Surprise showers drop 0.02 inches in Sutherland Heights; April won’t be rainless!

Well, those showers were a surprise to “me and my model” yesterday morning, at least the one I looked at just before going on the air. There were no upstream echoes in the clouds upwind of us, either, something normally seen before cool season showers get here. Fortunately, I was able to get in a prediction that rain would fall just as the drops began coming down.

However, there are a few times when that bit more lifting as the air moves from the lower ground to the southwest of us to here can trigger precip; the tops get that bit colder, form ice, voila, out drops the rain (and snow).  That’s probably what happened yesterday to cause a sudden development of light showers, “all quads” it seemed.

Cloud tops may not have gotten colder than about -10 C yesterday, too, and so our rain likely fell from ice crystals rarely seen in Arizona, hollow sheaths and needles (columnar crystals), which have to be in big aggregates before they can form a drop big enough to reach the ground.   So, not only are they rare here, but there also have to be a LOT of them )10s to 100s per liter in the clouds) for them to form big enough snowflakes so that a drop reaches the ground.  In fact, when columnar ice crystals form in clouds, they often do so prolifically so that, at least at the University of Washington where the present Arthur worked for about 30 years in airborne studies of clouds, needles and sheath crystals were always associated with the highest concentrations of ice crystals that we observed.   Some of those rare ice crystals were STILL forming in the clouds above us  near the time of the second sounding, shown below,  launched at 3:30 PM!  That was really shocking!

This ice crystal happenstance, and the surprise light showers,  made yesterday particularly worth commenting on from this cloud pulpit, if that’s what it is.  Some nice examples of needle and hollow column ice are shown here at CalTech.  These kinds of crystals are rare in Arizona because they require larger (greater than 23 microns in diameter2 in clouds at temperatures between -3 and -8 C.  Like the needle crystals themselves.  Since the clouds were shallow, one has to speculate WHY the cloud droplets might have been extra large.  It may have been that there were few of them (seems kinda unlikely this far from  clean oceanic air.  More likely, those clouds had large (micron-sized) dust particles in them, known to help form larger cloud droplets.  So, I’m guess those clouds were helped by dust so’s they could have larger droplets in them, ones big enough to help produce ice splinters consisting of needle and hollow sheath crystals at such high temperatures (higher than -10 C).

Our U of AZ model predicted soundings were pretty much what we saw, too, cloud capped by a stable layer that got stronger as the day went on, and the air drier above it.  Below, from IPS MeteoStar, these TUS soundings from yesterday morning and afternoon.

Baloon (I prefer this spelling) sounding launched from the U of AZ yesterday morning around 3:30 AM AST. Shows tops were marginal for ice in AZ at this time.
Baloon1  sounding launched from the U of AZ yesterday morning around 3:30 AM AST. Shows tops were marginal for ice in AZ at this time.
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The baloon sounding from the 3:30 PM AST launch from the U of AZ.  Where the lines pinch together is the top of the boundary layer where cloud tops were, -10 C.  A few shocking rain drops fell in Catalina (stop light at Oracle and Golder Ranch Drive construction zone) soon after this launch (4:22 PM) from a shallow complex of Cumulus/Stratocumulus clouds. Unbelievable.  Carried out a little photo documentation when I saw them on my windshield to record the remarkable event.  It was pretty darn exciting, and I hope for you, too, if you felt a coupla drops about that time.

But let’s drag this out and look at yesterday’s clouds now…

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6:13 AM. Clouds much fatter than expected this morning, but nothing to worry about. Not gonna rain. Mod says so.
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6:14 AM. Looking to the north, not much, just some harmless Stratocumulus.  Maybe will go inside, not think for awhile….
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6:33 AM. Its raining over there, cloud has a tremendous amount of ice in tops (that smooth area to the right and above Pusch Ridge. Unbelievable, since the tops aren’t that high!
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6:34 AM. Another soft turret filled with ice trailing light rain! That softness suggests to this observer that they are not the usual ice crystals, but rather jillions of needle and hollow column (sheath) crystals, too.
7:38 AM. By this time, the light showers were everywhere, especially in Oro Valley and drifted over Catalina for the next few hours. Here. light rain, not haze, produces crepuscular rays.
7:38 AM. By this time, the light showers were everywhere, especially in Oro Valley and drifted over Catalina for the next few hours. Here. light rain, not haze, produces crepuscular rays.
3:29 PM. By mid-afternoon, the clouds had gotten shallower, not warmer though, as bases rose more than the tops did. Tops were still around -10 C at this time, but the bases were higher, above the mountains, as you can see here. And, we had our usual "mountains shadows" (would be a nice motel name) light show.
3:29 PM. By mid-afternoon, the clouds had gotten shallower, not warmer though, as bases rose more than the tops did. Tops were still around -10 C at this time, but the bases were higher, above the mountains, as you can see here. And, we had our usual “mountains shadows” (would be a nice motel name) light show.

During the afternoon, a nice cloud “street” formed, came all the way from Mexico way, one that spawned a little more anomalous ice, and those few raindrops; see below for evidence if you don’t believe me.

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3:39 PM. Long cloud street trails over Catalina. Isolated veils of ice formed, with a few drops reaching the ground. See car front windshield below for the shocking evidence.
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4:20 PM. Raindrops fell on windshield as part of the long cloud street still in progress overhead. Grabbed camera immediately, of course, as this astounding event occurred! Was at the Golder Ranch Drive and Oracle intersection “war zone” as we would call it these days. (Hope they’re done before 2020. :}

BTW, here’s our cloud street as seen in the “visible” satellite imagery at 4 PM AST, just before the drops fell on my windshield.  You can see that it originated near the border with Mexico, as many things do.

Visible satellite imagery for 2300 UTC (4 PM AST) from the U of AZ.
Visible satellite imagery for 2300 UTC (4 PM AST) from the U of AZ.

The End, except I think there will be some more rain tomorrow morning, trace to quarter of an inch are the bounds, meaning about a tenth is the most likely amount from this cloud pulpit.

The End, again.

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1I prefer this spelling today; more “o’s” than “l’s” in that word make you think more of a thing full of air; maybe a few more “o’s” would help even more, too,  like “baloooon”…

2That size is considered “large” for a cloud droplet, and lab studies have shown that they splinter when they hit something, like soft hail, also called “graupel.”  Splintering is thought to lead to all those extra ice crystals at temperatures between -3 and -8 C.

By Art Rangno

Retiree from a group specializing in airborne measurements of clouds and aerosols at the University of Washington (Cloud and Aerosol Research Group). The projects in which I participated were in many countries; from the Arctic to Brazil, from the Marshall Islands to South Africa.

2 comments

  1. Hello, I was looking at your website and saw that epic photo of Zuma back in 1963! Do you have anymore photos from that big day or any other swells from back then?

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