Ch 9 Quiz: Air Masses & Fronts — Source Regions & Frontal Weather

Air mass: a large volume of air with horizontally uniform temperature, humidity, and stability, acquired from a source region.

A frontal zone

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A large body of air with roughly uniform temperature and humidity throughout, acquired from a source region

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A trough of low pressure

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The area between two isobars

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Classical air mass classification: Polar Maritime (mP), Polar Continental (cP), Tropical Maritime (mT), Tropical Continental (cT).

Polar Maritime, Polar Continental, Tropical Maritime, Tropical Continental

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Arctic, Antarctic, Equatorial, Stratospheric

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Cold, Warm, Wet, Dry

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Maritime, Continental, Monsoon, Trade

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Warm front: warm air mass actively advancing, sliding up over retreating cold air.

Any front associated with thunderstorms

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The boundary where warm air advances and overrides retreating cold air

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The boundary where cold air advances under warm air

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A stationary boundary

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Warm front slope: ~1:200 — 1 km rise per 200 km horizontal. Produces extensive cloud and precipitation bands up to 400-500 km ahead of the surface front.

1:200 (gentle)

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1:50

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1:100

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1:500

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Cold front: steeper slope (~1:50 to 1:100). Faster, more intense lifting produces narrow but intense weather bands: Cb, heavy showers, squalls, then rapid clearance.

Shallower

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Steeper (~1:50-1:100) — more rapid lifting, more intense but narrower weather band

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The same

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Always vertical

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Warm sector: wedge of warm moist air between warm and cold fronts. Weather: overcast, low stratus, possible drizzle. Eroded as cold front overtakes warm front.

The area behind the cold front

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The region of warm air between the warm front (ahead) and the cold front (behind) in a mid-latitude depression

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The area near the equator

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The centre of a high pressure system

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Occlusion: cold front (faster) catches the warm front. Warm sector air lifted off the surface. Cold occlusion: cold front air colder than pre-existing cold air.

Two cold fronts merge

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The cold front catches up with the warm front, lifting the warm sector air off the surface

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A warm front stalls

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High pressure builds between two fronts

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Stationary front: neither air mass advancing. Can produce persistent low cloud, fog, and light precipitation along its length for days.

Moves slowly eastward

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Shows little or no horizontal movement — wind blows parallel to it from both sides

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Always produces heavy rain

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Is the same as an occlusion

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Tropical Continental (cT): hot, dry, dusty. Originates over sub-tropical deserts. Brings heat waves to southern Europe and Middle East. Very low humidity.

Cold and moist

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Hot and dry — originates over sub-tropical deserts (Sahara for Europe)

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Cold and dry

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Warm and very moist

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Polar Maritime (mP): cold, moist air from polar oceans. Becomes unstable heated from below. Produces: heavy showers, hail, squalls, good visibility between showers.

Warm and dry

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Cold, moist, and unstable — produces convective showers, good visibility between them

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Hot and dry

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Stable and foggy

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The PFJS results from the strong horizontal temperature gradient across the polar front. Thermal wind relationship produces a narrow band of strong westerly winds at tropopause.

Weak pressure gradient

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Strong north-south temperature gradient — the thermal wind relationship produces a jet of 100-200 kt at the tropopause

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Surface low pressure only

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Equatorial weather

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Cold frontal passage: wind veers sharply (SW to NW in NH), temperature drops 5-10C, dew point falls, pressure rises, precipitation clears quickly.

Increase in temperature and dew point

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Wind shift (backing to veering in NH), sharp temperature and dew point drop, pressure trough then rise, passage of intense precipitation band

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Decrease in wind and cloud clearing slowly

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Fog development

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SIGMETs are issued by MWOs designated by ICAO. Cover: CB, TS, TURB, ICE, MTW, volcanic ash, tropical cyclones, and dust/sandstorms.

The airline

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National Met services — fronts, tropical cyclones, volcanic ash, space weather

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ICAO directly

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Pilots only

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Air mass modification: heat and moisture exchange with the underlying surface alter properties. Cold air over warm water becomes unstable; warm moist air over cold land becomes stable.

An identical surface

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A surface of different temperature or moisture — the air mass is gradually altered by sensible heat and moisture flux from the surface

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The ITCZ only

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A cold surface only

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Airmass storms: afternoon heating trigger, isolated cells. Frontal thunderstorms: mechanically forced by fronts, organised squall lines, can persist all night.

No difference exists

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Airmass storms form by surface heating in summer afternoons; frontal storms are triggered by mechanical lifting and can persist at night

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Frontal storms are always weaker

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Airmass storms only occur over oceans

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Norwegian Model: frontal wave on polar front -> low develops -> warm and cold fronts form -> warm sector -> cold front overtakes warm front -> occlusion -> fills and dissipates.

