Freezing Nucleus

What is Freezing nucleus?

A freezing nucleus is a specific type of particle that induces the growth of an ice crystal within a mass of supercooled water. Most ice crystals found in the atmosphere are believed to form through the action of these nuclei.¹²

Freezing nuclei are a category of ice nuclei, which are aerosols that facilitate the conversion of water vapor or liquid water into ice. Unlike deposition nuclei, which allow water vapor to deposit directly as ice, or contact nuclei, which trigger freezing upon touching the surface of a droplet, freezing nuclei must be absorbed into the liquid water to convert it to ice from the inside out.²

Because ice nuclei are often scarce in the atmosphere, liquid water can frequently remain in a liquid state even at temperatures below 0 °C (32 °F); this is known as supercooled water. In the absence of such nuclei, small cloud droplets typically do not freeze until temperatures reach approximately −39 °C (−38 °F).²

What is the difference between deposition and contact nuclei?

Deposition nuclei and contact nuclei are both types of ice nuclei that facilitate the formation of ice crystals in the atmosphere, but they differ in their physical interaction with water.²

• Deposition Nuclei: These function similarly to condensation nuclei. They allow water vapor to transition directly into a solid state, depositing as ice crystals onto the surface of the aerosol.²
• Contact Nuclei: These are involved in the conversion of existing supercooled water into ice. A contact nucleus triggers this phase change by physically touching the surface of a supercooled water droplet.²

In contrast, freezing nuclei (another type of ice nuclei) must be absorbed into the liquid water to initiate freezing from the inside out. While these processes are distinct, they all serve as essential initiation sites for ice formation in the atmosphere, especially when temperatures are above the threshold for homogeneous ice nucleation (approximately −39 °C or −38 °F).²

How does cloud seeding work to create precipitation?

Cloud seeding is the deliberate introduction of substances into clouds to induce or increase precipitation. The process varies depending on the cloud’s temperature.⁴⁵

Seeding Supercooled Clouds

Many clouds contain supercooled water—liquid droplets at temperatures below 0 °C—that remain stable because they lack sufficient ice nuclei. Seeding agents like silver iodide or dry ice are introduced to these clouds to act as artificial nuclei. These agents cause the water droplets to evaporate, with the resulting water vapor depositing onto the nuclei to form ice crystals. These crystals grow as surrounding water droplets attach to them, eventually becoming heavy enough to fall as rain or snow.⁴⁵

Seeding Warm Clouds

In clouds with temperatures above freezing, seeding agents such as sodium chloride or calcium chloride are used to provide condensation nuclei. These particles encourage the formation of giant cloud droplets that grow through coalescence, eventually falling and sweeping up smaller droplets to produce rain.⁴⁵

Why can water remain liquid below the freezing point?

Water can remain in a liquid state below its standard freezing point of 0 °C (32 °F) because of the absence of effective ice nuclei. This phenomenon is known as supercooling.⁷

In the atmosphere, the transition from liquid water to ice typically requires a surface—an ice nucleus—to initiate the formation of an ordered crystalline structure. When these particles are scarce, water molecules lack the necessary template to begin freezing, even at subfreezing temperatures.⁷

For small droplets, such as those found in clouds, the probability of forming an ice crystal by random chance is extremely low. Without ice nuclei, these droplets can remain liquid until temperatures reach approximately −39 °C (−38 °F), at which point homogeneous ice nucleation occurs spontaneously. If a “seed” (such as a dust particle or an artificial agent like silver iodide) is introduced into a supercooled liquid, it provides the structure needed for ice to form, triggering rapid crystallization.⁷²