PART 2 - Meteorology: Cloud Formation, Weather Fronts, and Precipitation Types

Convection

Vertical movement of warm, less dense air that rises, cools, condenses, and forms clouds and precipitation, often producing showers and storms.

Convergence

Air flowing toward a central point (low pressure), forcing it upward where it cools, condenses, and forms clouds.

Orographic Effect

Air forced up a mountain slope, cooling, condensing, and forming clouds/precipitation on windward side with dry conditions on leeward side (rain shadow).

Frontal Lifting

Warm air forced aloft when meeting cold dense air, causing cloud formation and precipitation.

Cirro-

Prefix describing high, thin ice-crystal clouds above ~20,000 feet.

Alto-

Prefix describing mid-level clouds between ~6,500-20,000 feet.

Strato-

Prefix describing low, layered, blanket-like clouds.

Cumulo-

Prefix describing puffy, vertically growing clouds associated with strong updrafts.

Nimbus

Suffix indicating precipitation-producing clouds.

Cumulonimbus

Towering thunderstorm cloud, producing heavy rain, lightning, hail, tornadoes.

Nimbostratus

Thick, layered gray cloud producing steady, long-duration rain.

Cirrus

Thin, wispy ice-crystal clouds that often indicate approaching warm fronts.

Cumulus

Puffy cotton-like clouds forming from convection; can grow into storms.

Stratus

Low blanket-like clouds covering large areas with drizzle and gloomy skies.

Fog

Ground-level cloud formed when air reaches saturation through cooling or moisture addition, reducing visibility below 1 km.

Radiation Fog

Fog formed overnight on clear, calm nights when the ground cools air to dew point.

Advection Fog

Fog formed when warm moist air moves over a cooler surface, cooling to saturation.

Upslope Fog

Fog formed when moist air is lifted up a slope and cools adiabatically to dew point.

Evaporation Fog

Fog formed when cold air moves over warm water, causing rapid condensation.

Steam Fog

Type of evaporation fog appearing like steam over lakes or rivers.

Valley Fog

Cold dense air drains into valleys, trapping moisture and forming fog.

Precipitation Fog

Fog formed when rain evaporates into cooler air near the ground, saturating it.

Warm Cloud Process (Collision-Coalescence)

Raindrops grow when droplets collide and merge inside warm clouds until heavy enough to fall.

Cold Cloud Process (Bergeron)

Ice crystals attract vapor and grow in subfreezing clouds, falling as snow or melting into rain.

Virga

Precipitation that evaporates before reaching the ground, common in dry air.

Condensation Nuclei

Tiny particles (dust, smoke, salt) that water vapor condenses onto to form droplets.

Snow

Frozen ice crystals that reach the ground without melting.

Freezing Rain

Raindrops freeze on contact with subfreezing surfaces, forming glaze ice.

Sleet

Raindrops that melt then refreeze mid-air into ice pellets before reaching the ground.

Wintry Mix

Combination of snow, sleet, and freezing rain caused by layered freezing/melting conditions.

Return Period/Interval

Statistical estimate of how often an extreme event occurs based on historical records.

PDI (Palmer Drought Index)

System measuring drought severity based on precipitation deficits and temperature.

Air Mass

Large body of air with uniform temperature and humidity characteristics.

Source Region

Geographic area where air masses form, typically with flat, calm, uniform surfaces.

cP (Continental Polar)

Cold, dry air mass from northern Canada; brings winter cold outbreaks.

mP (Maritime Polar)

Cool, moist air mass from northern oceans; causes cloudy, raw weather.

cT (Continental Tropical)

Hot, very dry desert air mass; causes heat waves and drought.

mT (Maritime Tropical)

Warm, humid air mass from Gulf/subtropics; fuels thunderstorms.

Air Mass Modification

Process by which an air mass changes characteristics as it moves over new surfaces.

Dryline

Boundary separating moist and dry air; common trigger for severe thunderstorms in the Plains.

Lake Effect Snow

Heavy snow formed when cold air passes over warm lakes and picks up moisture and heat.

Cold Front

Cold dense air rapidly displaces warm air upward, producing thunderstorms, heavy rain, wind shifts, and sharp cooling.

Warm Front

Warm air gradually rides over cool air, producing layered clouds and steady precipitation.

Stationary Front

Boundary where neither air mass moves; produces days of clouds and rain.

Occluded Front

Cold front overtakes warm front, lifting warm air off the ground and signaling cyclone maturity/weakened system.

Mid-Latitude Cyclone

Rotating low-pressure weather system with warm and cold fronts, producing widespread precipitation in mid-latitudes.

Cyclogenesis

Formation and strengthening of a mid-latitude cyclone along strong temperature gradients and jet stream support.

Cold Sector

Region behind a cold front, featuring cold, dry, windy conditions and clearing skies.

