Though HUGELY disappointing because only a trace of rain fell here as of 7 AM this morning, and only a little in the Canada del Oro wash watershed (amounts here), nevertheless, what a nice, classic passage of a cold front. A cold front, as it sounds, marks the advancing boundary of colder air that is displacing warmer air, and that went went by late yesterday afternoon. When it goes by, the wind direction changes almost instantaneously, the temperature begins to drop, often sharply at it did at 5 PM yesterday (see below), and the barometric pressure begins to rise.
But without measurements or satellite or other data, you yourself could have seen that invisible boundary approaching Catalina by the low, scruffy clouds that began to appear on the horizon to the northwest. Soon they were topping the Tortolita mountains, then the Catalinas. And you would have noticed that, unlike the clouds overhead, those lower clouds were advancing from the north. That evolving scene looked like this, finally ending up as a low overcast of Stratus clouds. The first shot below was at 4:40 PM, 20 minutes before the windshift and temperature plummet hit. The second shot is as the windshift was passing Golder Ranch Drive and shows the lower cloud bases associated with the cooler air racing south along the west side of the Catalinas. You can see that they are also connecting to the higher Stratocumulus layer. The third shot shows the Catalinas fully enveloped in the cooler air and lower clouds, and the last shot is of those much lower clouds (I would call them “Stratus”) over Catalina and Oro Valley, looking to the west.
You can also relive yesterday’s clouds and windshift from the vantage point of the University of Arizona’s timelapse film. You will see the windshift hitting there marked by puffs of dust from the NW and then those low scruffy clouds right behing beginning about 5:20 PM here.
So why didn’t it rain with all these clouds? What was missing? For almost every drop of rain that falls in Arizona, ice crystals are required to start the precipitation process going.
The formation of ice in clouds is a continuing scientific enigma, believe it or not. However, we know that they didn’t form, with brief exceptions yesterday afternoon when a few sprinkles (NOT “DRIZZLE”, dammitall! Sorry, lost control there for a second)…..formed in the higher deck of Stratocumulus clouds, and again last evening when it rained again for a few minutes.
The first thing you would guess then, since we are talking about the formation of ice in clouds, is that the tops of the clouds did not get cold enough, that is, were not high enough above us and upwind of us, for ice to form. That would be my best explanation for those periods where it was not raining, we had low clouds and they looked rather threatening for much of the time between 5 PM and dark. (After dark, some rain did briefly fall.)
However, the Tucson sounding launched yesterday afternoon around 4 PM shows that the tops were plenty cold enough; the top of the moist layer was about -20 C! (Note: soundings do not measure “clouds”, but rather humidity, from which we INFER clouds). Normally a considerable amount of ice would be expected in clouds having a top temperature that low. Tiny echoes did occur over and downwind of the Catalinas all around the time of that sounding which means that ice was forming precip here and there in the clouds we saw, and measurable precip was recorded in the CDO watershed.
Sure wish I could have been up there in our former research aircraft to check this out more! But, will have to leave this in a bit of an unsatisfactory way.
My apologies if this got a bit deeper than you really wanted to get into.