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