Stationary front only

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Frontal wave development to deepening depression to warm sector to occlusion to filling to dissipation

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Anticyclone only

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Only in polar regions

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Western Disturbances: shallow mid-latitude systems originating over the Mediterranean, traveling eastward along the jet stream. Bring cold cP air and moisture to NW India.

The Southern Ocean

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Central Asia and Tibet — continental polar air brings cold northerly winds (western disturbances)

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The Arctic directly

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The Pacific Ocean

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Post cold front (NH): cold unstable Polar Maritime air brings NW winds, intermittent heavy showers, Cumuliform cloud, excellent visibility between showers.

Warm and moist air

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Cold north-westerly winds, convective clouds (Cb, Cu), sharp visibility, and heavy showers — Polar Maritime air mass

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Fog and drizzle

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Light and variable winds

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Conveyor belt model: WCB = warm moist air rising poleward, producing cloud and precipitation. CCB = cyclonically turning cold air. Dry intrusion = descending stratospheric air.

A manufacturing analogy

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Three airstreams: warm conveyor belt (WCB), cold conveyor belt (CCB), and dry intrusion

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Trade wind circulation

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Monsoon progression

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Fohn wall: thick cloud on windward side of mountains. Clear Fohn gap on lee side where descending air warms and dries. Characteristic feature of Fohn events.

An isolated Cb

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An abrupt wall of cloud on the windward side of the Alps as moist air rises; clear Fohn gap on the leeward side

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A lenticular only

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Fog at mountain base

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Jet streaks: embedded wind maxima within the jet stream. Left exit region (NH) = preferred area for surface cyclogenesis (upper-level divergence).

Slower than average jet flow

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Locally enhanced wind maxima within the jet — entrance and exit regions associated with vertical motion and weather development

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Calm air

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No meteorological significance

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Secondary cold front: a second cold front following the primary, bringing an additional cold air surge. Can produce a second burst of convective weather 12-24 hours after the first.

Before the primary cold front

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Behind the primary cold front — a second surge of even colder air advancing

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During occlusion

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Only in tropical weather

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Air mass modification tracking: compare temperature/dew point at observation stations along the path. Stability indices (LI, K-index) track increasing instability.

Pressure only

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Temperature, dew point, lapse rate, and cloud types — a cold air mass over warm water becomes progressively more unstable

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Wind direction only

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Altitude of the tropopause only

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ITCZ vs. fronts: true fronts have a significant cross-frontal temperature gradient. ITCZ separates air masses of similar temperature. Primarily a CONVERGENCE zone.

It has no clouds

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It lacks a significant temperature gradient across it — it is a moisture/convergence boundary, not a sharp temperature discontinuity

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It is stationary

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It is below FL100

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Cold front temperature gradient: typically 5-15C across 50-200 km. This gradient drives the thermal wind (jet stream) above.

Negligible

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5-15 degrees C over 50-200 km — sharp enough to be analysed as a front on synoptic charts

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50C over 1 km

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The same temperature on both sides

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Frontogenesis: intensification of a temperature gradient (creating or sharpening a front). Key mechanisms: deformation, differential advection. Opposite = frontolysis.

Front destruction

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The process of front formation or intensification — caused by differential temperature advection, deformation, and boundary layer convergence

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Weather at the front

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Filling of a depression

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Warm occlusion: advancing cold air is warmer than pre-existing cold air ahead. Cold front undercuts the pre-existing cold air. Common in NW Europe and NW Pacific.

The occluded front is warm everywhere

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The cold air behind the cold front is WARMER than the cold air ahead of the warm front — cold front rides up over pre-existing colder air

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Two warm fronts merge

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No temperature gradient exists

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Thermal ridge (thickness ridge): ridge in the 1000-500 hPa thickness field. Indicates warm air in mid-troposphere. Associated with blocking anticyclones.

A surface low

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A ridge of warm temperature (thickness) in the mid-troposphere — downstream of surface warm sectors, associated with blocking

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Cold air mass

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Surface anticyclone only

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WDs: troughs/upper-level cyclonic circulations embedded in the subtropical westerly jet stream. Move W to E, bringing moisture from Mediterranean to NW India in winter.

Monsoon depressions

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Shallow extra-tropical cyclones from the Mediterranean/Caspian Sea tracking eastward along the subtropical jet — bringing winter rainfall to NW India

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Tropical cyclones from the Arabian Sea

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Bay of Bengal depressions

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Warm front geometry: slope 1:200. At 3000 ft AGL, surface front = 3000 x 200 = 600000 ft = ~100 nm ahead of aircraft.

600 nm ahead

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~100 nm ahead — 3000 ft times 200 = 600000 ft = ~100 nm

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3000 nm ahead

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At the aircraft position

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Ch 9 Quiz: Air Masses & Fronts — Source Regions & Frontal Weather

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