Ice in clouds
Middle clouds are typically all below 0°C, but they are still almost completely comprised of liquid droplets.
In fact there is virtually no ice in clouds warmer than -10°C, only a mix of ice and water in clouds at about -20°C to -30°C, and clouds are not completely ice until the temperature is down to about -40°C or lower.
Why?
Just as a very small amount of water vapor is unlikely to spontaneously condense into liquid even at the saturation point (100% relative humidity) unless there is a surface on which to condense, a small number of water molecules (such as water vapor in the air or liquid water in a cloud droplet) are unlikely to spontaneously crystallize even if the temperature is somewhat below freezing.
Though it is statistically unlikely for water molecules to crystallize, the chances can be improved a great deal in a few ways:
- Make it really cold: At -40°C and lower the water molecules are unenergetic enough than they will generally form crystals if air is saturated or freeze if already in liquid droplets
- Have a whole lot of water molecules: If there is a lot of water (like a bucketful), then the odds of getting some to randomly crystallize are much better than if your amount of water is miniscule (such as a cloud droplet), and if you can just getting freezing started...
- Start with something conducive to forming ice crystals: this is what typically happens in clouds that are not below -40°C
Water molecules crystallize much more readily if there is something there onto which to build the ice lattice. In a bucket or dog dish, it does not have to be that cold for freezing as some random crystallization within the water and off of microscopic imperfections on the container can begin the freezing and spread it throught. Ice crystals are excellent for making more ice crystals. But in the atmosphere, just as CCN (cloud condensation nuclei) are needed to begin condensation, IN (ice nuclei) with a structure similar to ice crystals are need to begin forming crystals by deposition or freezing. Tiny bits of clay and certain plant materials may serve as IN. They may begin freezing water by either already be contained in a liquid droplet or by bumping into a liquid droplet.
Why do some cumuliform clouds have very sharp outlines or edges while some have very fuzzy and indistinct outlines or edges?
Edges are sharp when that part of the cloud is comprised of water droplets, and the edges are fuzzy when that part of the cloud is comprised of ice crystals. Liquid droplets evaporate quickly if they stray outside the saturated air of the cloud, thus the transistion is very sharp. Ice crystals do not sublimate into vapor as quickly, thus they can move some distance away from the main part of the cloud before disappearing thus making the edge less distinct.