The question of how cold the air must be to freeze water suspended in the atmosphere is more complex than simply knowing the standard freezing point. While the temperature needs to be well below the familiar 32°F (0°C), the actual temperature required depends on the water’s form—whether it is a tiny, pure droplet in a cloud or a stream of liquid water thrown from a cup—and what is present in the air. Under specific circumstances, water can resist freezing even in extremely cold conditions, requiring either a physical trigger or a far lower temperature to initiate the solid state. Understanding this process involves looking at how water’s molecular structure changes and the role of impurities in the air.
The Baseline: Freezing Point and Supercooling
The theoretical freezing point of water is universally accepted as 32°F or 0°C at standard atmospheric pressure, but this only describes the temperature at which liquid water and solid ice can coexist in equilibrium. In the atmosphere, where water is often present as small, isolated droplets, this temperature is frequently insufficient to cause freezing. Pure water, free of impurities, can resist crystallization far below 0°C, a phenomenon known as supercooling.
Supercooling occurs because the water molecules lack a suitable surface to arrange themselves into the rigid, crystalline structure of ice. Suspended water droplets can remain in a liquid state down to temperatures approaching -40°F, or -40°C, before freezing spontaneously. Clouds at high altitudes often contain a mixture of ice crystals and supercooled liquid water, even when the air temperature is well below freezing. The smaller and purer the water droplet, the colder it can become while remaining liquid.
The Necessity of Nucleation Sites
To overcome the liquid state of supercooled water at warmer temperatures, a physical trigger known as a nucleation site is required. These sites are provided by Ice Nucleating Particles (INPs), which are airborne aerosols that act as a template for ice crystal growth. Without a nucleator, the water must rely on a rare, random alignment of its own molecules to form an ice embryo, which only reliably happens at the extremely low temperature of homogeneous nucleation.
Atmospheric INPs are a fraction of all aerosol particles, and they include substances like mineral dust, pollen, fungal spores, and certain types of bacteria. These particles allow water to freeze heterogeneously, meaning it freezes at a temperature warmer than the homogeneous limit. Tiny droplets containing an immersed nucleator can begin to freeze at temperatures between 23°F (-5°C) and -31°F (-35°C), depending on the nucleator’s chemical structure and surface properties.
Ice nucleation can occur through several distinct mechanisms. These include when a nucleator becomes immersed in a supercooled droplet, or when water vapor deposits directly onto the particle’s surface to form an ice crystal. The presence of these particles determines that most ice formation in the atmosphere, such as the initial formation of snow and ice in clouds, occurs at temperatures well above the -40°F threshold.
The Temperature Required for Instant Freezing
The spectacular visual effect of instantly freezing water, as seen when liquid is thrown into the air, requires conditions that accelerate the freezing process dramatically. For this rapid, visible transformation to occur, the air temperature generally needs to be in the range of -20°F to -40°F (-29°C to -40°C). The exact temperature depends less on the theoretical freezing point and more on the rate of heat transfer.
Boiling water is often used for this demonstration because the high temperature creates a large amount of water vapor and very small droplets when thrown. The small size of these droplets, combined with the extreme temperature difference between the water and the air, maximizes the rate of evaporative heat loss. As the hot water is dispersed, a significant portion flashes into vapor, rapidly removing latent heat from the remaining liquid droplets. This cools them far faster than room-temperature water.
While instant freezing can be observed around -22°F (-30°C), air temperatures need to be closer to -40°F (-40°C) for all the tossed water to freeze into ice crystals before hitting the ground. Below -40°F, the air is cold enough to induce homogeneous nucleation in nearly all the small droplets almost immediately upon contact. The visible cloud created at warmer temperatures, such as -14°F, is often primarily condensed water vapor, which is liquid fog, rather than solid ice crystals.