What condition is necessary for rime ice to form?

Rime ice formation requires temperatures that are at or below freezing, typically around -15 degrees Celsius or colder. This condition is crucial because rime ice occurs when supercooled water droplets in the atmosphere come into contact with surfaces that are at a temperature below freezing. The droplets freeze instantly upon contact, leading to the build-up of rime ice, which appears as a white, fluffy coating on objects.

In contrast, higher temperatures, whether above zero or within the range of 0 to -10 degrees Celsius, are insufficient for rime ice formation, as they don't allow supercooled droplets to freeze on contact. Warm and humid conditions are also not conducive to rime ice since the heat prevents the necessary freezing process from occurring. Therefore, the response indicating temperatures colder than -15 degrees Celsius aligns perfectly with the environmental conditions that facilitate the formation of rime ice.