The Sky Was Reporting in Eighths the Whole Time

To the version of me who logged four months of pressure and never once looked up:

You think you already know the sky. You’ve read FEW035 SCT120 BKN250 off a CYEG report before every flight for a couple of decades, cleared into the air or grounded by a string of letters. Here is what you never noticed, and it embarrasses me a little to write it down: those letters are cloud amounts, and they’re measured in eighths. FEW is one to two eighths of the dome covered. SCT — scattered — is three or four. BKN, broken, is five to seven. OVC is all eight. The sky quartered into eighths is called oktas, and it’s the counting system a person lying flat in their backyard uses with nothing but their own two eyes. You’ve been fluent in the ground observer’s vocabulary the whole time. You just used it to decide whether to fly, and never once to look.

So here’s the afternoon that catches you out. The VFR window slips — ceiling not lifting on schedule — and while you’re waiting, a lens-shaped stack of cloud assembles itself over the foothills to the west. Smooth as a turned bowl, stacked in plates like a poker chip tower. And you, who taught a four-hundred-dollar barometer its own altitude not so long ago and felt clever about it, cannot name the thing hanging over your own horizon.

It’s altocumulus lenticularis, and once you look it up you’ll want to un-know the ordinary version of it. That cloud is a standing mountain wave. It looks nailed to the sky — parked, motionless — but the air is tearing through it at speed. Moist air rides up the crest of an invisible wave in the lee of the Rockies, cools past its dew point, condenses into cloud on the way up, then evaporates on the way back down the far side. The vapour is constantly being made at the leading edge and unmade at the trailing one. The shape holds still while the material of it never stops moving. Glider pilots hunt these deliberately — the altitude records, fifteen kilometres and up, were all set in lee waves. The chinook arch you’ve watched your whole life from the valley is the downwind lip of exactly this machinery.

Learn one measuring trick early, because it turns the whole sky into an instrument. A fist held at arm’s length spans roughly ten degrees. A spread hand, thumb-tip to little-finger-tip, about twenty. So when a pale ring circles the sun through thin milky cirrostratus, you can check it: if the gap from sun to ring is a bit more than two fists, that’s the 22° halo, sunlight bending through the sixty-degree prism of hexagonal ice crystals five or six kilometres up. The number falls straight out of the crystal geometry. You are measuring optics with your hand.

Now the part I have to be honest with you about, past me, because you’ll want the day to resolve and it won’t. You’ll get greedy. You’ll try to log the whole sky at once, and a real sky is never one thing — it’s cirrus up high and stratocumulus lumping along underneath and a lenticular off west, three storeys of weather stacked in one glance. You’ll spend twenty minutes deciding whether a dappled patch is altocumulus or the higher, finer cirrocumulus (rule of thumb: a cloudlet a finger-width wide at arm’s length is the lower one; a fingernail’s width, the higher). And while you argue with yourself, the light will shift, the lenticular will thin to a smear, and the thing you were naming will simply stop being that thing. You’ll close the notebook with half a page filled and the sky already a draft ahead of you.

That’s fine. That might be the actual texture of it.

One more thing, and then I’ll let you go back to not looking up. The barometer you fought with, the pressure trace you’ve been logging — the sky writes the same forecast in a slower hand. High mares’-tail cirrus lowering and thickening to cirrostratus, then a watery altostratus sun, then grey nimbostratus and rain, is a warm front announcing itself a day or two out. You had the graph. You just never checked it against the ceiling it came from. Go outside. Bring the notebook. The instrument was over your head the entire time.

— me, four months from now, still getting the dappled ones wrong