Water freezing in the air involves complex atmospheric processes influenced by water purity, microscopic particles, and surrounding air conditions.
The Basic Freezing Point and Supercooling
Water typically freezes at 0°C (32°F) at standard atmospheric pressure. However, pure water can remain liquid below this point, a phenomenon known as supercooling.
Supercooling occurs because water molecules need a “nucleation site” to form ice crystals. Without impurities, dust, or rough surfaces, water resists freezing. Highly purified water can be supercooled to approximately -48.3°C (-54.9°F) before spontaneously freezing. When supercooled water is disturbed or contacts a nucleation site, it can freeze almost instantly.
The Role of Nucleation in Atmospheric Freezing
The freezing of water droplets in the air depends on nucleation, the process where ice crystals begin to form. Without nucleation, cloud droplets can remain liquid well below 0°C.
Two types of nucleation explain atmospheric ice formation. Homogeneous nucleation is the spontaneous freezing of pure water droplets, occurring at extremely cold temperatures, around -35°C to -40°C (-31°F to -40°F). Here, water molecules inherently align into an ice structure.
Heterogeneous nucleation is more common at warmer temperatures, generally between 0°C and -35°C. This process relies on tiny atmospheric particles called ice nuclei (IN). These particles, including dust, pollen, or soot, provide the surface for ice crystals to form. They act as catalysts, allowing ice to form at higher temperatures than homogeneous nucleation.
Atmospheric Conditions and Phenomena
The principles of supercooling and nucleation manifest in various forms of frozen precipitation and atmospheric ice. Humidity, altitude, and the vertical temperature profile of the atmosphere all play a significant role in determining how and when water freezes in the air. Different combinations of these factors lead to distinct weather phenomena.
Frost forms when surfaces cool to below 0°C and also below the dew point, allowing water vapor to directly deposit as ice crystals. This can occur even when the air temperature slightly above the surface is above freezing, as surfaces often cool more rapidly. Freezing fog consists of supercooled water droplets that remain liquid below 0°C, and these droplets freeze upon contact with objects, forming a feathery ice called rime. Freezing fog typically occurs when temperatures are between 0°C and -20°C.
Freezing rain happens when precipitation originates as snow or ice aloft, melts into rain as it falls through a warm layer, and then passes through a shallow sub-freezing layer near the ground. The raindrops become supercooled and freeze instantly upon impact with surfaces that are at or below 0°C. Sleet, also known as ice pellets, forms under similar conditions but with a deeper sub-freezing layer. Snowflakes melt into rain in a warm layer, but then refreeze into solid ice pellets before reaching the surface.
Snow itself forms when atmospheric temperatures are at or below 0°C, and there is sufficient moisture. Tiny ice crystals form and grow by absorbing additional water vapor, eventually becoming large enough to fall as snowflakes. Even if the ground temperature is slightly above freezing, snow can still reach the surface, as the melting process can cool the air immediately surrounding the snowflake.