About ice-in-clouds and APIPs (or high temperature contrails)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

How cold were those Cc clouds?  See below.

(Begin technical module)

In the mid- -20s C, around -15 F.  Height, about 21,000 feet above the ground here in Catalina.  Hope you got that estimate of cloud height right.

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Continuing…

Here’s what happened in the Cirrocumulus cloud layer in yesterday’s special day, a pretty rare one, after the jets flew through it:

 

Lessons to be learned from yesterday’s supercooled clouds and the aircraft interactions inside them:

• Cloud seeding works!  You CAN  make a supercooled, non-precipitating cloud produce a little precipitation that would not otherwise have occurred.

But in those situations where the clouds, say, are topping the Catalinas, they are often quite thin, and whether there is an economically worthwhile amount of precip is not known.  However, an experiment targeting those clouds would be the perfect “baseline” one in cloud seeding to establish how much we can wring out of non-precipitating clouds.   Things become kind of a mess when even randomized seeding takes on already precipitating clouds.

• “Overseeding”,  as here in these clouds when aircraft produce prolific numbers of ice crystals in a small volume,  it leads to tiny ice crystals with low  fallspeeds.  Sure, they fall out and leave a hole, but they virtually never reach the ground except in one a in billion cases when the very cold clouds are real low, practically on the ground.

• The Wegener-Bergeron-Findeisen mechanism produces precipitation.

Alfred Wegener, 1911, and  later Bergeron³ and Findeisen in the 1930s, came up with the hypothesis that adding ice to a supercooled cloud results in the growth of the ice crystal at the expense of the droplets.  They’ll tend to evaporate while ice is being added to the crystal via deposition of water vapor that was once liquid.  So, an awful lot, maybe most of the precipitation that falls on earth, involves “mixed phase” clouds.  This process has also been called the “cold rain process.”

However, let us not forget the two other processes that produce precipitation, the all ice process (no liquid required–helps produce “powder snow”, and the all liquid process, where cloud drops collide and grow into raindrops–the biggest measured drops in the world (about 1 cm in diameter) have formed soley through this process.  It is likely that most of the rain that falls in tropical locations like the Hawaiian Islands and in hurricanes is due to this process even when ice is present in the top part of storms.

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Later, we had some Altocumulus castellanus clouds with virga as the moist level lowered, though they were long gone before they could provide us with a nice sunset:

 

Still looking for scattered very light showers in the vicinity tomorrow as a Mr. Troughy goes by.

The End.

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¹Through the oral history tradition I learned while viewing the Washington Husky meltdown² at AZ stadium on Saturday from a Mr. Mark Albright that the Tucson weather balloon launch site has been moved from Davis-Monthan Airbase to the University of Arizona campus next to their weather department.

²Late in the proceedings, with about 2 min left and the Huskies starting a play, and in the lead, CM was visibly  moved to jump up and say, “Don’t hand the ball off!”, as a gift to Arizona fumble occurred simultaneously.  But, being bifurcated in his loyalties now that CM is in Arizona and not with the University of Washington, he had to be somewhat “glad” that the Cats maintained their somewhat suspect but great win-loss record.

³From the Historic Moments in Weather collection: