When is a contrail not a contrail?

When it’s a distrail (dissipation trail) …


Two of several concurrent distrails over Ballina on Saturday – the sky was full of cigars

The Cloud Appreciation Society explains distrails (also called ‘fallstreaks’) thus:

Short for ‘dissipation trail’, this is not so much a cloud, as a gap in cloud cover. It can appear when an aircraft happens to pass through a fairly thin cloud layer composed of ‘supercooled’ droplets. These are in unfrozen, liquid form, even though temperatures are well below 0degC. Water droplets can stubbornly refuse to freeze when there is a lack of air-borne particles to act as nuclei onto which the ice crystals can start forming. As an aircraft climbs or descends through one of these supercooled clouds, the turbulence of its wake and the many minute particles contained in its exhaust encourage the cloud’s droplets to freeze. This happens when some of the particles act as the nuclei onto which the droplets can start freezing. As the crystals form, they grow in size and fall below. Left behind, is a just gap in the cloud – the distrail.

The gap in the cloud soon got gappier …

Distrail2I looked for parhelia or sundogs, but didn’t see any. But this one had a touch of the 22 degree halo (or circumzenithal arc) in it …

Part of 22 degree halo

The Cloud Appreciation Society has a smartphone app you can download to help identify them while gazing skywards. If, like me, you don’t have such a phone, you can look at their gallery of wonderful cloud photos here or buy a copy of their book, The Cloudspotter’s Guide. NASA is even using the app to monitor clouds for research on their effects on temperature – see the article here.

I’ve been enjoying reading The Cloudspotter’s Guide lately, and so have become hyper aware of the things. There’s a great sense of humour in that book, as well as lots of information about the atmospheric conditions that cause different cloud formations. I know they wouldn’t object to what I’ve called this one …

The evil hell-bat from Planet Zog

The Evil Hell-Bat from Planet Zog

Is this the start of a Kelvin-Helmholtz formation?

Possible Kelvin-Helmholtz formation

Possible Kelvin-Helmholtz formation

It could be ‘merely’ a ‘cirrus uncinus’ (hooked cirrus or cirrus mares’ tails). I’d love to see a more defined Kelvin-Helmholtz like the ones here.

Needless to say, my eyes will be glued to the sky for a while as I come to grips with naming the clouds and figuring what they show about the otherwise invisible atmospheric conditions.

You can see more cloud and storm formations at the Northern Rivers Severe Weather Group here.

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2 Responses to When is a contrail not a contrail?

  1. Roz says:

    Thoroughly enjoy your roving writing, thank you. The Cloud book is one of the most interesting (and well written) books around. On my second reading now. You might also like The Wave Guide by the same author.

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