Europe they are impossible to see at any latitude north of Copenhagen. In comparison, a Parry arc is seen perhaps once a year. CHAs are common in the USA, rare in middle to north Europe.

Latitude is the key. The sun must be higher than 58° to form the arc. The further from the equator the less time that the sun is sufficiently high. The upper chart shows the dates for the Northern Hemisphere between which the the sun gets high enough. At the marginal dates the sun only grazes the required 58° for a few minutes close to noon. A second factor is the frequency of sunny days with some cirrus.

The lower chart shows the total number of hours each year that the sun is higher than 58° and therefore able to form a CHA. LA has nearly 5X as many hours as London. Factor in England's rainy climate and CHAs might be seen at least 10X more frequently in LA.