The question of whether snow is simply rain that has frozen is a common misunderstanding of winter precipitation. The definitive answer is no, because the process by which true snow forms is fundamentally different from how frozen rain develops. Meteorologists distinguish between snow, sleet, and freezing rain based entirely on the vertical temperature structure of the atmosphere, known as the temperature profile or sounding. Understanding these distinct formation mechanisms is necessary to accurately predict which type of precipitation will fall during a winter storm.
How True Snow Forms
Snow begins in the upper atmosphere through deposition, where water vapor transforms directly into ice without passing through a liquid phase. This occurs when water vapor molecules freeze onto microscopic particles, such as dust or pollen, which serve as ice nuclei. These initial ice crystals then grow by attracting more water vapor, forming intricate hexagonal structures.
For these developing ice crystals to reach the ground as snow, the atmospheric temperature must remain at or below the freezing point of 0°C (32°F) from the cloud base all the way to the surface. If the temperature profile is entirely sub-freezing, the snowflakes continue to grow through aggregation—colliding and sticking together—until they fall as snow. True snow is an ice crystal that has never melted, making it distinct from any form of frozen liquid water.
When Rain Freezes Before Impact (Sleet)
Sleet is composed of small, translucent ice pellets that bounce when they hit the ground. This precipitation type starts in the cloud as snow, but then encounters a specific three-layer temperature structure in the atmosphere. The snowflake first falls into a layer of air that is above freezing, causing it to melt completely or partially into a raindrop.
The crucial step for sleet formation is the subsequent fall through a deep layer of sub-freezing air, typically close to the ground. This cold layer allows the melted raindrop enough time to refreeze solid into an ice pellet before reaching the surface. This process involves the re-freezing of liquid water that occurred aloft, unlike the direct formation of snow.
When Rain Freezes On Impact (Freezing Rain)
Freezing rain is liquid rain that falls but freezes instantly upon contact with surfaces at or below 0°C (32°F). This condition requires a temperature profile similar to sleet, but with one difference in the layer closest to the ground. The initial snowflake melts entirely as it passes through a relatively deep layer of warm air above the ground.
The melted raindrop then falls through a shallow layer of sub-freezing air right at the surface. Because this cold layer is not deep enough, the raindrop does not have time to freeze into a solid pellet. Instead, the drop becomes “supercooled,” meaning it is liquid water existing below its normal freezing point. When these supercooled drops strike objects like roads or power lines, the contact triggers immediate freezing, creating a glaze of clear ice.
The Role of Atmospheric Temperature Layers
The specific type of winter precipitation that reaches the surface is determined by the vertical distribution of temperature, known as the atmospheric sounding. Subtle variations in the thickness and height of warm and cold air layers dictate the fate of the initial snowflake. Meteorologists use weather balloons to measure this vertical profile, which is the most important factor in forecasting winter precipitation.
If the entire atmospheric column is below freezing, the outcome is true snow. This is the simplest profile, requiring a consistent cold air mass throughout the troposphere. The presence of a warm layer aloft, where the temperature rises above freezing, introduces the complexity of mixed winter precipitation.
The difference between sleet and freezing rain hinges on the depth of the cold air layer near the surface. A deep sub-freezing layer, typically more than a few thousand feet thick, allows enough time for the melted drops to refreeze into ice pellets, resulting in sleet. Conversely, if that sub-freezing layer is shallow, the supercooled rain reaches the ground as a liquid that freezes on contact, producing the glaze of freezing rain. A temperature change of only a few degrees, or a slight shift in the height of these layers, can cause a precipitation type to transition over a short distance.