Why do some water droplets in clouds remain liquid at temperature less than zero degrees?
== == Clouds do freeze! Planes without anti-icing equipment
depend on that to stay flying. Flying into a cloud at or near
freezing is a bad idea if you have no method of shedding the ice
that will build up very quickly. The closer to freezing the faster
the ice tends to build up. Subcooling is an actual issue at temps
near freezing. Once the temperature goes about 10 degrees C below
freezing the ice buildup is minimal. Flying into clouds that cold
means the water inside is already frozen. Frozen particles of water
tend to be abrasive to paint, but do not stick.
Freezing is a pretty complicated phenomenon. The molecules slow
down from the lowered temperature, but they aren't always able to
stick together long enough to solidify. If there is a key to start
with, such as another bit of ice, the freezing can start there and
spread to the rest of the liquid. The larger the liquid body, the
more likely it is that there will be something for the freezing to
get started on. Clouds, which consist of small droplets, can have a
hard time getting going. It's common for clouds to get to -10°C
before freezing. This is responsible for many phenomena, particular
freezing rain, where the rain falls as a liquid but solidifies as
soon as it hits a surface, like a road or a power line. When a
large deck of subcooled but unfrozen droplets gets goes over the
edge and starts freezing, the ice sucks the moisture out of the
surroundings, leaving a hole. There's a picture and an explanation
at the link. It's also why hot water pipes are more likely to burst
than cold water pipes. The hot water has had the air driven out, so
it is more resistant to freezing. The cold water pipe is less
likely to subcool, so when it freezes it tends to do it slowly,
giving the water time to escape before solidifying. Hot water pipes
subcool, then freeze suddenly, resulting in high internal pressure
and bursting.
