Why do some water droplets in clouds remain liquid at temperature less than zero degrees?

Answer 1

== == 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.

Answer 2

Ice crystals require something to start growing on (a nucleus), such a sdirt/dust etc. If the droplet is pure, there is no nucleus to start off the crystallisation process. Hence a drop of water becomes supercooled. Water will spontaneously freeze at -42C (approx) if it lacks the nucleus.

Answer 3

if you dissolve something in water it will lower the meltingpoint of the water. as example if you dissolve salt in water it's melting point will be below 0°C

that's why they use it to make roads icefree.