Way back about 30 years ago, when I was a passionate and dedicated hang gliding Instructor, (with passionate and dedicated students), I only had an East facing training hill, which limited the number of days we could get out.
One of the ways we increased the number of training days was to get out very early and set up the gliders before the sun rose enough to hit the hill (which was a little bit later than actual sunrise). Then, when the sun first hit the hill, there would often be a little bit of upslope flow that would allow for a few flights before whatever the prevailing direction for the day set in.
The flow was so light that often the windflags would not stir enough to give any reliable indicator of direction, so I used soap bubbles to find suitable launch windows. Yes, Virginia, it is harder to launch in “still air” when the bubbles are going down the hill than when they are going up the hill.
As it turned out, the soap bubbles provided a fair amount of entertainment beyond the flying activities of the day. They allowed new students to better visualize air flow. Blow bubbles upwind of a glider wing and watch the ones above the wing speed up while the ones below slow down - to provide an excellent demonstration of the Bernoulli effect. Or watch the airflow (and turbulence) around obstacles and terrain features. And then there were other surprising things that were pretty cool - like a bubble hitting a dew-drenched thistle. Instead of popping the bubble, it would often come to rest with the sharp thistle points inside of, or even poking through it.
As it also turned out, we ended up blowing bubbles on some very cold mornings, which brings us to the point of this long winded post. When it was well below freezing, the newly blown bubbles would transition fairly rapidly from clear to milky and opaque. When one popped it did not disappear as they normally do, but simply deflated, like a balloon with no air, and would then flutter to the ground.
No longer being a dedicated instructor (neither part any longer applies :-), it has been a long time since I have seen, or even
thought of, frozen bubbles. Fortunately, Jean thought about them on this cold morning, and a balcony off of a warm living room gave ample opportunity to play and take photos of frozen bubbles, as you can see in this collage.