Meteorology Lec 5 - Precipitation

Precipitation

Any form of water (liquid or solid) that falls from a cloud and reaches the ground

Cloud Droplet

Typically ~20 μm in diameter; must grow to form precipitation

Raindrop

Typically ~2000 μm (2 mm) in diameter; ≥0.5 mm to be classified as rain

Condensation Nucleus

Tiny particle (e.g., salt, dust) that enables cloud droplet formation; ~0.2 μm

Equilibrium Vapor Pressure

Saturation vapor pressure where condensation equals evaporation

Curvature Effect

Smaller droplets require higher supersaturation to maintain equilibrium due to surface tension

Solute Effect

Dissolved substances (e.g., salt) lower equilibrium vapor pressure, aiding condensation

Hygroscopic Nucleus

Particle that attracts water vapor, allowing condensation at RH <100%

Terminal Velocity

Constant fall speed achieved when air resistance equals gravity

Coalescence

Merging of cloud droplets upon collision to form larger drops

Warm Cloud

Cloud with temperatures above freezing; precipitation forms via collision-coalescence

Cold Cloud

Cloud with temperatures below freezing; precipitation forms via ice-crystal process

Supercooled Droplet

Liquid water droplet existing at temperatures below 0°C

Ice Nucleus

Particle that initiates ice crystal formation in cold clouds

Bergeron Process

Growth of ice crystals at the expense of supercooled droplets due to lower saturation vapor pressure over ice

Accretion

Growth of ice crystals by freezing of supercooled droplets upon contact; forms graupel

Aggregation

Clumping of ice crystals to form snowflakes

Graupel

Soft, icy pellet formed by accretion; precursor to hail

Cloud Seeding

Artificial introduction of nuclei to enhance precipitation

Warm Cloud Seeding

Using hygroscopic particles (e.g., salt) to promote droplet growth

Cold Cloud Seeding

Using ice nuclei (e.g., silver iodide) to enhance ice crystal formation

Drizzle

Precipitation consisting of drops <0.5 mm in diameter; falls from stratus clouds

Virga

Precipitation that evaporates before reaching the ground

Rain Shower

Short-duration rain from convective clouds (e.g., cumulus)

Cloudburst

Very heavy, sudden rain shower

Continuous Rain

Steady rain from nimbostratus clouds covering a large area

Raindrop Shape

Small drops (

Rainfall Intensity

Amount of rain per unit time (e.g., mm/hour)

Very Light Rain

Scattered drops that do not wet surfaces; trace to 2.5 mm/hour

Light Rain

2.5–7.5 mm/hour; puddles form slowly

Moderate Rain

7.5 mm/hour; puddles form rapidly, streams in gutters

Heavy Rain

Sheets of rain, roaring noise on roofs; often from thunderstorms

Snow

Ice crystals in hexagonal shapes that aggregate into snowflakes

Fallstreaks

Ice crystals falling from high cirrus clouds; visible as virga

Sleet

Ice pellets formed when melted snow refreezes in a deep subfreezing layer

Freezing Rain

Supercooled liquid rain that freezes on contact with cold surfaces

Freezing Drizzle

Freezing rain with drops <0.5 mm

Hail

Layered ice balls formed in severe thunderstorms; can be golf-ball-sized or larger

Hailstone

Individual unit of hail; grows by accretion in strong updrafts

Rain Gauge

Instrument that collects and measures rainfall

Standard Rain Gauge

Funnel collector with measuring tube; amplifies depth 10x for precision

Tipping Bucket Rain Gauge

Collects rain in small buckets that tip at 0.01 inch increments; records electronically

Weighing-Type Rain Gauge

Measures rainfall by weight; allows remote recording

Radar

Radio Detection and Ranging; uses microwaves to detect precipitation

Echo

Radar return signal indicating precipitation intensity and location

Doppler Radar

Measures velocity of precipitation using frequency shift (Doppler effect)

Doppler Shift

Change in wave frequency due to motion of source or observer

Himawari-8

Japanese geostationary weather satellite providing images every 10 minutes

FY-2E Fengyun

Chinese geostationary meteorological satellite; backup to MTSAT

COMS

Korean Communication, Ocean, and Meteorological Satellite; images every 15 minutes

PAGASA

Philippine Atmospheric, Geophysical and Astronomical Services Administration

Supersaturation

RH >100%; required for tiny droplet growth due to curvature effect

Collision Efficiency

Likelihood of droplets colliding and coalescing; depends on size and charge

Cloud Thickness

Affects droplet growth time; thicker clouds produce larger drops

Updraft Strength

Influences droplet suspension time and growth; stronger updrafts support hail formation

Liquid Water Content

Amount of liquid water in cloud; key factor in precipitation formation

Rime Ice

Opaque, porous ice formed by rapid freezing of supercooled droplets

Clear Ice

Transparent ice formed by slow freezing; allows air bubbles to escape

Rain Shadow

Dry area on leeward side of mountains due to descending air

Melt Layer

Altitude where snow melts into rain; critical for sleet/freezing rain formation

Subfreezing Layer

Air layer below 0°C; depth determines precipitation type (sleet vs. freezing rain)