Category Archives: Lower tangent arc

More odd optics; a Cirrostratus halo with a suggestion of a lower tangent arc

7:25 AM arrives as usual. Seeming radiating bands of Altostratus stream toward Catalina (little too thick and gray to be “Cirrus”). They also appeared so smooth as to resemble lenticular clouds.
1:14 PM. Since its summer again in Arizona in late October, not surprising to see afternoon Cumulus clouds originating in air from the Tropics clustering over our mountains again.
1:38 PM. The curved brightening at the bottom of this otherwise normal halo is likely a “lower tangent arc.”
1:40 PM. Zooming in with excitement here. Is it really a lower tangent arc? Not positive, but will go with that description anyway.
2:59 PM. That Cirrostratus thickened into Altostratus (usually, btw, due to the bottom of the cloud lowering) and at the same time the lower moisture, evidenced by these Cumulus and Stratocumulus clouds, also increased. However, none of these lower clouds got cold enough to produce ice, so likely tops were warmer than -10°C (14°F).

DSC_90074:32 PM.  Mix of Altostratus and Altocumulus clouds, with just remnants of the lower Cu.  Here it appears that a liquid layer of Altocumulus now resides at the bottom of the Altostratus, or may be embedded in it.  The globular masses in the middle of this photo represent droplet clouds that appear to have merged into a plate.  In the distance, the telltale sign of lowering tops:  only droplet clouds, with a occasional splash of virga can be seen.

DSC_90115:23 PM.  Just a little before a great sunset,  which I missed due to a social engagement, the shallow Altocumulus droplet clouds are plainly evident around the sun’s position.  Above to center, the backside of the deeper Altostratus clouds with much higher tops,  is about to pass over us.  Here you can see, how much lower the Altocumulus cloud fragments are than the Altostratus layer as they are  illuminated by the sun (upper center highlighted clouds).

All in all, a pretty pleasant day with interesting clouds passing by, though ultimately disappointing since at one time, this was to be a day with showers here.  Oh, well, that’s weather forecasting for you.

Another minimal chance for showers comes up in the middle of next week…

The End