Small now, but what portent for the summer weeks ahead when it’s big brothers will show up over the Catalina Mountains; a real milestone, so pretty. This photo was yesterday, at 2:45 PM, when it was 101 F! Cloud name: Cumulus humilis or Cumulus fractus.
Later, smoky Altocumulus filled in before and at sunset as a wisp of tropical air also drifted into Arizona. Unfortunately, that was it for our hint of the summer rain season as this morning there were no clouds in sight.
If you look carefully at the orangish sky around the Altocumulus, you can see something that looks like very faint cloud ghosts, patches of whiter sky. Those are moist areas at the same level as the Altocumulus where the air is nearing saturation, and some of the aerosol (smog) particles have fattened up as they “deliquesce”, absorb some water vapor, and when they get larger as a result, scatter more of the sun’s light. That produces a faint ghost of a cloud that may be ready to appear if the air gets more moist. Typically the relative humidity is 60-80 percent when particles begin “deliquescing.” Some of our greatest summer days are humid, with low cloud bases on the Catalinas, and yet there is no evidence of haze whatsoever. That means the air must REALLY be clean!
There was a suggestion of virga in clouds downwind of the Catalinas at sunset, but it was marginal if even there. So, what was the temperature of those Altocumulus? Those clouds could still be below freezing, but a good guess would be warmer than -10 C (14 F). The TUS sounding indicated that they were at -12.5 C, marginal for ice crystals to form (and precipitation to fall out).
