A freezing nucleus is any particle that, when found in a body of supercooled water, triggers the formation and growth of an ice crystal around itself. It's believed that the majority of ice crystals in the atmosphere originate from freezing nuclei.

When water vapor in the atmosphere condenses or sublimates (transition from vapor to water or ice), it typically requires the presence of tiny particles known as condensation or freezing nuclei. These micrometric particles serve as sites where water molecules can gather and form droplets or ice crystals. Without these nuclei, water vapor would have difficulty transitioning into liquid or solid phases. These particles can include dust, salt, pollen, or other atmospheric aerosols.