Fall streaks

Fallstreaks at sunset, Juan-les-Pins
Click to enlarge
© The Boon Companion, 2016

At first glance, that’s just some nice sunset cloud over Juan-les-Pins in France. But there’s quite a lot going on in that picture, which you’ll see better if you click to enlarge it. (I’ll post some zoomed views of sections of the whole image below.)

The sun has just set, off to the left of the photograph. Its light is making it easier to figure out the heights of the various cloud layers. At right, we have some deep red clouds—low enough to be illuminated only by the sunset glow. At the left side of the picture, about halfway up, we have a line of clouds that look bright yellow—they’re a bit higher, and still seeing the setting sun. And at top left there are some fluffy white wisps—the highest clouds of all, still receiving full daylight illumination.

If I zoom in on the low, red clouds first, you’ll get a better view of what’s going on:

Fallstreaks detail 1
Click to enlarge

There are vertical streaks descending from the base of the cloud, which are then curving noticeably leftwards.

These downward extensions from the cloud are called fall streaks. As the air cools with sunset, the clouds are starting to release a little rain, but it’s evaporating before it reaches the ground. Another name for this phenomenon is virga, from the Latin for “rod”, and you can see the rod-like central cores in the fall streaks.

Why are they curving? The raindrops are accelerated vertically by gravity, but also pushed sideways by the wind. The larger the drop, the faster and more nearly vertically it falls. Small drops drift downwards at lower speeds, buoyed by air resistance, and therefore end up being pushed sideways more for a given vertical descent. But the drops are evaporating as they fall—big drops at the top of the fall streak turn into small drops lower down, so we see the streak bending more and more downwind the farther it falls below the parent cloud.

OK. If we look over at the yellow clouds now, we see something different:

Fallstreaks detail 2
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More fall streaks, but pointing in the opposite direction. So the wind must be blowing in a different direction where these clouds are—either because it varies with height, or because there’s some local swirl caused by the land to the right of the picture.

Now look up at the highest, whitest clouds—they look like altocirrus to me, so will contain ice crystals rather than rain. They also have developed fall streaks, but rather remarkable ones:

Fallstreaks detail 3
Click to enlarge

The streaks go left initially and then turn abruptly right, making a neat right-angle in the air. Those falling ice-crystals aren’t doing much evaporating, since they seem to be travelling in pretty straight lines, but they have certainly fallen across a very abrupt transition in wind direction between the high cirrus and the lower clouds.

So there’s often quite a remarkable show going on up there, if you’re paying attention.

Fall streaks can produce some remarkable effects, if they rain out locally from an extended sheet of cloud:

Hole-Punch Cloud
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Original photo by H. Raab. Used under Creative Commons Attribution-Share Alike 3.0 Unported licence

A little area of supercooled liquid droplets in the cloud layer has converted to ice, and that area is snowing out in the form of virga, leaving a patch of blue sky behind. These are called fallstreak holes, or hole-punch clouds.

Why do they form? The seem to be associated with aircraft passing through the cloud layer. The airflow around the plane’s wings (and propellers) causes a little local expansion of the air, which causes it to cool—if that’s enough to cause freezing, a fall streak is induced. And the vertical movement of air associated with the raining out of that initial fall streak causes a wave of up-and-down movement in the surrounding cloud, allowing the freezing effect to propagate outwards to produce a neat, almost circular hole. *

And if an aircraft hangs around in the vicinity of the cloud layer, rather than simply ascending or descending through it, you can get some rather spectacular linear hole-punch clouds, like the one in this video (posted by a birder, which explains the owl that appears in the bottom right corner):

* Heymsfield et al. Formation and Spread of Aircraft-Induced Holes in Clouds. Science 2011; 333: 77-81

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