Warm Sector

Warm moist air between cold and warm fronts, often unstable with thunderstorms.

Cool Sector

Cooler region ahead of a warm front with widespread cloudiness and steady rain.

Comma Cloud Signature

Satellite image pattern indicating a mature mid-latitude cyclone.

Occlusion

Stage where cold front catches warm front, cutting off energy supply and weakening the cyclone.

Colorado Low

Mid-latitude cyclone forming east of Rockies, often producing severe winter storms.

Gulf Low

Moist cyclones forming over Gulf of Mexico that can produce flooding rains.

Hatteras Low

Cyclone forming off North Carolina coast, often leading to nor'easters.

Alberta Clipper

Fast, dry, cold cyclones moving southeast from Canada, producing light snow and strong winds.

Nor'easter

Coastal low-pressure storm bringing heavy snow, coastal flooding, high winds to Northeast U.S.

Single-Cell Thunderstorm

Short-lived storm with distinct updraft, mature, and dissipating phases.

Multi-Cell Thunderstorm

Cluster of storms formed by outflow boundaries, sustaining longer-lived severe weather.

Supercell Thunderstorm

Rotating thunderstorm with strong persistent updraft (mesocyclone), capable of producing major tornadoes.

Mesocyclone

Rotating updraft within a supercell, precursor to tornado formation.

Outflow Boundary

Leading edge of cooler air from downdraft that can trigger new storms.

Anvil Cloud

Flattened cloud top of a thunderstorm spread by upper-level winds.

Squall Line

Organized line of intense thunderstorms ahead of a cold front.

Severe Thunderstorm

Storm producing hail ≥1", winds ≥58 mph, and/or tornadoes.

Watch

Conditions are favorable for severe weather; stay alert.

Warning

Severe weather is occurring or imminent; take shelter immediately.

Hail

Ice that forms in strong updrafts, gaining layers as it cycles within the storm.

Wet Growth

Clear hail layers formed by water freezing slowly in abundant moisture.

Dry Growth

Opaque hail layers formed by rapid freezing of droplets.

Lightning

Electrical discharge balancing differences in charge within clouds or between cloud and ground.

Thunder

Sound wave caused by rapid expansion of superheated air from lightning.

Stepped Leader

Initial invisible channel of negative charge descending from a cloud before lightning strike.

Return Stroke

Visible upward current creating the bright flash of lightning.

Dart Leader

Subsequent lightning channel that recharges the pathway and produces multiple flashes.

Anvil Crawler

Lightning traveling horizontally across the top of thunderstorm anvils.

Waterspout

Tornado occurring over water, usually weaker than land tornadoes.

Derecho

Long-lived windstorm caused by fast-moving bow-shaped thunderstorms, producing widespread damage.

Microburst

Powerful localized downdraft producing damaging straight-line winds over small areas.

Haboob

Large dust storm produced by thunderstorm outflow winds in arid regions.

Dust Devil

Small rotating column of air formed on hot, dry surfaces; non-thunderstorm phenomenon.

Weather Balloon

Instrument package launched twice daily to measure temperature, humidity, wind, and pressure in upper atmosphere.

Radar

Uses radio waves to detect precipitation location, movement, intensity, and rotation.

Satellite

Observes cloud cover, storm development, and temperature from space, especially over oceans.

UTC/Zulu Time

Coordinated universal time used on weather maps; Knoxville = 4-5 hours behind depending on daylight saving.

Model Output Statistics (MOS)

Computer-based forecast data offering temperature, precipitation, and cloud predictions for local areas.

Tornado

Narrow, violently rotating column of air extending from thunderstorms to the ground.

Wall Cloud

Lowered, rotating cloud base beneath a mesocyclone; often where tornadoes form.

Rear Flank Downdraft (RFD)

Dry descending air wrapping around a mesocyclone, helping tighten rotation.

Hook Echo

Radar signature showing rotation and potential tornado development.

Enhanced Fujita Scale (EF Scale)

Damage-based tornado rating scale from EF0 to EF5 estimating wind speed.

Tornado Climatology

Study showing most tornadoes occur in central U.S. due to unique geography and air mass collisions.

Tropical Cyclone

Warm-core rotating storm system fueled by ocean heat, with no attached fronts.

Eye

Calm, clear center of a tropical cyclone with sinking air.

Eyewall

Ring of strongest winds, heaviest rain surrounding the eye.

Storm Surge

Coastal flooding caused by hurricane winds pushing ocean water onshore.

Hurricane Ingredients

Warm water ≥80°F, low wind shear, moist environment, instability, Coriolis force.

Hurricane Strengthening

Occurs over warm water with low shear and high moisture.

Hurricane Weakening

Occurs when storms encounter land, cold water, dry air, or high wind shear.

Cabo Verde Hurricane

Long-track hurricanes originating near West Africa that can impact Caribbean and U.S.