I have found hyale to be exceptionally elusive. It does not occur in Var where I spend most of my time in France, so I have little experience of it. There are records further north in the PACA region, although confusion with Berger's Clouded Yellow (C. alfacariensis) may well be a major factor  in misidentifications, alfacariensis being widespread and common throughout the region. There are records shown in the Hautes-Alpes in the excellent PACA Atlas (https://naturographe-editions.fr/boutique/atlas-des-papillons-de-jour-et-zygenes/), and CEN-PACA, the publishers, have been assiduous in validating records.

Before 2022 I had photographs of what were most probably hyale, but none that were definitive. In late August 2022 I happened perchance to spot a field of Lucerne (Medicago sativa) in central France with several yellow butterflies flitting about in it. This was very promising as Lucerne is the larval hostplant, and hyale seems never to stray far from it. There were large numbers of hyale flying there, and I took photographs of as many as possible to get a feel for the variation in this species, making the assumption that they were all hyale and that some were not alfacariensis. I revisited this site in 2023 and it was still flourishing.

It is almost impossible to differentiate hyale and alfacariensis by external characteristics of the underside and they always have closed wings, and a view of the upperside is usually only visible during courtship.

In reality, the only totally sure method for identification is by the examination of male genitalia. I am of the opinion that this should only be undertaken in extremis for scientific studies where accurate identification is necessary, and not by enthusiasts that just wish to know and record what they have seen. Personally, I do not catch or interfere with any species.

I compared the 20+ images taken in August 2022, which I shall assume were all hyale, with 20+ images of alfacariensis, known to be such as they were photographed in Var, where hyale does not fly.

The larvae of these two species are different and in a recent study by Tim Cowles has bred them both from ova, comparing the larvae at each instar. Tim Cowles' study can be found here: http://www.european-butterflies.org.uk/downloads/EIG13.pdf

It seems to be less common than before. It is highly migratory and seems to move around, but the replacement of Lucerne as a crop by Maize may be a factor, and it may be that the Lucerne is harvested at the wrong time.

Females observed egg-laying may be the most reliable indicator:

alfacariensis = mainly on Horseshoe Vetch (Hippocrepis comosa) or Crown Vetch (Coronilla varia).

hyale = mainly on Lucerne, but also on White Clover (Trifolium repens)

The larval hostplants are different and the only completely reliable method of differentiating them (without capture and examination) is to observe the female egg-laying and identify the plant.

Location: where they were seen with reference to reliable distribution maps for each species. This is circumstantial and would only have credence if clearly outside the normal range for either species.

A review of (trusted) books and websites reveals a number of clues to identification (reference to the images shown in T&L may be necessary):

Colour of uph discal spot:

only apparent from an upperside view

alfacariensis = bright orange-yellow

hyale = pale orange-yellow

I only have one clear upperside image of either species, hyale 50342, and in this image it certainly appears that this spot is quite pale.

Degree of upf basal shading:

only apparent from an upperside view

alfacariensis = narrow, extending along the lower margin

hyale = broader and extending further upward, along the costa. 50341 seems to tick this box as well.

Extent of uph black marginal mark:

only apparent from an upperside view

hyale = said to be stronger than alfacariensis

Smoothness of the internal edge of the upf apical mark:

alfacariensis = said to be slightly less smooth than hyale

Curvature of forewing margin:

alfacariensis = not straight, curved to some degree

hyale = straight

In the majority of the hyale I studied, the forewing margin was straight, a few had a slight curvature, and two (assuming they were hyale) had a more noticeable curvature. The reverse was true for alfacariensis in that nearly all had a greater or lesser degree of curvature and a few had straight margins. So, it appears this clue is a matter of probabilities, not an absolute rule.

Curvature or pointedness of forewing apex:

alfacariensis = curved to some degree, not pointed

hyale = quite noticeably pointed

In the majority of the hyale I studied, the apex was quite pointed, a few had a slight curvature, and two (assuming they were hyale) had a more noticeable curvature – and they were the specimens that had some curvature of the margin. The reverse was true for alfacariensis in that nearly all had a greater or lesser degree of apical curvature and a few had quite pointed apices. So, like the straightness of the forewing margin, it appears this clue is a matter of probabilities, not an absolute rule.

Colour of unh:

alfacariensis = said to be a brighter yellow than hyale

There seemed to not be a lot of difference here.

Degree of grey scales on unh:

alfacariensis = greater degree of grey scaling than hyale

Both species were quite variable, but on balance the scaling of alfacariensis was greater, so there appears to some validity in this clue.

Colour of the surround of the unh white discal spot:

This appears only to be referenced in the excellent new book on identification published by Gard Nature.

hyale = slightly darker in colour than alfacariensis

There seemed to be more consistency between specimens, but on balance the surround was darker for hyale.

In reality, there was no great consistency between the books and the features quoted were subject to some degree of variation. It is probably true to say that if it doesn’t have a straight margin and a pointed apex it is probably not hyale, but the converse is not always true as some known alfacariensis have straight margins and pointed apices.

observations

I believe this is a male hyale as the forewing outer margin is quite straight and the apex is maybe just about pointed enough for hyale. The ground colour is a pale lemon yellow indicating hyale. It was seen in northern Italy, where hyale does occur.