Winter precipitation comes in several forms, and while both snow and sleet are common, they represent distinct physical processes in the atmosphere. These types of frozen water are frequently confused, yet they pose different challenges and accumulate uniquely. The differences lie not just in appearance, but in the specific journey each takes through the vertical layers of the atmosphere. This article clarifies the nature of snow and sleet and explains the meteorological conditions that create each one.
Defining Snow and Sleet
Snow is precipitation composed of delicate ice crystals that form into recognizable snowflakes. These hexagonal crystals aggregate as they descend, resulting in the light, intricate structures characteristic of fresh snowfall. Snow is a collection of individual or clustered ice crystals that maintain their frozen state throughout their fall.
Sleet is defined as translucent or opaque ice pellets. These pellets are essentially raindrops that have frozen solid while still in the air. Sleet consists of distinct, hard particles of ice that typically bounce or make a tapping sound when they strike the ground. This physical state distinguishes it from the fluffy, crystalline structure of snow.
The Atmospheric Layers Required for Formation
The difference between snow and sleet is determined by the vertical temperature profile of the air column. For precipitation to fall as snow, the temperature in every layer from the cloud base to the surface must remain at or below the freezing point of 32°F (0°C). When this condition is met, the ice crystals descend directly to the ground without melting, preserving their crystalline structure.
Sleet requires a complex, three-layer temperature structure to form. Precipitation begins as snow high in the atmosphere where temperatures are below freezing. As the snow falls, it encounters an elevated layer of air, often called a warm nose, where the temperature rises above 32°F. In this warm layer, the snowflakes melt entirely or partially, turning into liquid water droplets.
The liquid then continues to fall until it passes into a deep layer of sub-freezing air, situated just above the surface. This deep, cold layer provides enough time for the melted raindrops to fully refreeze into small, solid ice pellets before they impact the ground.
Differences in Ground Impact and Accumulation
Snow and sleet behave differently in terms of accumulation and hazard. Snow, due to its low density and intricate structure, creates a light, fluffy blanket with a high volume-to-weight ratio. This soft accumulation is relatively easy to remove and blankets surfaces uniformly, often resulting in deep drifts.
Sleet, conversely, consists of dense, pre-frozen ice pellets that have a high weight-to-volume ratio. Because the precipitation is already solid when it hits the ground, it does not typically accumulate into the deep, soft drifts characteristic of snow. Instead, the small ice pellets often compact quickly or mix with other surface moisture to create a slushy or icy layer.
The immediate consequence of sleet is the creation of slick, icy conditions, especially on roads and walkways. The hard pellets bounce and then settle, forming a thin, dense layer of ice that is hazardous for travel and walking immediately upon impact. The sound of sleet hitting a window or car is also a distinct, sharp tapping noise, unlike the quiet, muffled fall of fresh snow.