I. Q. Test (“Ice Q”, that is)

Yesterday was a great day both for airborne researchers studying the onset of ice in clouds, and for my followers to test their “ice” Q detecting abilities, to come up with a clever play on words there.

What was so great about yesterday’s clouds?

Well, they were real cold, bases up around 9,000 feet above Catalina (about 12,500 feet above sea level) at -7° C (19° F).   Excellent.  Nice data point.

Cloud tops?

This is what was pretty great for you and me; they didn’t overshoot much, the clouds were pretty flat, not very deep, not a lot of flight time needed climbing to cloud top to see what it was around here.   That means that if you are flying around up there sampling clouds for ice content, that the tops you smashed with your aircraft were pretty much the ones at the temperature that the ice crystals you ran into later formed at.  Remember, when cloud tops first rise up, they usually have little detectable ice (the ice crystals are too small for your instruments, or, they haven’t formed yet, takes a little time.

When there are big overshooting tops,  an inexperienced, well, crummy researcher in an aircraft finding the ice, as it is usually found, lower down in the cloud, might put the origin of the ice at the temperature of the collapsed top, not at the lower temperature where it formed and the original top reached up to.

So, the lack of much overshooting made it a great day to assign the ice you found to the right cloud top temperatures.

What else was great?

It was a marginal day for ice formation here in the Catalina area, so you get a good data point on when ice starts to form in clouds given that base temperature.  As the cloud deepens upward, more ice would be expected with the lower temperatures.

And, as noted by Ludlum way back in the 1950s, and by Prof. Battan right here at the University of Arizona which I did not attend, btw, that level at which ice and precip onsets changes from day to day (largely related to how warm (crazy isn’t it?) the cloud base temperature is.  On days with warmer cloud bases, the ice onset temperature is also higher.    For example, in summer here, its not unusual to have ice onset between temperatures of -5° and -10° C (23° and 14° F) when bases are warmer than about 10° C.

Anybody still out there?

So, yesterday, with the deepest Cumulus clouds around 2,000 to three thousand feet thick right in our area (they were deeper elsewhere), tops were running around -15° C, this temperature, as you know, leads to the formation of plate-like crystals, hexagonal plates, stellars (Christmas card crystals), maybe some spatial dendrites (stick out in different directions) if the latter crystals were in the Cumulus cloud long enough.  If the concentrations of ice get high enough, you’ll get “snowflakes”, interlocking dendritic crystals.   A single, good-sized snowflake might have 20-50 individual dendritic crystals.

Is anybody still out there?

Below some shots from yesterday afternoon when there were traces of ice spewing out of local clouds.  Did you see those regions and note them in your cloud diaries, that’s the important question.

3:23 PM. A nice view of the overall scene around here with our small Cumulus clouds (Cumulus humilis and mediocris). You see a house in the distance off this dirt road. Pretty say when you think that in America some people still live on dirt roads.
3:49 PM. Oh, there’s a nice little Cumulus toward the Charoleau Gap. Doesn’t seem to have any ice…. Let us look closer, and, of course, we look for ice to appear at the downwind edge where cloudy air has been in the cloud the longest, see if anything is falling out.
3:49 PM zoomed view. Oops there it is, a little ice, single crystals, concentrations likely lower than 1 per liter of air. You wouldn’t expect to find any “aggregates” here, since they require higher concentrations of ice to bump into each other and lock together. Wow, this is an incredible amount information based on a little hazy spot in a photograph!
4:13 PM. Lets look over here toward the south. OK, there’s an obvious ice haze beyond the Catalinas, but what about the cloud in the middle? See anything coming out the downwind (right) side?
4:13 PM zoomed view of the fall of ice crystals out of this cloud. This patch of ice haze is so obvious it would have been pretty embarrassing for you not to have noticed it, made a note about it.   Of course, we care so much about ice because that’s where nearly all of our rain here in Arizony comes from, as you know, recalling the work of Wegner, Bergeron, and Findeisen, where it was shown that an ice particle in a water cloud will grow at the expense of the droplets around it.  For a time, it was thought that all precip of consequence was due to that process, but not so.  Ask Hawaiians.  Or powder snow lovers about storms consisting only of little dry ice crystals, no water drops in those clouds.

Stormy weather still ahead as noted here I don’t know how long ago.  April looking more and more to be a generous month of rain here in Catalina.  But will those showers be too late for May flowers?

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.