Pretty Altocumulus

5:30 AM.  Altocumulus opacus virgae.
5:30 AM. Altocumulus opacus virgae.  TUS sounding indicates the highest tops were about -13 C (9 F), just about where ice would be forming in concentrations of a few per liter, which is likely what the concentrations are in the virga shown in this photo.  The thickest portions of the Altocumulus were about 3,000 feet (1 km).   In the upper portions of those clouds,  droplet sizes would be the largest since that’s where they have been lifted to the highest level.  The larger the drops, the higher the temperature at which ice onsets in them.  Lenticular clouds, marked by mainly laminar flow, not vertical flow as in these Altocumulus clouds, would NEVER form ice at -13 C since the drops remain extremely small, as an interesting extra note for you.
6:30 PM, exactly 12 h later...
6:30 PM, exactly 12 h later… Nice example of Altocumulus castellanus, no virga or ice visible.  Estimated height above the ground, 12,000 feet.  Sounding says tops were warmer that -5 C, too warm for ice crystals to form.  The highest temperature at which natural ice has formed in clouds is about – 4 C,  but those clouds always have drizzle or rain drops that can freeze at this highest temperature.  Our clouds, except in summer on a few days with extra “warm” cloud bases (greater than about 10 C, 50 F), do not have drizzle or rain drops in them before reaching the -4 C level.

Today’s clouds

“Dewpoints up!”, akin to “surf’s up!” as said by surfers to one another.   52 F dewpoint here in Catalina, generally, 40s to low 50s throughout southern AZ now.  So, we have a good chance for some decent Cumulus and small Cumulonimbus clouds around in the middle of this afternoon and evening. Bases will still be way up there, 14-15 kft above the ground and near the freezing level, so rain at the ground can’t be much, and coverage will be minimal.

Dries out tomorrow, too, so today’s our only hope for a while for rain in the area.

Interaction between Little Crissy Hurricane and the westerlies continues to disappoint as Little Crissy’s moist plume is carried to the south and east of Arizona. There is some indication that that a residual, barely alive low level circulation, as it completely dissipates off Baja around the 19th, might still result in a surge of moist air as a new upper trough moves into the interior of the SW then. More wishful thinking?

The End