Cirrus, old and young

Our deep blue sky, loaded with interesting Cirrus clouds yesterday, and generally low in contrail impact, makes southeast Arizona a haven for Cirrus cloud watchers.  Though no one site is completely immune from them, a sky like this, so free of contrails, is impossible on most of the Atlantic Seaboard due to air traffic.  It was just so pretty here yesterday with so few contrails!  Here are a few shots:

10:01 AM. Mostly Cirrus fibratus advance toward Catalina.
11:30 AM. Tenuous strands of CIrrus fibratus, ones underneath  higher, scattered Cirrus clouds. Long strands like these require hours to form, and moist air for a great depth below the much higher, spawning flecks, now long gone.
2:05 PM. The upstream tail where the much of the Cirrus clouds form approaches.
The lack of long strands, and little specs of cloud tell you that these Cirrus clouds are quite young.
5:19 PM. Patchy young Cirrus clouds with an older contrail at right. This photo shows how contrails eventually evolve to resemble natural Cirrus clouds.

The weather ahead:  cold front’s a coming

Its been well predicted for 1-2 weeks that a cold front would pass through our area on Dec 9th-10th.  At times the models had substantial rain here, but it’ll be a dry blast from the north Sunday night.  What makes us here in Catalina a little different in experiencing this frontal passage is that the high pressure behind the front pushes air along the Catalina mountains from the north here, and we often get quite a windy episode, 15-30 mph likely Sunday night.  But, because we have no official weather reporting stations, and the Catalina Mountains block that north wind from Tucson so that they don’t get it, so us here in Catalina are about the only folks that know its quite windy, with a chill in the air.

This cold front is part of a large scale pattern change in the jet stream that is taking place, one that will vastly increase our chances of rain in the weeks ahead as storms barge into the Pacfic Coast farther south than they have and head this way.  No longer will we be in a stagnant condition where day after day the weather is almost exactly the same;  not much wind, temperatures above normal.  Instead there will be occasional windy episodes as storms get close, temperatures closer to normal, and we hope, one that gets far enough south for a good rain.

Some rain is showing up now overnight on the 13th-14th.  In our rain frequency chart, the peak rain days in December were the 11th-13th, deemed a statistical fluke from this keyboard in the 35 year record.

But, here’s a rain threat materializing in the very window.  Hmmmm.  Further, if you look at the spaghetti plot for this time period, a trough is guaranteed in this region on the 13th-14th, though getting circumscribed by the jet stream that is, its south of us as the trough goes by is what’s marginal with this trough situation, a requirement for almost all winter rain here.

The End.


By Art Rangno

Retiree from a group specializing in airborne measurements of clouds and aerosols at the University of Washington (Cloud and Aerosol Research Group). The projects in which I participated were in many countries; from the Arctic to Brazil, from the Marshall Islands to South Africa.