Types of Precipitation

Why This Matters

Precipitation isn't just "stuff falling from the sky"—it's the visible result of complex atmospheric processes that you'll be tested on throughout your atmospheric science course. Each precipitation type reveals something about the temperature profile of the atmosphere, the stability conditions present, and the microphysical processes occurring within clouds. When you see sleet hitting your windshield, you're actually observing evidence of a specific vertical temperature structure that meteorologists use to forecast hazardous weather.

Understanding precipitation types connects directly to broader concepts like air mass interactions, frontal boundaries, convective dynamics, and phase changes of water. Exam questions often ask you to work backward—given a precipitation type, what atmospheric conditions must be present? Don't just memorize that "sleet is frozen rain." Know why it freezes, where in the atmosphere that freezing occurs, and what synoptic setup produces those conditions. That's what separates a 5 from a 3.

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Liquid Precipitation: Warm Profiles and Coalescence

These precipitation types remain liquid throughout their journey to the surface, indicating that temperatures stay above freezing through the entire atmospheric column—or at least through the critical lower layers where precipitation forms and falls.

Rain

• Forms through collision-coalescence or the Bergeron process—droplets grow by colliding with others or by ice crystals collecting supercooled water until gravity overcomes updraft support
• Requires above-freezing temperatures from cloud base to surface, or a deep enough warm layer to completely melt any ice that forms aloft
• Intensity classifications (light, moderate, heavy) directly relate to vertical motion strength and moisture availability—key variables in precipitation forecasting

Drizzle

• Droplet diameter less than 0.5 mm—forms in shallow stratus clouds with weak updrafts that can't support larger drop growth
• Associated with stable atmospheric conditions and persistent overcast; often indicates warm advection or marine layer influence
• Low precipitation rates but can persist for hours, making it significant for visibility reduction and accumulated moisture input

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Frozen Precipitation: Cold Profiles Throughout

When temperatures remain below freezing from cloud level to the surface, precipitation reaches the ground in solid form. The specific type depends on how ice crystals form and grow within the cloud.

Snow

• Ice crystals form via deposition when water vapor deposits directly onto ice nuclei at temperatures below $-10°C$—the Bergeron process dominates in mixed-phase clouds
• Crystal habit (shape) depends on temperature and supersaturation—dendrites form near $-15°C$, plates and columns at other temperatures
• High albedo reflects incoming solar radiation, creating a positive feedback loop that reinforces cold conditions (snow-albedo feedback)

Graupel

• Forms through riming—supercooled water droplets freeze on contact with falling snowflakes, creating opaque, soft pellets 2-5 mm in diameter
• Indicates convective activity even in winter; the presence of supercooled liquid water and sufficient updrafts signals instability
• Often precedes or accompanies thundersnow—a key indicator of intense vertical motion in cold-season convection

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Phase-Change Precipitation: Temperature Inversions and Warm Layers Aloft

These precipitation types undergo phase changes during their descent, revealing complex vertical temperature structures—typically a warm layer aloft sandwiched between cold air at cloud level and cold air at the surface. This setup is classic for warm fronts and freezing rain events.

Sleet (Ice Pellets)

• Raindrops freeze completely before reaching the surface—requires a warm layer aloft (to melt snow into rain) and a deep subfreezing layer below (to refreeze drops)
• Subfreezing layer must be deep enough (typically >1000 feet) to allow complete refreezing; shallower cold layers produce freezing rain instead
• Bounces on impact and accumulates like sand—creates traction issues but less structural icing than freezing rain

Freezing Rain

• Remains liquid until contact with subfreezing surfaces—indicates a shallow cold layer at the surface insufficient to freeze falling drops
• Temperature inversion signature: warm air (often $>0°C$) overrunning a shallow dome of cold air ($<0°C$) trapped at the surface
• Most dangerous winter precipitation type—ice accumulation causes power outages, tree damage, and creates nearly frictionless road surfaces

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Convective Precipitation: Severe Weather Signatures

Strong updrafts in convective systems can suspend precipitation particles long enough for dramatic growth. These precipitation types signal atmospheric instability and often accompany severe weather.

Hail

• Forms in supercell thunderstorms with updrafts exceeding 25-50 m/s—strong enough to suspend growing ice stones against gravity
• Growth occurs through accretion as hailstones cycle through regions of supercooled water; alternating clear and opaque layers reveal multiple trips through the updraft
• Size correlates with updraft strength—severe hail ($\geq 1$ inch diameter) is a criterion for severe thunderstorm warnings and indicates extreme instability

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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, and what atmospheric variable determines which one reaches the surface?

2. A winter storm produces graupel followed by heavy snow. What does the graupel tell you about atmospheric conditions at that time compared to when snow dominated?

3. Compare and contrast the formation mechanisms of snow and hail. Both are frozen precipitation—why does one indicate severe weather while the other does not?

4. You observe drizzle persisting for several hours under overcast skies. What can you infer about atmospheric stability and cloud characteristics?

5. FRQ-style: Given a sounding showing a surface temperature of $-5°C$, a warm layer at 850 mb reaching $+4°C$, and cloud-top temperatures of $-20°C$, predict the precipitation type and explain your reasoning using the temperature profile.