Ch 8 – Airmasses, Fronts & Cyclones

Ch 8 – Airmasses, Fronts & Ch 8 – Airmasses, Fronts & Cyclones Cyclones

Ch 8 – Airmasses, Fronts & Ch 8 – Airmasses, Fronts & CyclonesCyclones• IntroductionIntroduction

– The general circulation and monsoon The general circulation and monsoon discussed in the previous chapter have discussed in the previous chapter have very large horizontal dimensions and very large horizontal dimensions and long time scales.long time scales.

– Another notable characteristic is that Another notable characteristic is that seasonal, large-scale features, such as seasonal, large-scale features, such as highs and lows, tend to remain highs and lows, tend to remain stationary (Lester, 2006).stationary (Lester, 2006).


– In this chapter, we look at another In this chapter, we look at another collection of circulations that are collection of circulations that are different in at least three ways: they are different in at least three ways: they are smaller in size, have shorter life times, smaller in size, have shorter life times, and they have significant movement.and they have significant movement.

– These are extratropical cyclones and These are extratropical cyclones and tropical cyclones that, in their most tropical cyclones that, in their most intense form, are known as hurricanes intense form, are known as hurricanes (Lester, 2006).(Lester, 2006).


– They are the “weather makers” that have They are the “weather makers” that have significant effects on aviation activities.significant effects on aviation activities.

– Many of the flight hazards discussed so far Many of the flight hazards discussed so far are related to cyclones.are related to cyclones.

– When you complete Chapter 8, you will be When you complete Chapter 8, you will be familiar with the causes and structures of familiar with the causes and structures of extratropical cyclones and tropical extratropical cyclones and tropical cyclones, and the weather they produce cyclones, and the weather they produce (Lester, 2006).(Lester, 2006).


– You will also have been introduced to a You will also have been introduced to a conceptual model of each type of conceptual model of each type of cyclone, which will prove to be cyclone, which will prove to be invaluable in the interpretation of invaluable in the interpretation of meteorological observations, analyses meteorological observations, analyses and forecasts (Lester, 2006).and forecasts (Lester, 2006).

Ch 8 – Airmasses, Fronts & Ch 8 – Airmasses, Fronts & CyclonesCyclones• Section A – Extratropical CyclonesSection A – Extratropical Cyclones

– The Polar FrontThe Polar Front• AirmassesAirmasses• FrontsFronts• Extratropical Cyclones Structure and DevelopmentExtratropical Cyclones Structure and Development

• Section B – Tropical Cyclones & Hurricanes Section B – Tropical Cyclones & Hurricanes – ClimatologyClimatology– Development and BehaviorDevelopment and Behavior– Structure and WeatherStructure and Weather

Ch 8 – Airmasses, Fronts & Ch 8 – Airmasses, Fronts & CyclonesCyclones• Section A: Extratropical CyclonesSection A: Extratropical Cyclones

– Extratropical cycloneExtratropical cyclone – an extratropical – an extratropical cyclone is a macroscale low-pressure cyclone is a macroscale low-pressure disturbance that develops outside the tropics.disturbance that develops outside the tropics.

– Frontal lows or frontal cyclonesFrontal lows or frontal cyclones – – extratropical cyclones draw their energy from extratropical cyclones draw their energy from temperature differences across the polar front, temperature differences across the polar front, so they are also known as frontal lows or so they are also known as frontal lows or frontal cyclones.frontal cyclones.

Ch 8 – Airmasses, Fronts & Ch 8 – Airmasses, Fronts & CyclonesCyclones• Wave cyclones and frontal wavesWave cyclones and frontal waves – frontal – frontal

lows or frontal cyclones move from west to east lows or frontal cyclones move from west to east as macroscale eddies embedded in the prevailing as macroscale eddies embedded in the prevailing westerlies. westerlies. – These disturbances distort the polar front into These disturbances distort the polar front into

a wave shapea wave shape– therefore they are also referred to as wave therefore they are also referred to as wave

cyclones and frontal waves.cyclones and frontal waves.

Ch 8 – Airmasses, Fronts & Ch 8 – Airmasses, Fronts & CyclonesCyclones• Polar front modelPolar front model – the important characteristics of – the important characteristics of

the development and structure of a frontal low are the development and structure of a frontal low are represented by the polar front model.represented by the polar front model.

• AirmassesAirmasses – an airmass is a large body of air that – an airmass is a large body of air that has fairly uniform temperature, stability and has fairly uniform temperature, stability and moisture characteristics.moisture characteristics.

