Rain freezes at 32 degrees Fahrenheit (zero degrees Celsius). However, the true behavior of falling rain is complex, often defying this common expectation due to unique atmospheric conditions. Rain is defined as liquid water falling from the sky, but the process of freezing is not always initiated solely by the air temperature dipping below the freezing point. For rain to become a hazard, it must pass through a specific vertical temperature structure that causes it to behave in an unexpected and hazardous way.
The Baseline Freezing Point of Water
The standard freezing point of pure water is 0°C (32°F) under normal atmospheric pressure. This phase change occurs when the water molecules slow down enough to arrange themselves into the crystalline structure of ice. Natural rainwater is not perfectly pure; it contains various microscopic particles like dust and aerosols. These contaminants typically act as ice nucleating agents, providing the necessary template for the initial formation of an ice crystal.
The presence of these nuclei means that natural rain usually begins to freeze near the standard 32°F mark. However, dissolved impurities, such as salts, can slightly lower the freezing point through a process called freezing-point depression.
Understanding Supercooling
The mechanism that allows rain to remain liquid at temperatures far below 32°F is known as supercooling. Supercooling occurs when liquid water is cooled below its standard freezing temperature without turning into a solid. For an ice crystal to form, water molecules must cluster together around a nucleation site, which acts as the seed for crystallization.
If the water is exceptionally pure and lacks these microscopic particles, it can remain in a liquid state, even when the temperature drops significantly below zero. Water droplets can avoid freezing until they reach approximately -40°F, the point where water molecules spontaneously form ice without an external seed. This is known as the homogeneous nucleation point.
Rain becomes “freezing rain” when these supercooled liquid droplets strike an object, such as a road or power line. The impact provides the necessary physical shock and nucleation site, instantly triggering the liquid to freeze into a solid glaze of ice, creating the slick accumulation characteristic of an ice storm.
The Atmospheric Profile for Freezing Rain
The formation of freezing rain requires a specific vertical temperature structure, often called a temperature inversion. The process begins with precipitation forming as snow or ice crystals in the cold upper atmosphere. As the snow falls, it encounters a deep layer of warm air where the temperature is above freezing.
This elevated warm layer causes the snowflakes to completely melt and turn into liquid raindrops. The liquid precipitation continues its descent until it reaches a shallow layer of sub-freezing air trapped near the Earth’s surface. This cold air layer is typically only a few hundred to a couple of thousand feet thick.
This sub-freezing layer near the ground must be too shallow to allow the rain droplets enough time to fully freeze before reaching the surface. The droplets are cooled below 32°F but remain liquid, entering the supercooled state. This liquid rain then falls onto surfaces that are also at or below freezing, resulting in the immediate formation of a clear ice glaze.
Why Freezing Rain Differs from Sleet
Freezing rain is frequently confused with sleet, but the difference depends on the depth of the sub-freezing air layer near the ground. Both phenomena begin with snow melting into rain as it falls through an elevated warm layer.
For freezing rain, the cold layer near the surface is thin, meaning the liquid droplets are supercooled but remain liquid until impact. In contrast, sleet forms when the sub-freezing layer near the ground is deep and extensive. This deep layer allows the supercooled raindrops sufficient time to refreeze while still suspended in the air column.
These refrozen drops hit the ground as small, translucent ice pellets that usually bounce upon impact. Sleet is precipitation that freezes before it hits the ground, while freezing rain is supercooled liquid that freezes on contact with the surface. The difference in the vertical temperature profile determines whether a winter storm produces ice pellets or dangerous, surface-glazing ice.