When snow begins to melt as it falls and then refreezes into small, hard pellets before reaching the ground, it is known as sleet (or ice pellets). This phenomenon is a distinct type of winter precipitation, but the question can also refer to snow that melts on the ground and subsequently freezes into ice.
Understanding Sleet (Ice Pellets)
Sleet is an intriguing atmospheric occurrence that results from a specific temperature profile in the atmosphere. It forms under conditions where snowflakes do not remain frozen throughout their entire fall:
- Formation Process: Sleet forms when snowflakes descend from a cloud and encounter a shallow layer of warm air (above freezing) somewhere in the middle of the atmosphere. In this warm layer, the snowflakes only partially melt, transforming into slushy drops. As these partially melted drops continue their descent, they then fall through a deep layer of freezing air located closer to the Earth's surface. Within this crucial cold layer, the slushy drops refreeze completely, turning into small, solid ice pellets that are typically transparent or translucent.
- Impact on the Ground: When sleet reaches the ground, these frozen raindrops bounce upon impact, creating a distinctive sound. Unlike snow, which is soft and powdery, or freezing rain, which freezes after hitting a surface, sleet arrives as already frozen precipitation. It generally does not adhere to surfaces as readily as freezing rain but can accumulate and make surfaces slippery.
Differentiating Winter Precipitation Types
It's helpful to distinguish sleet from other common types of winter precipitation that involve melting and refreezing processes:
| Precipitation Type | How it Forms | Appearance & Impact |
|---|---|---|
| Sleet (Ice Pellets) | Snowflakes partially melt in a warm layer, then refreeze into solid ice in a deep freezing layer before hitting the ground. | Small, hard ice pellets that bounce upon impact. |
| Freezing Rain | Snowflakes melt completely into rain in a deep warm layer, then fall through a shallow freezing layer near the surface, freezing on contact with cold surfaces. | Liquid rain that turns to a glaze of ice upon impact, coating everything. This is often more destructive than sleet due to the weight of the ice. |
| Snow | Precipitation remains frozen as ice crystals throughout its fall through a consistently freezing atmosphere. | Soft, fluffy flakes that accumulate. |
| Refrozen Surface Ice | Snow or slush on the ground melts due to rising temperatures or rain, then solidifies into ice when temperatures drop below freezing. | Varies from visible sheets of ice to nearly invisible "black ice." Can be extremely slippery and dangerous for walking and driving. |
Refrozen Snow and Ice on Surfaces
Beyond the atmospheric phenomenon of sleet, the phrase "snow that starts to melt and then freezes again" can also refer to snow that has already accumulated on the ground and then undergoes a melt-freeze cycle. This leads to various forms of surface ice:
- Process: This occurs when a layer of snow on the ground, or slush created from melted snow, experiences a rise in temperature above freezing, causing it to melt. If temperatures then drop below freezing again, this melted water or slush will solidify, transforming into a hard, often dense, layer of ice.
- Common Forms of Surface Ice:
- Refrozen Slush: Often seen after a day where temperatures rise above freezing, causing snow to become slushy. As night falls and temperatures drop, this slush hardens into a very uneven and slippery ice layer.
- Black Ice: A particularly dangerous type of refrozen ice. It's a thin, transparent layer of ice that forms on roadways and sidewalks, making them appear merely wet rather than icy. It's almost invisible, hence the name "black ice," as it allows the dark surface beneath to show through.
- Glazed Ice: A thicker, more visible layer of ice that can result from freezing rain, or from a substantial amount of refrozen melted snow, creating a smooth, slick surface.
Navigating Icy Conditions Safely
Both sleet and refrozen surface ice present considerable hazards for travel and daily activities.
Impacts on Daily Life
- Transportation: Sleet can reduce visibility and create challenging driving conditions due to reduced traction. Refrozen ice on roads, especially black ice, drastically increases the risk of accidents by making vehicles lose grip on the pavement.
- Walking: Sidewalks and pathways covered in sleet or refrozen ice become extremely slippery, leading to a high risk of falls and injuries.
- Infrastructure: Heavy accumulations of sleet or freezing rain can weigh down tree branches and power lines, causing power outages and property damage.
Safety Tips During Icy Weather
When encountering conditions where snow has melted and refrozen, or sleet is falling, it's crucial to take precautions:
- Driving Safely:
- Reduce Speed: Always drive slower than the posted speed limit to give yourself more time to react.
- Increase Following Distance: Allow significantly more space between your vehicle and the one ahead.
- Avoid Sudden Movements: Brake, accelerate, and steer gently to prevent skidding.
- Beware of Specific Areas: Bridges, overpasses, and shaded areas freeze first and often remain icier longer.
- Walking Safely:
- Wear Appropriate Footwear: Choose shoes or boots with good traction.
- Walk Slowly and Deliberately: Take small, shuffling steps, keeping your center of gravity over your feet.
- Use Handrails: Utilize railings when available on stairs or ramps.
- Clear Pathways: Use salt, sand, or ice melt on your sidewalks and driveways to improve traction.
- Preparation:
- Monitor Weather Forecasts: Stay informed about current and upcoming weather conditions.
- Emergency Kit: Keep an emergency kit in your vehicle, including blankets, a shovel, and sand or cat litter for traction.
For more comprehensive winter safety information, consult resources from organizations like the National Weather Service. Understanding these different forms of icy precipitation and surface conditions is vital for staying safe during cold weather events.
