I DIDN’T know whether to look down or look up on my earlyish Saturday morning walk to get a paper last weekend.

Down in the River Tees was a heron, directly beneath Croft bridge. It was standing in the fastest flowing part of the river, in broken water, staring upstream, hoping the current would drive fish towards it. It would have been perfectly still but it was slowly pawing at the water with its left leg.

The fish seemed to know it was there, because I saw a couple of them jumping in the shallows, silvery streaks against the pebbles, as they tried to make their way around it.

Up was a distinctive cloud formation. Straight lines of fluffy white clouds set against a blue sky. To me, they looked like the underside of a newly ploughed field; other people saw it as a fingerprint on the sky while others described it as grains of rice piled into rows.

It was visible briefly over south Durham and North Yorkshire – on this page on Tuesday, we published Sheila Harris’ picture of billowing rolls of cloud over Darlington, and today you’ll see Neil Barker’s picture taken at Scruton, near Northallerton, where the pattern looks more like the markings left by the tide on a beach.

I discover that there are 10 different types of cloud, 34 subtypes and an infinite number of varieties, but this appears to be an altocumulus undulatus formation.

Of the 10 cloud types, cumulus are the classic clouds: white and fluffy, the kind kids draw. In Latin, “cumulo” means “heap”, and they are heaps of puffiness.

Cumulus clouds are low level, between 2,000ft and 7,000ft, but altocumulus are just above them, and should not be mistaken from cirrocumulus which are higher still, the wispy streaks above 14,000ft.

“Undulatus” comes from the Latin “unda” meaning “wave or ripple”, and Saturday’s waves of cloud were indeed rippling across the blue sky.

They appear to have been caused by the wind blowing at different speeds and directions above and below the clouds. This “wind shear” causes turbulence for a plane, and it forces the heaps of altocumulus caught in the middle between the winds to bunch up into ridges. Altocumulus undulatus often forms when two weather fronts come up against each other, and it seems to be a feature of May’s skies.

When I returned from the paper shop, the distinct furrows of clouds were beginning dissipate into unremarkable fluffy patches, but the heron was still there, still slowly pawing at the water in the hope of catching its breakfast.