Meteorology Lec 4 - Stability and Cloud Development

Atmospheric Stability

Condition of equilibrium where air resists or encourages vertical motion

Stable Equilibrium

Air returns to original position after being displaced

Unstable Equilibrium

Air continues moving away from original position after displacement

Adiabatic Process

Temperature change in air parcel without heat exchange with surroundings

Dry Adiabatic Rate

Cooling/warming rate of unsaturated air: 10°C per 1000 m

Moist Adiabatic Rate

Cooling/warming rate of saturated air: ~6°C per 1000 m (varies with T and P)

Environmental Lapse Rate (ELR)

Rate at which surrounding air temperature changes with altitude

Absolutely Stable Atmosphere

ELR < moist adiabatic rate < dry adiabatic rate; resists vertical motion

Absolutely Unstable Atmosphere

ELR > dry adiabatic rate > moist adiabatic rate; encourages vertical motion

Conditionally Unstable Atmosphere

Moist adiabatic rate < ELR < dry adiabatic rate; unstable only if saturated

Neutral Stability

ELR equals adiabatic rate; air neither rises nor sinks persistently

Subsidence Inversion

Warming layer formed by sinking air, stabilizing the atmosphere

Causes of Stability

Air aloft warms or surface air cools (e.g., radiational cooling, cold advection)

Causes of Instability

Air aloft cools or surface air warms (e.g., solar heating, warm advection)

Cloud

Visible aggregate of water droplets or ice crystals suspended in air

Convection

Vertical air motion due to heating, forming cumulus clouds

Thermal

Rising bubble of warm air that may form a cumulus cloud

Condensation Level

Altitude where rising air becomes saturated and clouds form

Entrainment

Mixing of cooler environmental air into a rising cloud, enhancing cooling and evaporation

Orographic Uplift

Forced lifting of air over topographic barriers, forming orographic clouds

Rain Shadow

Dry region on leeward side of mountains due to descending, warming air

High Clouds

Bases above ~6000 m; e.g., Cirrus, Cirrostratus, Cirrocumulus

Middle Clouds

Bases 2000–8000 m; e.g., Altostratus, Altocumulus

Low Clouds

Bases below ~2000 m; e.g., Stratus, Stratocumulus, Nimbostratus

Clouds with Vertical Development

e.g., Cumulus, Cumulonimbus; form through convection

Cirrus (Ci)

High, wispy ice-crystal clouds; indicate fair weather or approaching change

Cirrostratus (Cs)

High, thin layer clouds producing halos; precede storms

Cirrocumulus (Cc)

Small, high cumuliform patches; "mackerel sky"

Altostratus (As)

Middle-layer gray clouds; often precede continuous precipitation

Altocumulus (Ac)

Middle cumuliform clouds; indicate fair weather or thunderstorms

Nimbostratus (Ns)

Low, dark rain clouds; produce continuous precipitation

Stratocumulus (Sc)

Low, lumpy layer clouds; common in patches or waves

Stratus (St)

Low, uniform gray layer; resembles fog, may drizzle

Cumulus (Cu)

Puffy fair-weather clouds with flat bases

Cumulus Humilis

Wide, shallow cumulus; fair weather

Cumulus Mediocris

Moderate vertical development

Cumulus Congestus

Towering cumulus; can produce showers

Cumulonimbus (Cb)

Thunderstorm cloud; produces hail, lightning, tornadoes

Cumulonimbus Incus

Anvil-top cumulonimbus due to high-level winds

Mammatus Clouds

Pouches hanging from cloud bases; often after thunderstorms

Lenticular Clouds

Lens-shaped clouds formed downwind of mountains

Contrails

Condensation trails from aircraft engines

Nacreous Clouds

Polar stratospheric clouds; iridescent

Noctilucent Clouds

Mesospheric clouds; visible at twilight

Pileus Clouds

Cap clouds atop growing cumulus

Cloud Ceiling

Height of lowest cloud layer

Ceiling Balloon

Measures cloud ceiling by ascent rate into clouds

Ceiling Light Projector

Measures cloud base height using light beam and clinometer

Okta

Unit of cloud cover; 1 okta = 1/8 sky covered

Clear Sky

0 oktas; no clouds

Partly Cloudy

1–4 oktas covered

Mostly Cloudy

5–7 oktas covered

Overcast

8 oktas; completely covered

Dew Point Lapse Rate

~2°C per 1000 m; rate at which dew point decreases with height

Radiational Cooling

Cooling of air/clouds by emitting IR radiation, increasing stability

Cold Advection

Influx of colder air, stabilizing atmosphere

Warm Advection

Influx of warmer air, destabilizing atmosphere

Mackerel Sky

Pattern of cirrocumulus or altocumulus resembling fish scales

Scud

Stratus fractus; ragged low clouds under nimbostratus

Towering Cumulus (TCu)

Cumulus congestus with strong vertical growth

Halo

22° ring around sun/moon caused by ice crystals in cirrostratus

Anvil

Flattened top of cumulonimbus due to wind shear

Cloud Base Height

Determined by surface T and Td; larger T-Td difference = higher base

Trigger Mechanisms

Forces that lift air: convection, topography, convergence, fronts

Convergence

Air flows together, forcing uplift

Frontal Lifting

Air lifted along weather fronts

Neutral Stability

ELR equals dry or moist adiabatic rate

Layer Clouds

Form in stable conditions (e.g., stratus, altostratus)

Cumuliform Clouds

Form in unstable conditions (e.g., cumulus, cumulonimbus)

Orographic Clouds

Form due to air lifting over mountains

Evaporation in Clouds

Cools air and promotes sinking around cloud edges

Condensation in Clouds

Releases latent heat, fueling further uplift

Cloud Dissipation

Occurs when sinking air dominates or moisture is depleted

Cloud Classification

Based on height and appearance

Cloud Identification

Uses color, texture, weather context, and sky coverage

Weather Prediction

Clouds indicate upcoming conditions (e.g., cirrostratus → rain in 12-24h)

Solar Heating

Destabilizes lower atmosphere by day

Radiational Inversion

Stabilizes atmosphere at night due to surface cooling

Advection Fog

Forms when warm moist air moves over cooler surface

Steam Fog

Forms when cold air moves over warm water

Radiation Fog

Forms from nocturnal cooling

Upslope Fog

Forms from adiabatic cooling on slopes

Precipitation Fog

Forms from rain evaporating into cold air

Absolute Instability

Rare; requires ELR > 10°C/1000 m

Conditional Instability

Common in troposphere; requires saturation to trigger instability

Subsidence

Sinking air warms adiabatically, creating inversions

Lifting Condensation Level (LCL)

Height where RH reaches 100% and cloud forms

Level of Free Convection (LFC)

Height where parcel becomes warmer than environment

Equilibrium Level (EL)

Height where parcel temperature equals environment; cloud top

Dry Convection

Uplift without condensation; no clouds form

Moist Convection

Uplift with condensation; forms clouds and storms

Latent Heat Release

Powers further uplift in saturated air

Wind Shear

Changes cloud shape (e.g., anvil formation)

Cloud Droplets

~10–20 μm; require nuclei to form

Ice Crystals

Form in clouds below -40°C or via ice nuclei

Cloud Seeding

Adding nuclei to enhance precipitation

Virga

Precipitation that evaporates before reaching ground

Cloud Atlas

Reference for cloud identification and classification

PAGASA

Philippine agency for weather observations and forecasts

Ceiling Measurement

Important for aviation and weather reporting