The most interesting clouds–technical discussion ahead; skip now if you have a headache

Well, to me, anyway, those flat, thin ones yesterday (Stratocumulus, and Cumulus fractus, humilis, mediocris, if you want some names).  The thicker versions of these clouds were shedding ice and snow but weren’t very cold and that’s what made them interesting.  Take a look.  If your eye is calibrated you can see that light snow is falling on the Catalina Mountains in the first two photos, and in the third photo, one that better illustrates how thin these clouds were (probably 2,000 to 3,000 feet thick is all), snow is seen falling in the distance from the cloud above and to the right of the white roof of the shed in the foreground.  Quite remarkable.


According to our TUS sounding data, these clouds, at least at 5 AM LST, were topping out at only around -10 Celsius (14 F), a very marginal temperature for ice formation in clouds in Arizona.  By the afternoon, the TUS sounding was suggesting the clouds near the sounding balloon were as warm as -3 C (28 F), too warm for natural ice formation.  (Where the temperature and dewpoint temperature lines come together, right now, over me.)

If you go to the movies, the time lapse one from the U of AZ here, you will see that the clouds were coming toward TUS from the Catalina Mountains by the time of the afternoon sounding, and therefore its not valid for the heights of those clouds over the mountains–the air has sunk coming off them.  So, the best we can do is probably with the morning sounding and assume the clouds were topping out at the -10 C level with bases about 0 C to -2 C.  You can get base height from where they were intersecting the Catalina Mountains (the cloud, not the snow!).  That was at about 6,000 to 7,000 feet elevation, roughly 4,000 feet above ground level here in Catalina.

So, we had pretty cold bases, and not very cold tops.  When we see ice coming out of clouds like these (and I would estimate from airborne experience at the U of WA, that those concentrations of ice coming out of those clouds were a few to 10 per liter).  If that estimate is correct, then we would say that these clouds exhibited “ice multiplication” or “ice enhancement.”  That’s because we don’t expect concentrations that high in clouds with tops around -10 C.  That is,  unless something “extra” is going on to add to the very few ice crystals that might have formed without “multiplication”; perhaps only 1 in 100 liters at -10 C.

So wha happened?

Our most accepted theory is that the very few lead to the many in collisions with cloud drops.  Those cloud drops, when a bit larger than usual (24 microns in diameter for a number), shed ice splinters, and those splinters go on to populate the cloud.  It takes a long time because the itty bitty splinters have to grow to sizes where they can collide with cloud drops, too.  This ice enhancing mechanism, referred to the Hallet-Mossop riming splintering mechanism, named after two scientists named Hallett and Mossop (hahaha), is known to only occur in the temperature range of -2.5 C to -8 C.

Those temperatures were indeed found in our little clouds.  So, too much ice explained!

Not quite.

In little, cold-based clouds deep into the continent as we are here in Arizona, it would NOT be expected that the droplets inside those thin clouds would be large enough (as big as 24 microns in diameter) so that ice splinters would occur when colliding with the rare ice crystal, the 1 in 100 liters one.

So, that’s the mystery of yesterday.   These clouds DID look kind of “maritime” to me.  In maritime clouds, such as those found in onshore flow along our coasts, drops reach that critical size for riming and splintering just above the bottom of the cloud.  That’s because the air is clean over the ocean as a rule,  and so there are few particles for the moisture to condense on, leading to larger drops right away, and many fewer in a liter than in continental clouds.   A typical maritime cloud has less than 100,ooo drops per liter, whereas a “continental” cloud would have several hundred to a half a million drops per liter and even more in some polluted areas.  So, as a drop, its hard to be a big deal in a “continental” crowd like THAT!

Best thought is that the heavy rains of late cleaned the air hereabouts and those clouds that looked soft and “maritimey” yesterday REALLY were like maritime clouds found over the oceans away from land.  Maritime clouds, as ours might have been,  have large drops in them, and even drizzle and raindrops before reaching the freezing level.  Therefore,  ice forms in maritime clouds at the highest known temperatures for natural ice formation, -4 C to -7 C.

The End.  You might want to rest for awhile if you got this far.

The weather ahead?  More rain, continued below normal temperatures as another “cut off” low rolls off the jet stream table in the north Pacific and falls into the Southwest and Baja area in the next coupla days.

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.