Droplets of supercooled water often exist in stratus and cumulus clouds. Under some atmospheric conditions, snow crystals may encounter supercooled water droplets. Freezing rain is also caused by supercooled droplets. The crystallization of ice from supercooled water is generally initiated by a process called nucleation. This can also be observed in the nucleation of ice in supercooled small water droplets. This is seen for example in the nucleation of ice in supercooled small water droplets. They are iridescent when the constituent supercooled water droplets are all about the same size. It is similar in appearance to clear ice, which forms from supercooled water droplets. However, the drops themselves do not freeze, a phenomenon called supercooling (or forming "supercooled drops"). The result of the glass transition is a kinetically static state of a supercooled liquid. It is a reversible change of the solid and liquid states of supercooled liquids. Traffic scientists liken such a situation to the sudden freezing of supercooled fluid. In all cases the water supercooled, reaching a temperature of typically before spontaneously freezing. When water is supercooled to temperatures below , it must freeze. When this happens, it is now supercooled liquid water.