Weather and climate 2

Air Parcel

Imaginary volume of air that does not exchange energy with its surroundings

Adiabatic Process

A process where no heat is exchanged; temperature changes due to expansion or compression

Diabatic Process

A process where heat is added or removed (e.g., radiation, conduction)

First Law of Thermodynamics

Energy is conserved; temperature changes due to heat or work

Expansion (Rising Air)

Air expands as pressure decreases, causing cooling

Compression (Sinking Air)

Air compresses as pressure increases, causing warming

Dry Adiabatic Lapse Rate (DALR)

Rate of temperature change for unsaturated air; 10°C per km

Saturated Adiabatic Lapse Rate (SALR)

Rate of temperature change for saturated air; ~5–9°C per km

Latent Heat

Energy released during condensation, slowing cooling

Dew Point

Temperature at which air becomes saturated

Lifting Condensation Level (LCL)

Height where rising air becomes saturated

Environmental Lapse Rate (ELR)

Actual temperature change in the surrounding atmosphere

Stable Atmosphere

Air resists vertical movement; ELR < SALR

Unstable Atmosphere

Air accelerates upward; ELR > DALR

Conditionally Unstable

Stable when dry, unstable when saturated

Condensation Nuclei

Tiny particles (dust, salt) that water condenses onto

Collision-Coalescence

Process where larger droplets collide and merge with smaller ones

Collector Drop

Large droplet that falls faster and collects smaller droplets

Terminal Velocity

Maximum falling speed when gravity and drag are balanced

Bergeron Process

Ice crystal growth at expense of supercooled water droplets

Supercooled Water

Liquid water below freezing temperature

Deposition

Water vapor changing directly to ice

Riming

Supercooled water freezing onto ice crystals (forms graupel)

Aggregation

Ice crystals sticking together to form snowflakes

Rain

Liquid drops ≥ 0.5 mm diameter

Drizzle

Liquid drops < 0.5 mm diameter

Sleet

Rain that refreezes before reaching ground

Freezing Rain

Rain that freezes on contact with surfaces

Hail

Ice formed by repeated cycling in strong updrafts

Pressure Gradient Force (PGF)

Force that moves air from high to low pressure

Isobars

Lines of equal pressure on a map

Coriolis Force

Apparent force causing deflection of moving air (right in NH)

Geostrophic Wind

Wind flowing parallel to isobars when PGF = Coriolis

Friction Force

Surface drag that slows wind and alters direction

Convergence

Air flowing together; causes rising motion

Divergence

Air spreading apart; causes sinking motion

Hydrostatic Balance

Balance between upward pressure force and gravity

Air Mass

Large body of air with uniform temperature and moisture

Maritime Tropical (mT)

Warm, moist air mass

Continental Tropical (cT)

Warm, dry air mass

Maritime Polar (mP)

Cold, moist air mass

Continental Polar (cP)

Cold, dry air mass

Continental Arctic (cA

Extremely cold, dry air mass

Front

Boundary between two different air masses

Cold Front

Cold air advances and lifts warm air rapidly

Warm Front

Warm air rises gradually over cold air

Stationary Front

Boundary between air masses that is not moving

Occluded Front

Cold front overtakes warm front

Dryline

Boundary between dry and moist air masses

Mid-Latitude Cyclone

Large low-pressure system in mid-latitudes with fronts

Cyclogenesis

Formation of a mid-latitude cyclone

Polar Front Boundary between cold polar air and warm tropical air

Occlusion Stage

Stage when cold front overtakes warm front

Upper-Level Divergence

Air spreading aloft, lowering surface pressure

Anti-cyclone

High-pressure system with sinking air and fair weather

Tropical Cyclone

Intense low-pressure system over warm ocean water

Hurricane

Name for tropical cyclone in Atlantic

Typhoon

Name for tropical cyclone in Pacific

Eye

Calm center of hurricane with lowest pressure

Eyewall

Region of strongest winds and heaviest rain

Rainbands

Spiral bands of precipitation around storm

Storm Surge

Rise in sea level caused by storm winds and pressure

Tropical Depression

Weak system with winds <38 mph

Tropical Storm

Organized system with winds 38–73 mph

Hurricane Stage

System with winds >73 mph

Wind Shear

Change in wind speed/direction with height; disrupts storms