Nature Studies by Michael McCarthy: A glorious burst of the warm south

It's an extraordinary event, is it not? This miniature summer granted us weeks after what was meant to be the summertime, but was a chilly washout, is over and gone. Not long – five days only of cloudless skies, blistering sunshine and balmy evenings – but five days of such splendour as we never had in all the rest of the year, and October starts tomorrow.

People seem to be surprised but intensely gratified, as if they had been left a fat legacy by an unloved relative. People want to make the most of it, they want to enjoy the mini-summer to the full, by beach and by barbecue, they want to squeeze every last drop out of it; and if you're minded to do that, in the three days we have left of colour and warmth and light, and you're interested in the natural world, I have a slightly different suggestion for you: look for a butterfly.

Hardly any photos do justice to the clouded yellow, because, like several other British butterflies such as the grayling or the brimstone, it always perches with its wings closed. This means that when the insect is at rest and able to be photographed properly, only the wing undersides are visible, and they are very different from the uppersides, glimpsed in their turn only when the butterfly is in flight, or in a painting, or when a dead specimen is mounted in a display case.

Greenish-yellow, the undersides, with a white spot in the centre of the hindwing – attractive enough. But the upper wings, which confer on the creature its identity and character, are something again. They are yellow too, but of a darker and much more arresting tone. There's a hint of burnt orange in there. You might say, sulphurous. Mustardy, maybe, with the whole effect enhanced by broad black wing margins.

Compare it with the other two British butterflies which are predominantly yellow, the brimstone itself and the swallowtail. The brimstone, the first butterfly of early spring, is a pallid lemon; the swallowtail, creature of June and midsummer, is bright canary, banana-yellow maybe; but in contrast the clouded yellow is approaching the burnished colour of goldenrod, that feathery backstop of the herbaceous border. When a clouded yellow is in flight, it is an old gold guinea that is bowling through the air towards you.

It seems to me these tones closely correspond – perhaps by chance – to the feeling of the season when they are seen. The brimstone reflects the hesitant sunlight of spring, and the swallowtail the glaring sunlight of June; but the clouded yellow reflects perfectly the mature sultry light of late summer, and let it be said, the sultry warmth, as well. It has a Mediterranean feel, as if it were a piece of the warm south, visiting Britain.

Which is exactly what it is. The clouded yellow is one of Britain's three regular butterfly migrants, heading to us each year, like the painted lady and the red admiral, from breeding grounds in southern Europe and even North Africa. Sometimes, in so-called "clouded yellow years", it arrives in enormous numbers – tens of thousands – and these have occasionally been witnessed as great yellow clouds crossing the Channel – during the last war, one was seen and at first thought by soldiers manning the coastal defences to be a cloud of German poison gas.

But clouded yellow years are infrequent – the most recent was in 2000 – and usually only a few hundred arrive, and they can be very hard to catch sight of. Two years ago, in trying to see all of Britain's 58 butterfly species in a single season, I spent the whole summer vainly searching for a clouded yellow, gradually becoming obsessed with it, and found one only on the very last day; it was the final species of the 58 and my elation was unbounded when at last I glimpsed gold above Steyning in Sussex at the stroke of noon on 31 August.

For me it will always symbolise late summer; and this weekend, the warm air from Africa, which is giving us our latest summer ever, will bring an influx of clouded yellows with it, and on the chalk downlands of the south, from Sussex to Dorset, they will be nectaring on the last wild flowers, the knapweed and the devil's-bit scabious.

Head up there, why don't you? You can head for the beach (but then that will be like any other day at the beach) or you can get out the barbecue (but then that will be like any other barbecue); or you can embark on a quest, just a single day's quest, and if you find what you're looking for, that splendid golden butterfly, you will have taken this amazing, unexpected, glorious, preposterously late miniature summer, and seized the very heart of it.

Migrants winging it across the Atlantic

News of an even scarcer migrant: a monarch butterfly from North America was spotted and photographed at Ringstead Bay in Dorset on Wednesday by a couple, Shelley Cunningham and Shane Austin, walking the south-west coastal path; yesterday they took their pictures in to the charity Butterfly Conservation in nearby Lulworth, who confirmed the identification. Monarchs migrate 2,500 miles every year from the northern USA to Mexico; just occasionally, they also get across the Atlantic.

m.mccarthy@independent.co.uk; Twitter.com/mjpmccarthy