Ice pellets are a common form of winter precipitation consisting of small, hard balls of translucent or opaque ice. These particles are typically spherical or irregular, measuring less than five millimeters in diameter. Unlike other types of frozen precipitation, ice pellets arrive at the surface already frozen solid and bounce upon impact. This phenomenon requires a specific temperature profile within the atmosphere to form.
How Ice Pellets Are Formed
The formation of ice pellets relies on a unique three-layer atmospheric structure. Precipitation begins high in the atmosphere as snow, forming in a cold layer where temperatures are well below freezing. As these snowflakes descend, they encounter a thick layer of warmer air where the temperature rises above \(0^\circ\) Celsius. The snowflakes melt completely into raindrops as they pass through this elevated warm layer.
The rain then continues its descent into a deep layer of sub-freezing air, which extends down to the ground. This bottom cold layer must be deep enough and cold enough to allow the liquid raindrops sufficient time to refreeze completely before they reach the surface. If this lowest sub-freezing layer is about 1,500 feet or deeper, the melted raindrops will turn back into solid ice particles, becoming ice pellets. The resulting pellet may be transparent if it was a fully melted raindrop, or more opaque if it was a partially melted snowflake that refroze.
Distinguishing Ice Pellets from Other Precipitation
Ice pellets are often confused with freezing rain and hail, but their formation and behavior are distinctly different. The depth of the cold air near the ground is the primary factor separating ice pellets from freezing rain. Freezing rain occurs when the lowest cold layer is too shallow, meaning raindrops do not have enough time to refreeze completely before hitting the ground. Instead, the liquid raindrops become supercooled, remaining liquid even though their temperature is below freezing. They freeze instantaneously upon contact with surfaces like roads, trees, or power lines, creating a dangerous glaze of ice.
Ice pellets, conversely, are already solid upon impact and produce a noticeable rattling sound. Hail is also a form of solid ice, but its formation process is entirely different, involving powerful summer thunderstorms. Hailstones grow through repeated cycles of being carried upward by strong updrafts into the cold upper parts of a storm cloud. They accumulate layers of ice before their weight overcomes the updraft and they fall to the earth, often reaching sizes much larger than ice pellets.
Understanding Meteorological Terminology
The term “ice pellets” is the official designation used by the National Weather Service in the United States, yet the public frequently uses the term “sleet” interchangeably. This interchangeability can lead to confusion because, in many other parts of the world, including the United Kingdom and Canada, “sleet” refers to a mixture of falling rain and snow.
Ice pellets generally pose a lower risk of severe icing compared to freezing rain, which coats surfaces with an adhering layer of ice. Because the pellets are solid upon arrival, they tend to accumulate like granular snow rather than forming a cohesive sheet of ice on surfaces. While they still contribute to slick driving conditions, ice pellets do not typically cause the widespread power outages that result from the weight of freezing rain on infrastructure.