Final answer:
Freezing rain happens when supercooled water droplets hit a cold surface and freeze upon impact. They can remain in a liquid state even below their normal freezing point due to the absence of a nucleus for ice crystal formation.
Step-by-step explanation:
Freezing rain (or drizzle) occurs when falling water drops are supercooled; thus, drops partially freeze when they strike cold surfaces.
Supercooling of water droplets has significant effects on the Earth's climate. The process of supercooling describes a state where water remains liquid even at temperatures where it normally would be frozen. Water droplets in clouds must aggregate around a nucleus, such as a dust particle or an ice crystal, to form precipitation. These water droplets can remain as a supercooled liquid down to about -10°C, inhibiting their conversion into ice crystals, which are better nuclei for rain formation.
In weather events like freezing rain, these supercooled droplets fall towards the Earth's surface, and upon contact with cold surfaces, they transition from a supercooled state to ice. Supercooling can also be manipulated, such as in cloud seeding using dry ice, which cools the water droplets further, causing them to crystallize and act as nuclei for raindrop formation, thereby potentially inducing precipitation in areas that would otherwise remain dry.