How ice forms
In normal circumstances, the freezing point of water is zero degrees Celsius. But for ice crystals to form, ice nucleating particles are needed (e.g. dust particles). The more similar the surface structure of a material is to ice, the more suitable it is as a nucleating particle.
Highly purified water can be cooled to below -30 degrees in a laboratory before it freezes. When liquid water has a temperature of below zero degrees, this is known as “supercooled water”. The freezing point of water can be lowered by adding salt (or sugar), a fact that is exploited by the winter road maintenance services to prevent or delay ice formation on roads.
In summary, ice can form in the presence of sufficient moisture, ice nucleating particles and sub-zero temperatures. Because the density of ice is lower than that of liquid water (known as the water density anomaly), ice floats on the surface of water. Only about 10% of the ice volume protrudes from the water, leaving the remaining 90% of the ice volume below the water’s surface.