Sleet, officially known as ice pellets, is a unique type of winter precipitation that undergoes a complex transformation within the atmosphere. It is defined as precipitation that freezes into small, solid balls of ice before reaching the ground surface. This process requires a specific arrangement of warm and cold air layers positioned vertically above the earth. The journey from a cloud to a bouncing ice pellet explains why this form of precipitation is distinct from both snow and rain.
The Essential Vertical Temperature Profile
The formation of sleet depends entirely on a layered thermal structure in the atmosphere, often described as a temperature inversion. Precipitation begins high in the atmosphere, where temperatures are well below the freezing point of 0°C (32°F), allowing snow crystals to form and begin their descent. These snowflakes then encounter a layer of warmer air, which is the middle section of the required three-layer structure.
This elevated warm layer, where temperatures rise above 0°C, must be deep enough to melt the falling snowflakes completely or partially into raindrops. This warm air is often situated thousands of feet above the ground, creating a band of liquid water suspended high in the atmosphere. This melting process converts the initial solid snow into a liquid state.
Beneath the warm air, the liquid droplets enter a second, sub-freezing layer of cold air that extends down to the surface. This lowest layer must be sufficiently deep and cold to allow the melted raindrops time to refreeze completely before impact. As the liquid water falls through this deep layer of below-0°C air, it solidifies into small, translucent ice pellets.
How Sleet Differs from Freezing Rain and Snow
The specific thicknesses of the atmospheric layers differentiate sleet from other common forms of winter precipitation like snow and freezing rain. Snow occurs when the temperature remains at or below 0°C throughout the entire atmospheric column from the cloud base to the ground. In this scenario, the snowflakes never encounter a warm layer deep enough to melt them, allowing them to remain in their original crystalline form.
Freezing rain requires a deep warm layer that melts the snow completely into rain. However, the sub-freezing layer near the ground is too shallow to allow the liquid droplet time to refreeze while falling. The supercooled liquid water then strikes cold surfaces on the ground, such as pavement or trees, and instantly freezes upon contact, forming a slick glaze of ice.
Sleet exists in a unique middle ground between these two extremes, characterized by the “sandwich” of warm air between two cold layers. The distinction is the depth of the freezing layer closest to the ground. For sleet, this layer is deep enough to ensure the complete refreezing of the droplet into a hard ice pellet before it reaches the surface.
Characteristics and Behavior Upon Impact
Once the precipitation has completed its journey, it arrives at the surface as a small, hard, translucent ball of ice. These ice pellets typically measure less than five millimeters in diameter. The impact of these solid pellets creates a distinct, audible sound, often described as a “tinkling” or “pattering” when they strike objects like car windshields or jackets.
A characteristic of sleet is its behavior upon reaching the ground, where the pellets typically bounce off hard surfaces. Because the water droplet has already completely frozen in the air, the pellets do not adhere to surfaces in the same way that freezing rain does. While an accumulation of sleet can certainly create slick conditions, it generally does not form the solid, heavy glaze of ice that is characteristic of a freezing rain event.