Understanding Precipitation: Formation and Types

Sep 9, 2024

Lecture on Precipitation Formation and Types

Introduction to Precipitation

  • Precipitation refers to any form of water, termed hydrometeors, that falls from the sky.
  • Main Factors for Precipitation:
    • Water availability: Quantified by the mixing ratio.
    • Cloud base height: Lower clouds increase the likelihood of precipitation reaching the surface.
    • Droplet Size and Growth: Larger droplets form when held in suspension longer, influenced by updrafts.

Cloud Types and Droplet Formation Processes

  • Types of Clouds:
    • Cumulonimbus Clouds: Tall, strong updrafts, large precipitation.
    • Stratus Clouds: Shallow, weak updrafts, small precipitation.
  • Warm vs Cold Clouds:
    • Warm Clouds: Above freezing, liquid water, droplet growth through collision and coalescence.
    • Cold Clouds: Below freezing at some altitude, droplet growth through aggregation and accretion.

Precipitation Types and Characteristics

  • Snowflake Formation:
    • Dendrites: Form in moderately cold clouds with high mixing ratios.
    • Supercooled Water: Exists below freezing; contributes to accretion.
  • Raindrop Dynamics:
    • Size Limitations: Droplets split at 4-5 mm due to shear.
    • Terminal Velocity: Larger raindrops fall faster.
    • Largest Recorded Drops: 10 mm in tropical clouds (Brazil).

Precipitation Intensity

  • Intensity Measurement: Volume per time (e.g., 2 inches per hour).
  • Definitions:
    • Drizzle: Small droplets, low intensity.
    • Rain: Larger droplets, intensity varies (light, moderate, heavy).
    • Snow: Intensity is similar to rain, with terms like light, moderate, heavy.

Wind and Snow Measures

  • Wind Influence:
    • Squalls: Winds of 25 mph for at least a minute.
    • Blizzards: Winds of 35 mph, reduced visibility for an hour.
  • Snow Measurement:
    • Snow Water Equivalent (SWE): More accurate than snow depth.

Other Forms of Precipitation

  • Sleet and Freezing Rain:
    • Sleet: Refrozen pellets after liquid passes through a cold layer.
    • Freezing Rain: Liquid freezes upon contact with surfaces.
  • Graupel: Soft hail formed from supercooled water rimming around a snow crystal, smaller than hail.
  • Hail:
    • Formed from accretion; larger hailstones result from strong updrafts.
    • Recent notable hail events include fatalities and significant property damage.
    • Largest recorded hailstone in the US: 8 inches in diameter, South Dakota, 2010.

Conclusion

  • Precipitation processes are complex and dependent on several atmospheric conditions.
  • Understanding these processes helps in weather prediction and mitigating precipitation-related hazards.