Here they are, reflecting the heat island of Mt. Lemmon yesterday, repeated narrow surges of heat and cloud sprouting upward, and only one reaching the level where ice formed, and a little snow fell out–second photo. Go here to the U of A fubball-practicing Wildcats Atmospheric website to see the whole interesting sequences of pulses yesterday.
Note frizzy stuff at left and below residual cloud patch in the second photo. That’s ice that formed because the top of the cloud reached temperatures well below freezing, and is a good example of the threshold level at which ice formed yesterday because there is only a small amount coming out of this cloud. Had that top ascended another couple of thousand feet, it is likely that it would have been all frizzy and fibrous; completely ice.
The height at which cloud tops begin to form substantial ice tends to change day to day and much of that due to how warm the bottoms of the clouds are. The warmer the bottoms of the clouds, the higher the temperature at which ice first forms in clouds! The highest temperature at which ice has been observed to form in any cloud is around -4 C (25 F), and then only when the cloud has formed rain already by an all liquid process called coalescence where cloud droplets merge to form bigger droplets, and eventually those colliding-merging drops reach sizes where they qualify as drizzle or rain drops. Here in Arizona, clouds occasionally form ice in clouds with tops warmer than -10 C, but mostly they have to be below -10 C. It happens, too, but it is rare that clouds here form rain by the collision-coalescence process.
It may seem odd that ice does not form in clouds when they get colder than 0 C (32 F), but rather at lower temperatures, sometimes much lower. This is a mystery that is still being investigated to this day.
When did we find out the complexity of ice formation in clouds, to continue a bit of a lecture?
We really found this out in Project Whitetop, a large, sophisticated and randomized cloud seeding experiment in Missouri carried out in the early 1960s under the aegis of the University of Chicago. When researchers went up in aircraft to examine clouds on not seeded days, they they found that the Cumulus clouds already had some ice in them in cloud tops that had never been colder than -10 C. This was quite a surprise since nobody really thought ice formed much in all clouds until the tops were at least around -20 C (-4 F). This was because measurements on the ground of artificial clouds in cloud chambers chilled to -20 C were almost always ice free until that temperature. Nature’s trick?