• Airmass source regionAirmass source region – an airmass is generally – an airmass is generally identified by its airmass source regionidentified by its airmass source region– that is, by the geographical area where it developsthat is, by the geographical area where it develops

•Common airmass types are Arctic (A), Polar (P), Common airmass types are Arctic (A), Polar (P), and Tropical (T).and Tropical (T).

Ch 8 – Airmasses, Fronts & Ch 8 – Airmasses, Fronts & CyclonesCyclones• Cold airmassCold airmass – once an airmass leaves its source – once an airmass leaves its source

region, it is also classified according to its region, it is also classified according to its temperature relative to the ground over which it is temperature relative to the ground over which it is movingmoving– A cold airmass is colder than the ground over A cold airmass is colder than the ground over

which it is moving.which it is moving.• Warm airmassWarm airmass - once an airmass leaves its source - once an airmass leaves its source

region, it is also classified according to its region, it is also classified according to its temperature relative to the ground over which it is temperature relative to the ground over which it is movingmoving– A warm airmass is warmer than the ground over A warm airmass is warmer than the ground over

which it is moving.which it is moving.

Ch 8 – Airmasses, Fronts & Ch 8 – Airmasses, Fronts & CyclonesCyclones• FrontFront – as you know from your earlier reading – as you know from your earlier reading

about the causes of vertical motions, that about the causes of vertical motions, that boundary is called a front. boundary is called a front. – Fronts are hundreds of miles long and have Fronts are hundreds of miles long and have

lifetimes similar to those of airmasseslifetimes similar to those of airmasses– They are classified according to their They are classified according to their

movement.movement.

Ch 8 – Airmasses, Fronts & Ch 8 – Airmasses, Fronts & CyclonesCyclones• ****WHEN AN AIRMASS IS STABLE, IT IS COMMON ****WHEN AN AIRMASS IS STABLE, IT IS COMMON

TO FIND SMOKE, DUST, HAZE, ETC., TO FIND SMOKE, DUST, HAZE, ETC., CONCENTRATED AT THE LOWER LEVELS, WITH CONCENTRATED AT THE LOWER LEVELS, WITH RESULTING POOR VISIBILITY****RESULTING POOR VISIBILITY****

Ch 8 – Airmasses, Fronts & Ch 8 – Airmasses, Fronts & CyclonesCyclones• ****ONE OF THE MOST EASILY RECOGNIZED ****ONE OF THE MOST EASILY RECOGNIZED

DISCONTINUTIES ACROSS A FRONT IS A CHANGE DISCONTINUTIES ACROSS A FRONT IS A CHANGE IN TEMPERATURE****IN TEMPERATURE****

Ch 8 – Airmasses, Fronts & Ch 8 – Airmasses, Fronts & CyclonesCyclones• Cold frontCold front – fronts are assigned a name according – fronts are assigned a name according

to whether the cold airmass is advancing (cold front) to whether the cold airmass is advancing (cold front) or the warm front is advancing (warm front).or the warm front is advancing (warm front).

• Warm frontWarm front - fronts are assigned a name according - fronts are assigned a name according to whether the cold airmass is advancing (cold front) to whether the cold airmass is advancing (cold front) or the warm front is advancing (warm front).or the warm front is advancing (warm front).

• Stationary frontStationary front – if the airmasses show no – if the airmasses show no appreciable movement, the front is designated a appreciable movement, the front is designated a stationary front (or quasi-stationary front).stationary front (or quasi-stationary front).

• Occluded frontOccluded front – in the situation where a cold front – in the situation where a cold front overtakes a warm frontovertakes a warm front– the result of which is termed an occluded front.the result of which is termed an occluded front.

Ch 8 – Airmasses, Fronts & Ch 8 – Airmasses, Fronts & CyclonesCyclones• Frontal slopeFrontal slope – refers to the ratio of the altitude – refers to the ratio of the altitude

of the top of the cold air at some point in the cold of the top of the cold air at some point in the cold airmass to the horizontal distance of that point on airmass to the horizontal distance of that point on the surface from the nearest edge of the airmassthe surface from the nearest edge of the airmass– ratio of the altitude (Z) of the frontal surface to ratio of the altitude (Z) of the frontal surface to

the distance (X) from the surface position of the distance (X) from the surface position of the frontthe front•frontal slope = Z / X frontal slope = Z / X

Ch 8 – Airmasses, Fronts & Ch 8 – Airmasses, Fronts & CyclonesCyclones• Frontal zoneFrontal zone – a front is not a thin line as shown – a front is not a thin line as shown

on a chart, but is actually a narrow frontal zone on a chart, but is actually a narrow frontal zone through which there is a rapid transition of through which there is a rapid transition of conditions from one airmass to the other.conditions from one airmass to the other.

