Precipitation Forms

Why This Matters

Understanding precipitation forms is essential for interpreting weather systems, forecasting, and explaining how atmospheric conditions translate into the weather we experience on the ground. You're being tested on your ability to connect temperature profiles, atmospheric stability, and phase changes to the specific type of precipitation that results. This isn't just about knowing what falls from the sky—it's about understanding the vertical structure of the atmosphere that produces each form.

The key to mastering this topic is recognizing that precipitation type depends on the temperature profile from cloud to surface. A single storm system can produce rain, freezing rain, sleet, and snow simultaneously in different locations based solely on where warm and cold air layers exist. Don't just memorize definitions—know what atmospheric conditions each precipitation type reveals and how meteorologists use these forms as diagnostic tools for understanding air mass interactions.

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Liquid Precipitation: Direct Condensation to Surface

These forms remain liquid throughout their journey from cloud to ground, indicating above-freezing temperatures through the entire atmospheric column. The collision-coalescence process dominates in warm clouds, while the Bergeron process operates in mixed-phase clouds.

Rain

• Droplets exceed 0.5 mm in diameter—formed through collision-coalescence in warm clouds or ice crystal melting in cold clouds
• Requires above-freezing temperatures from cloud base to surface, indicating no sub-zero layers in the vertical profile
• Primary freshwater delivery mechanism for most mid-latitude and tropical regions, directly tied to the hydrologic cycle

Drizzle

• Droplets smaller than 0.5 mm in diameter—fall slowly due to minimal mass and low terminal velocity
• Associated with stratus clouds and stable atmospheres—weak updrafts prevent droplet growth through collision-coalescence
• Indicates persistent low-level moisture without significant vertical development, common in marine and coastal environments

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Frozen Precipitation: Ice Crystal Formation Aloft

These forms begin as ice in the cloud and remain frozen to the surface, requiring sub-freezing temperatures throughout the atmospheric column. The Bergeron process—where ice crystals grow at the expense of supercooled water droplets—drives snowflake formation.

Snow

• Ice crystals form directly from vapor deposition when temperatures remain below freezing from cloud to ground
• Crystal structure varies with temperature and humidity—dendrites form near $-15°C$, plates and columns at other temperatures
• High albedo (up to 90%) reflects incoming solar radiation, creating positive feedback loops that reinforce cold air masses

Graupel

• Soft ice pellets formed through riming—supercooled water droplets freeze onto falling snowflakes, coating them
• Indicates convective instability and the presence of supercooled liquid water in clouds, often preceding thundersnow
• Density between snow and hail—compressible and opaque white, unlike the hard, layered structure of true hail

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Winter Mixed Precipitation: Temperature Inversions at Work

These forms result from complex vertical temperature profiles where warm and cold layers alternate. Temperature inversions—warm air overriding cold surface air—create the conditions for freezing rain and sleet.

Freezing Rain

• Falls as liquid but freezes on contact with surfaces at or below $0°C$, creating dangerous ice glaze
• Requires a specific temperature profile—warm layer aloft (above freezing) deep enough to completely melt falling snow, with a shallow cold layer at the surface
• Most hazardous winter precipitation type—ice accumulation causes power outages, tree damage, and creates nearly invisible road hazards

Sleet

• Ice pellets that freeze before reaching the ground—bounces on impact and accumulates like sand
• Indicates a colder or deeper sub-freezing surface layer than freezing rain, allowing complete refreezing during descent
• Audible on impact—the "ticking" sound distinguishes it from rain or snow, useful for real-time weather identification

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Convective Precipitation: Severe Weather Indicators

These forms require strong vertical motion and are associated with cumulonimbus clouds and atmospheric instability. Powerful updrafts suspend hydrometeors long enough for significant ice accumulation.

Hail

• Layered ice stones formed in severe thunderstorms—updrafts repeatedly loft ice through supercooled water zones
• Size indicates updraft strength—golf ball-sized hail ($\approx 4.4 \text{ cm}$) requires updrafts exceeding $50 \text{ m/s}$
• Cross-sections reveal growth history—alternating clear and opaque layers show wet and dry growth phases in different cloud regions

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Precipitation That Doesn't Reach the Surface

Not all precipitation completes its journey to the ground. Evaporation rates depend on the temperature and humidity of sub-cloud air.

Virga

• Precipitation that evaporates before reaching the surface—visible as wispy streaks beneath cloud bases
• Indicates dry air below cloud level—common in arid climates and during temperature inversions
• Can produce microbursts—evaporative cooling creates downdrafts that accelerate toward the surface, posing aviation hazards

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Quick Reference Table

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Self-Check Questions

1. Which two precipitation types both require a warm layer aloft melting snow, and what determines which one forms?

2. A weather station reports soft, white pellets that compress when squeezed and preceded a thundersnow event. What precipitation type is this, and what does it indicate about atmospheric stability?

3. Compare and contrast hail and graupel in terms of formation process, cloud type, and what each reveals about updraft strength.

4. You're given a vertical temperature profile showing: surface at $-5°C$, a layer at $+4°C$ from 1000-1500 m altitude, and $-10°C$ above 2000 m. What precipitation type would you forecast, and why?

5. How does virga serve as a diagnostic tool for meteorologists, and what hazard can it indicate for aviation even though it never reaches the ground?