Though both sleet and hail are forms of frozen precipitation, they originate from entirely different atmospheric processes and occur under distinct weather conditions. Their formation mechanism provides the clearest distinction, separated by differences in the vertical temperature profile and the type of cloud in which they develop. Understanding these meteorological differences explains why one is a winter occurrence and the other is associated with summer thunderstorms.
The Formation Process of Sleet
Sleet, formally known as ice pellets, requires a specific vertical temperature structure in the atmosphere to form. This structure begins with sub-freezing air high up, where precipitation starts as snow or ice crystals. As these crystals descend, they encounter a relatively deep layer of air above the freezing point, typically between 5,000 and 10,000 feet. This warm layer causes the snowflakes to melt completely into liquid raindrops.
The rain then continues to fall toward the surface, where a deep layer of sub-freezing air is trapped near the ground. This final, cold layer must be substantial enough, often at least 1,500 feet thick, to allow the liquid droplets time to refreeze before reaching the ground. The resulting precipitation is small, translucent pellets of ice that are essentially frozen raindrops. Sleet formation is an indicator of a temperature inversion, a setup common ahead of a warm front during the cold season.
The Formation Process of Hail
Hail formation is fundamentally different from sleet, relying on the powerful dynamics within severe thunderstorms. Hailstones originate within large cumulonimbus clouds, characterized by immense vertical development and intense, sustained updrafts. The process begins when small ice particles are swept upward by these strong currents into the upper regions of the cloud where temperatures are well below freezing.
As the developing hailstone is suspended high above, it collides with supercooled water droplets, which remain liquid even below 0 degrees Celsius. These droplets instantly freeze upon contact, a process called accretion, causing the hailstone to grow in size. The powerful updrafts, which can reach speeds of 110 mph, cycle the hailstones through the cloud’s freezing and non-freezing layers.
This repeated cycling allows the hailstone to accumulate multiple concentric layers of ice, similar to the rings of a tree. Growth continues until its weight becomes too great for the updraft to support. Once gravity overcomes the lifting force of the air current, the large, dense hailstone plummets to the earth. The size of the resulting hailstone is directly proportional to the strength and duration of the updraft within the thunderstorm.
Observable Differences and Seasonal Timing
The differing formation mechanisms lead to easily observable physical and temporal distinctions between the two types of precipitation. Sleet consists of small, uniform ice pellets, typically less than 5 millimeters in diameter, that are often translucent. In contrast, a hailstone must be larger than 5 millimeters to be officially classified as hail, and can grow into irregular, dense lumps of ice up to the size of a grapefruit or larger.
Sleet is a phenomenon of cold weather, occurring almost exclusively in the winter months when the necessary atmospheric temperature profile is present. It is not a sign of severe weather, and the pellets generally bounce upon impact, creating a distinct tapping sound. Hail, however, is a product of intense convective storms, making it most common during the warmer months of spring, summer, and fall.
The hardness and mass of hail give it the potential to cause significant damage to crops, vehicles, and structures, a capability sleet lacks. Cutting a hailstone open often reveals a layered structure that records its journey through the cloud, a feature absent in the uniformly frozen raindrop that is sleet.