Ch 8 – Airmasses, Fronts & Ch 8 – Airmasses, Fronts & CyclonesCyclones•CyclogenesisCyclogenesis – vertical motions – vertical motions

provided by large mountain chains and provided by large mountain chains and latent heat derived from moist air latent heat derived from moist air during condensation can also enhance during condensation can also enhance cyclone development (cyclogenesis)cyclone development (cyclogenesis)– These two processes frequently work These two processes frequently work

together to produce frontal lows on the together to produce frontal lows on the east slopes of the Rocky Mountains.east slopes of the Rocky Mountains.

Ch 8 – Airmasses, Fronts & Ch 8 – Airmasses, Fronts & CyclonesCyclones• Wave cyclonesWave cyclones – these cyclones do not – these cyclones do not

necessarily develop beyond the incipient stage. necessarily develop beyond the incipient stage. – These so-called stable waves simply move These so-called stable waves simply move

rapidly along the polar front, finally dissipating. rapidly along the polar front, finally dissipating.

Ch 8 – Airmasses, Fronts & Ch 8 – Airmasses, Fronts & CyclonesCyclones• ****A VARAITION THAT WILL ALWAYS OCCUR ****A VARAITION THAT WILL ALWAYS OCCUR

WHEN FLYING ACROSS A FRONT IS A CHANGE IN WHEN FLYING ACROSS A FRONT IS A CHANGE IN THE WIND****THE WIND****

Ch 8 – Airmasses, Fronts & Ch 8 – Airmasses, Fronts & CyclonesCyclones• Incipient stageIncipient stage – this is also called the wave – this is also called the wave

cyclone stage because the previous stationary cyclone stage because the previous stationary front has been distorted into a wave shape in front has been distorted into a wave shape in response to the developing circulationresponse to the developing circulation

Ch 8 – Airmasses, Fronts & Ch 8 – Airmasses, Fronts & CyclonesCyclones• Warm sectorWarm sector – the triangular region of warm air – the triangular region of warm air

between the fronts and to the south of the between the fronts and to the south of the cyclone is called the warm sector.cyclone is called the warm sector.

• Wind ShearWind Shear – the change of wind speed and / or – the change of wind speed and / or wind direction over a distance. Shears may be wind direction over a distance. Shears may be vertical, horizontal or both.vertical, horizontal or both.

• Cyclonic wind shearCyclonic wind shear – changes in the wind – changes in the wind speed or direction correspond with what you speed or direction correspond with what you would find as you cross a low-pressure area.would find as you cross a low-pressure area.

Ch 8 – Airmasses, Fronts & Ch 8 – Airmasses, Fronts & CyclonesCyclones• Anticyclonic wind shearAnticyclonic wind shear – what you would expect – what you would expect

when crossing a high-pressure areawhen crossing a high-pressure area– The wind shear across a front (which often lies in The wind shear across a front (which often lies in

a trough of low pressure) is cyclonic.a trough of low pressure) is cyclonic.• Deepening Deepening – as the cyclone progresses – as the cyclone progresses

northeastward, the central pressure continues to fallnortheastward, the central pressure continues to fall– This is an indication that the cyclone is This is an indication that the cyclone is

deepening. The winds around the cyclone deepening. The winds around the cyclone increase in response to the greater pressure increase in response to the greater pressure gradient.gradient.

Ch 8 – Airmasses, Fronts & Ch 8 – Airmasses, Fronts & CyclonesCyclones• Occlusion processOcclusion process – the warm sector air is – the warm sector air is

pushed aloft by the occlusion process and the pushed aloft by the occlusion process and the cyclone enters the occluded stage.cyclone enters the occluded stage.

• FillingFilling – the central pressure begins to rise (the – the central pressure begins to rise (the cyclone is filling) as the frontal low enters the cyclone is filling) as the frontal low enters the dissipating stage of its life cycledissipating stage of its life cycle

Ch 8 – Airmasses, Fronts & Ch 8 – Airmasses, Fronts & CyclonesCyclones• Short Wave TroughsShort Wave Troughs – the upper-level troughs – the upper-level troughs

which correspond t developing frontal lows are which correspond t developing frontal lows are smaller scale than long waves (1,000 nm vs. smaller scale than long waves (1,000 nm vs. 3,000 nm or more).3,000 nm or more).– These are also called short wave troughs and These are also called short wave troughs and

these disturbances move toward the east these disturbances move toward the east much more rapidly than long wave troughs much more rapidly than long wave troughs averaging about 600 nm per day.averaging about 600 nm per day.

