Miscellaneous precipitation refers to various forms of precipitation that do not fit into the common categories of rain, snow, sleet, or hail. These types of precipitation occur under specific atmospheric conditions and can have unique characteristics.
1. Drizzle
a. Definition:
- Drizzle: Light liquid precipitation consisting of very small, uniformly sized water droplets with a diameter less than 0.5 mm. Drizzle typically falls from low clouds like stratus clouds.
b. Formation:
- Cloud Composition: Drizzle forms in clouds where the cloud droplets coalesce and grow large enough to fall to the ground, but remain small and light.
- Air Stability: It usually occurs in stable air masses where there is little vertical air movement.
c. Components:
- Water Droplets: Very fine and closely spaced water droplets.
d. Example:
- Example: Drizzle is common in coastal and maritime climates, where moist, stable air masses prevail, such as in Seattle, Washington.
e. Characteristics:
- Visibility: Drizzle can reduce visibility but usually does not significantly accumulate.
- Impact: Often makes surfaces wet and can create slippery conditions on roads and sidewalks.
2. Freezing Rain
a. Definition:
- Freezing Rain: Rain that falls as liquid but freezes upon contact with cold surfaces, forming a layer of ice.
b. Formation:
- Temperature Profile: Freezing rain occurs when a warm layer of air is sandwiched between two cold layers. Raindrops form in the warm layer and fall through the cold air near the ground, where they freeze upon impact.
- Supercooling: The rain remains in a supercooled liquid state until it contacts a surface.
c. Components:
- Supercooled Liquid Droplets: Liquid water droplets that are below freezing temperature.
d. Example:
- Example: Freezing rain often occurs in regions experiencing a winter storm with a warm air intrusion, such as the northeastern United States during a nor’easter.
e. Characteristics:
- Ice Accumulation: Can cause significant ice accumulation on trees, power lines, and roads, leading to hazardous conditions and power outages.
- Impact: Creates dangerous travel conditions and can cause structural damage due to the weight of the ice.
3. Graupel
a. Definition:
- Graupel: Also known as soft hail or snow pellets, graupel is precipitation that forms when supercooled water droplets freeze onto falling snowflakes, creating small, soft pellets.
b. Formation:
- Accretion Process: Graupel forms in convective clouds where snowflakes encounter supercooled water droplets that freeze on contact.
- Temperature Range: Typically forms in temperatures just below freezing.
c. Components:
- Snow Crystals and Ice: Snowflakes encased in a rime of ice.
d. Example:
- Example: Graupel often falls during thunderstorms or intense winter storms with strong updrafts, such as in the Rocky Mountains.
e. Characteristics:
- Texture: Soft and crumbly, unlike hard hailstones.
- Impact: Generally not harmful but can indicate the presence of convective instability in the atmosphere.
4. Ice Pellets (Sleet)
a. Definition:
- Ice Pellets (Sleet): Small, translucent balls of ice that form when raindrops or partially melted snowflakes refreeze before reaching the ground.
b. Formation:
- Temperature Profile: Ice pellets form when there is a layer of warm air above a layer of subfreezing air near the surface. Precipitation melts in the warm layer and refreezes in the cold layer.
- Layer Thickness: The refreezing layer must be thick enough to completely refreeze the liquid droplets.
c. Components:
- Frozen Droplets: Small, round, clear or translucent ice particles.
d. Example:
- Example: Ice pellets commonly occur during winter storms in the central United States when warm air aloft is present over a subfreezing surface layer.
e. Characteristics:
- Sound: Ice pellets make a distinctive “pinging” sound when they hit surfaces.
- Impact: Can accumulate on roads, creating hazardous driving conditions.
5. Virga
a. Definition:
- Virga: Precipitation that evaporates or sublimates before reaching the ground, often visible as streaks or shafts of precipitation extending from clouds.
b. Formation:
- Dry Air: Virga occurs when precipitation falls into a layer of dry air, where it evaporates due to low humidity and high temperatures.
- Sublimation: In cold conditions, ice crystals can sublimate directly into water vapor.
c. Components:
- Water Droplets/Ice Crystals: Precipitation that partially or completely evaporates or sublimates.
d. Example:
- Example: Virga is common in desert regions or during high-altitude flights, where dry air layers are prevalent.
e. Characteristics:
- Visual Effect: Appears as streaks descending from clouds but not reaching the ground.
- Impact: Can indicate dry conditions aloft and sometimes lead to downdrafts or microbursts.
6. Diamond Dust
a. Definition:
- Diamond Dust: Minute ice crystals that form near the ground in extremely cold conditions, creating a sparkling, dust-like appearance.
b. Formation:
- Supercooled Air: Forms in very cold, clear air when water vapor sublimates directly into ice crystals without the need for clouds.
- Stable Conditions: Typically forms under stable, calm atmospheric conditions.
c. Components:
- Ice Crystals: Tiny, hexagonal ice crystals.
d. Example:
- Example: Common in polar regions and during cold winter days in places like Antarctica and the Arctic.
e. Characteristics:
- Appearance: Appears as a shimmering, glittering effect in the air.
- Impact: Generally benign, creating beautiful visual effects without significant impact on visibility or travel.
Conclusion
Miscellaneous precipitation forms such as drizzle, freezing rain, graupel, ice pellets, virga, and diamond dust each have unique characteristics and formation processes. These types of precipitation add complexity to weather patterns and have various impacts on the environment, transportation, and human activities.
