For those less weather-watchfully-endowed as a CM or CMJ is, these from last evening between 6:02 and 6:10 PM:
Yes, Virginia, it did rain on the Lemmon yesterday, and that very light rain crept across SH-Catalina just after 6 PM yesterday, eventually crossing Oro Valley with a little baby sprinkle.
Drops were pretty numerous, pretty small, and fairly uniform in size, making me think about what was up there doing it. The shaft was always thin coming down the mountains toward us, and being thin means the top ain’t too high, as you know. Maybe, it was one of those rare ice multiplication days that happens here every so often, ice that forms at temperatures not terribly far below freezing. Going to movies now to see if the U of AZ time lapse can shed light on the buildup–thinking aggregates of needles and sheaths that melted into raindrops and ice multiplication in a top only as cold as -10 C or so.
OK, looked at movie, can’t see a protruding high top and so I am concluding that I am correct in the assertion that an unusual event happened in AZ, ice formation a plenty at temperatures of -10 C. It happens, but requires larger cloud drops in our clouds than usual, maybe some drizzle drops that froze, became graupel (soft hail).
Now I will look at the TUS sounding for yesterday afternoon and see that it confirms my thoughts, tidying up a nice story of cloud microphysics. After looking at it, will post it since it is supportive of the above conjectures, otherwies I would not have posted it. From the Cowboys this for Tucson yesterday afternoon:
Gotta go now, ride a horse, more later maybe….looks like we have those pesky Altocumulus clouds, though ones not as thick as yesterday’s which took into mid-afternoon to burn off, and kind of wrecked our rain chances.
Needles and hollow sheath ice crystals only form when the temperatures in cloud are warmer than -10 C (14 F). Normally in AZ we do not see ice forming at those temperatures because the conditions for their formation, generally involving very large cloud drops and drizzle drops in clouds at those temperatures are rare. This is because we usually have high concentrations of cloud droplets and those higher concentrations lead to itty bitty drops, ones less than 30 microns in diameter at temperatures higher than -10 C. So, another thing that we can guess about yesterday’s clouds is that the droplet concentrations might have been lower than usual, and that the drops in the clouds got larger than 30 microns in diameter.