Clear ice is formed when:

Clear ice is formed when supercooled water droplets impact a surface and disrupt the liquid layer. This process occurs when droplets that are still in liquid form despite being below freezing come into contact with a cold surface, leading to an immediate freezing reaction that creates a smooth, clear layer of ice.

In these instances, the supercooled droplets do not freeze until they hit something, which can cause them to freeze rapidly as they spread out across the surface. The result is a hard, transparent layer of ice that is particularly hazardous, especially for aviation, transportation, and outdoor activities.

Other processes like warm rain falling on a cold surface or direct condensation to ice have different characteristics and often lead to various types of ice that may be more opaque or slushy. These scenarios do not produce the same clear, hard ice that supercooled droplets do when they strike and freeze instantly.