An interesting day from a cloud modification standpoint, one that doesn’t happen very often

Every once in a great while, we have days where fairly thick clouds do not produce even a sprinkle, even though their tops are a little below  freezing, but not quite cold enough for natural ice to form.  Yesterday was one of those days.

And it was a day you, a cloud maven junior member,  could likely have done something about it:  rented a small plane or helicopter capable of flying up to around 15,000 kft ASL,  taking a bag of commercially available dry ice pellets, then drop them into the fattest, highest Cumulus tops you saw while nipping them in VFR flight mode, and, “violet!”,  ice would have formed along the path of the falling dry ice pellets!

So what were the ingredients that  made yesterday so special for a little renegade cloud seeding?

The clouds that did not rain were pretty thick for ones that didn’t rain naturally, maybe 5,000 to 6,000 thousand feet thick in their maximum “overshooting” tops, and temperatures at top were a little below freezing, but warmer than -10° C.  At lower top temperatures ice would likely have formed naturally.  Here’s the annotated TUS sounding from yesterday afternoon from IPS MeteoStar:

Ann 2016110500Z_SKEWT_KTUS
The 00 Z (5 PM AST) rawinsonde from TUS, typically launched about 3:30 PM AST. The upward pointing arrow shows what the in-cloud temperature would have been like. The lapse rate, from aircraft measurements is virtually never along the “pseudoadiabatic temperature line (one of which is where the horizontal arrow heads are), but somewhere between that and the dry adiabatic temperature lines that show the temperature drop in a rising dry parcel of air (one of which is where the upward pointing arrow begins). Cumulus protrusions carry the boundary layer air from the surface, that air that forms the Cumulus clouds, into the stable,, and warmer overlying air. So, protruding tops sink like a stone; don’t stay long at their lowest temperature, also hurting the chances that ice will form. That’s why you have to do it.

Here’s how it works:  the dry ice pellets, themselves at -72° C, will chill the air it comes in contact with to -40° C, resulting in the formation of jillions of tiny ice crystals in each pellet’s wake, which are then spread over a wider region in the following minutes due to turbulence in the cloud.  In essence, each pellet is creating a tiny,  vertical “contrail” in that cloud, as least in those upper parts of the cloud below freezing.  (Bases yesterday were a little above freezing, around 2° C, while the highest afternoon tops locally appeared to run between -5° and -10° in clouds that were forming in more haze and smoke than usual (wonder if you noticed that?)  Haze and smoke tend to reduce droplet sizes, and in doing that, make it harder to form ice and rain, especially in marginal clouds for that, such as we had yesterday.

What happens next is that the “supercooled” water in the cloud evaporates around those crystals due to the dry ice bombardment, while the crystals take up that evaporated vapor.   When the crystals get large enough, they may collide with some remaining cloud droplets, if there are any around.  Usually all those crystals that have formed will not left too many droplets in their vicinity.

As the crystals grow in size, and because they are in such high concentrations, they will bump into one another and form clusters of ice crystals we call snowflakes.    Cloud Maven Person has, along with Professor Doctor Lawrence F. Radke, the latter the  “Flight Scientist” in those days with the University of Washington Huskies’ Cloud Physics Group1 in the late 1970s,  made snowflakes the size of pie plates (fluffy light ones without a lot of water content) in Cumulus clouds like yesterday’s here.

IMO you would have created not something of much importance, but rather just an annoying sprinkle or very light shower for those out hiking,  horseying around on their horses, biking the trails,  on an otherwise perfect day for outdoor activities.

One of the problems, long known about in such seeding experiments as could have taken place yesterday, is that the cloudy air is moving THROUGH the cloud, exiting at the downwind location.  That is, lower clouds in particular, move SLOWER than the air itself2.

So, you drop some dry ice in a nice turret, the air you dropped it is, along with that turret’s air, will be moving downwind and is going to go out into clear air eventually.    So, if the crystals don’t stay in a turret and upward moving air, but goes out the side of the cloud or into “shelf clouds” like yesterday, those crystals/snowflakes aren’t going to grow much, and will remain “light and fluffy” even though they could be huge because they are like “powder snow” not a lot of water mass in them.   When they melted at cloud base, they might end up being just drizzle-sized drop (less than 500 microns across) or very small raindrops.  So, that’s why you would likely have gotten just a sprinkle or very light rain shower had you done some unlawful, renegade cloud seeding yesterday.  Remember, just like when you hike in the State Land Trust areas, you need a permit to seed legally.

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Yesterday’s clouds

7:02 AM. Tall, but not so tall as to form ice, Cumulus clouds boil up off the Catalinas. Made you think some Cumulonimbus clouds might form later on. They didn't.
7:02 AM. Tall, but not so tall to form ice, Cumulus clouds boil up off the Catalinas. Made you think some Cumulonimbus clouds might form later on. They didn’t.
10:26 AM. Disorganzied Cumulus still lurking on on the Catalinas, but the main thing here is how much smoke was in the air when you might have been expecting a very clean morning due to the previous evening’s thunderstorms and rains. Very upsetting to this smoky scene.
10:27 AM. Looking NE at some Cumulus congestus, no ice evident from this view, but I would not rule it out, Forgot to check radar to see if there was an echo with this cloud.
2:10 PM. By mid-afternoon skies started to look a little threatening with a Cumulus congestus having formed over and extending downwind from the Tortolita Mountains.
2:14 PM. Looking for ice to appear in the oldest top portions, now evaporating, top of photo. None seen.
2:39 PM. Someone needs to get up there on top of this Cumulus congestus, drop a little dry ice in it. Nothing came out, though I thought I would feel a drop at any moment! Some sort of birds can be seen, lower right.
3:19 PM. Looking downwind at part of the base of this Cumulus congestus cloud line that sat over Catalina for awhile. The top has the base, to the left is the “shelf cloud of Stratocumulus spreading out from other tops and drifing downwind. If precip falls out of the shelf cloud, once part of a turret, you can see I hope that it would fall out of a thicker column of dry air.


4:01 PM. The scene before a lot writing appears on this photo.
The same photo with a lot of writing on it.
The same photo with a lot of writing on it.

The weather way ahead

While warm weather returns to AZ over the next week to12 days or so, there is now, and this goes with climo, a big trough that barges into all of the West Coast in two weeks.

When I say climo, I mean that there  is a noticeable tendency for this to happen in mid-November in the longterm upper air records so that in some areas of California, for example, there is a modest increase in the chance of rain in mid-month over other times in the month.  These kinds of things in weather are termed, “singularities” like the supposed, “January thaw” back East.  This mid-November annual trough passage may be related to the increasing speed of the jet stream in the Pacific as winter approaches, something that changes the spacing between the troughs.  Pure speculation.

But in any event, be on the lookout for a major change in weather here between the 17th and 20th of November.  Something like this is starting to show up in the models.

The End

1Later renamed the Cloud and Aerosol Research Group.

2Something that was  even noticed in small tradewind Cumulus in the Pacific in the 1950s by Joanne Malkus (later, Joanne Simpson) and her colleagues.

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.