Rain generally does not freeze at 33 degrees Fahrenheit (1 degree Celsius) because this temperature is above water’s standard freezing point. The behavior of water can be more complex under certain atmospheric conditions.
The Baseline: Water’s Freezing Point
Water’s standard freezing point is 32 degrees Fahrenheit (0 degrees Celsius) at typical atmospheric pressure. At this point, water molecules slow down sufficiently, allowing their attractions to arrange them into a fixed, hexagonal crystalline structure. At 33 degrees Fahrenheit, water molecules retain enough kinetic energy and movement to remain in their liquid state. They are still moving freely and sliding past each other, rather than locking into the rigid structure of ice. Therefore, under normal circumstances, water at 33 degrees Fahrenheit remains liquid.
The Science of Supercooling
Water can sometimes remain in a liquid state even when its temperature drops below its standard freezing point, a phenomenon known as supercooling. This occurs because the formation of ice crystals requires not only cold temperatures but also the presence of nucleation sites. These sites are tiny impurities, dust particles, or rough surfaces that provide a template for water molecules to begin arranging into an ice structure. In the absence of such nucleation sites, water molecules can continue to move freely even when cooled below 32 degrees Fahrenheit. Highly purified water, or water in very clean environments, can supercool significantly, sometimes to temperatures as low as -55 degrees Fahrenheit (-48.3 degrees Celsius).
However, this supercooled state is unstable. If supercooled water is disturbed, for example by shaking or by the introduction of a nucleation site, it will rapidly freeze. The water molecules quickly arrange into a crystalline structure, releasing latent heat and solidifying almost instantly.
When Rain Freezes on Impact
Supercooling is relevant for understanding freezing rain. Freezing rain typically begins as snow or ice crystals higher in the atmosphere that melt into liquid as they fall through a warmer layer of air, then encounter a shallow layer of air very near the ground that is below freezing. The raindrops cool rapidly in this sub-freezing layer but do not have enough time or nucleation sites to freeze into ice pellets before reaching the surface. When these supercooled raindrops strike surfaces that are at or below freezing, such as roads, trees, or power lines, the physical impact and the presence of nucleation sites on these surfaces trigger immediate freezing, forming a layer of clear ice, known as glaze ice. This process is distinct from sleet, where precipitation freezes into ice pellets while still in the air, or snow, which remains frozen from cloud to ground.