• Closed lowClosed low – an occluded cyclone at the surface – an occluded cyclone at the surface usually corresponds with a closed low aloftusually corresponds with a closed low aloft

Ch 8 – Airmasses, Fronts & Ch 8 – Airmasses, Fronts & CyclonesCyclones• ****WHILE FLYING CROSS COUNTRY IN THE ****WHILE FLYING CROSS COUNTRY IN THE

NORTHERN HEMISPHERE, IF YOU EXPERIENCE A NORTHERN HEMISPHERE, IF YOU EXPERIENCE A CONTINUOUS LEFT CROSSWIND WHICH IS CONTINUOUS LEFT CROSSWIND WHICH IS ASSOCIATED WITH A MAJOR WIND SYSTEM, YOU ASSOCIATED WITH A MAJOR WIND SYSTEM, YOU ARE FLYING TOWARD A LOW-PRESSURE AREA ARE FLYING TOWARD A LOW-PRESSURE AREA AND GENERALLY UNFAVORABLE WEATHER AND GENERALLY UNFAVORABLE WEATHER CONDITIONS****CONDITIONS****

Ch 8 – Airmasses, Fronts & Ch 8 – Airmasses, Fronts & CyclonesCyclones• OverrunningOverrunning – along the warm front, warm, – along the warm front, warm,

moist, stable air moves over the retreating wedge moist, stable air moves over the retreating wedge of cold air in what is frequently described as a of cold air in what is frequently described as a gentle up-gliding motion or overrunning.gentle up-gliding motion or overrunning.

• Warm front occlusionWarm front occlusion – the warm front remains – the warm front remains on the ground because the cold air ahead of the on the ground because the cold air ahead of the warm front is much colder than the air behind the warm front is much colder than the air behind the cold frontcold front

• Cold front occlusionCold front occlusion – cold front remains on the – cold front remains on the groundground

Ch 8 – Airmasses, Fronts & Ch 8 – Airmasses, Fronts & CyclonesCyclones• ****INDICATORS OF AN APPROACHING WARM ****INDICATORS OF AN APPROACHING WARM

FRONT ARE STEADY PRECIPITATION WITH FRONT ARE STEADY PRECIPITATION WITH STRATIFORM CLOUDS****STRATIFORM CLOUDS****

• ****A COMMON IN-FLIGHT HAZARD ASSOCIATED ****A COMMON IN-FLIGHT HAZARD ASSOCIATED WITH WARM FRONTS IS PRECIPITATION-INDUCED WITH WARM FRONTS IS PRECIPITATION-INDUCED FOG****FOG****

• ****IN A COLD FRONT OCCLUSION, THE AIR ****IN A COLD FRONT OCCLUSION, THE AIR AHEAD OF THE WARM FRONT IS WARMER THAN AHEAD OF THE WARM FRONT IS WARMER THAN THE AIR BEHIND THE OVERTAKING COLD THE AIR BEHIND THE OVERTAKING COLD FRONT****FRONT****

Ch 8 – Airmasses, Fronts & Ch 8 – Airmasses, Fronts & CyclonesCyclones• ****WHEN THE COLD AIRMASS FOLLOWING A ****WHEN THE COLD AIRMASS FOLLOWING A

COLD FRONT IS MOIST AND UNSTABLE, IT IS COLD FRONT IS MOIST AND UNSTABLE, IT IS CHARACTERIZED BY CUMULIFORM CLOUDS AND CHARACTERIZED BY CUMULIFORM CLOUDS AND SHOWERY PRECIPITATION****SHOWERY PRECIPITATION****

• ****A RIDGE OR HIGH-PRESSURE AREA IS ****A RIDGE OR HIGH-PRESSURE AREA IS CHARACTERIZED BY DOWNWARD MOTION**** CHARACTERIZED BY DOWNWARD MOTION****

Ch 8 – Airmasses, Fronts & Ch 8 – Airmasses, Fronts & CyclonesCyclones• Frontal cloud bandFrontal cloud band – a low-level cold frontal – a low-level cold frontal

cloud band is found to the south of the cyclone cloud band is found to the south of the cyclone and east of the trough line.and east of the trough line.

• Comma cloudComma cloud – on the east side of the upper – on the east side of the upper trough are a comma cloud composed of middle trough are a comma cloud composed of middle clouds and a broad curved band of jet stream clouds and a broad curved band of jet stream cirrus located just to the right of the jet axis.cirrus located just to the right of the jet axis.

Ch 8 – Airmasses, Fronts & Ch 8 – Airmasses, Fronts & CyclonesCyclones• Section B: Tropical Cyclones and HurricanesSection B: Tropical Cyclones and Hurricanes

– Tropical cycloneTropical cyclone – a mesoscale, cyclonic circulation – a mesoscale, cyclonic circulation that develops in the tropical easterliesthat develops in the tropical easterlies•the term tropical cyclone covers a number of the term tropical cyclone covers a number of

similar tropical disturbances which are classified similar tropical disturbances which are classified according to their maximum sustained wind according to their maximum sustained wind speeds:speeds:

– Tropical disturbance – < 20 knotsTropical disturbance – < 20 knots– Tropical depression – 20 to 34 knotsTropical depression – 20 to 34 knots– Tropical storm – 35 to 64 knotsTropical storm – 35 to 64 knots– Hurricane – > 64 knotsHurricane – > 64 knots

Ch 8 – Airmasses, Fronts & Ch 8 – Airmasses, Fronts & CyclonesCyclones• Storm surgeStorm surge – an abnormal rise of water due to a – an abnormal rise of water due to a

tropical cyclonetropical cyclone– Saffir-Simpson scale of damage potential of Saffir-Simpson scale of damage potential of

hurricanes (Figure 8-19).hurricanes (Figure 8-19).• Hurricane eyeHurricane eye – the hurricane eye is the circular, – the hurricane eye is the circular,

nearly cloud-free region approximately 10 to 20 nm nearly cloud-free region approximately 10 to 20 nm in diameter that is located in the center of the storm.in diameter that is located in the center of the storm.

• Eye wallEye wall – the eye wall is the cloudy region – the eye wall is the cloudy region embedded with many thunderstorms immediately embedded with many thunderstorms immediately adjacent to the eye adjacent to the eye

Ch 8 – Airmasses, Fronts & Ch 8 – Airmasses, Fronts & CyclonesCyclones• RainbandsRainbands – the rainbands that spiral into the – the rainbands that spiral into the

storm are also lines of convergence characterized storm are also lines of convergence characterized by thunderstorms and showery activityby thunderstorms and showery activity

• Hurricane watchHurricane watch – a hurricane watch is issued – a hurricane watch is issued when hurricane conditions are expected in a when hurricane conditions are expected in a particular area within a day or more.particular area within a day or more.

• Hurricane warningHurricane warning – issued when the arrival of – issued when the arrival of those conditions is expected within the next 24 those conditions is expected within the next 24 hourshours

SummarySummary• Extratropical cyclones are important large Extratropical cyclones are important large

scale disturbances that move eastward in scale disturbances that move eastward in the middle latitudes.the middle latitudes.

• They draw their energy from the polar They draw their energy from the polar front and involve the movement of large front and involve the movement of large airmasses and fronts near the earth’s airmasses and fronts near the earth’s surface, as well as the development of surface, as well as the development of troughs and jet streaks aloft (Lester, troughs and jet streaks aloft (Lester, 2006).2006).

SummarySummary• The average wind, cloud, and precipitation The average wind, cloud, and precipitation

patterns that evolve during the life cycle of patterns that evolve during the life cycle of the extratropical cyclone are captured by the extratropical cyclone are captured by the polar front model.the polar front model.

• In contrast, slightly smaller scale tropical In contrast, slightly smaller scale tropical cyclones develop and move westward in low cyclones develop and move westward in low latitudes.latitudes.

• Some of these develop into highly Some of these develop into highly destructive hurricanesdestructive hurricanes

• Tropical cyclones, which are not Tropical cyclones, which are not characterized by fronts and airmass characterized by fronts and airmass contrasts, instead draw their energy from contrasts, instead draw their energy from warm waters and die over cold waters or warm waters and die over cold waters or land (Lester, 2006).land (Lester, 2006).

SummarySummary• Structurally, the extratropical cyclone has Structurally, the extratropical cyclone has

a cold core and intensifies with height.a cold core and intensifies with height.• Tropical cyclones have warm cores and Tropical cyclones have warm cores and

weaken with height.weaken with height.• Both of these circulations are critical for Both of these circulations are critical for

aviation.aviation.• Nearly every aviation weather flight hazard Nearly every aviation weather flight hazard

can be present at one time or another can be present at one time or another during the lifetimes of these phenomena, during the lifetimes of these phenomena, including thunderstorms, windshear, including thunderstorms, windshear, turbulence, icing, and IMC (Lester, 2006).turbulence, icing, and IMC (Lester, 2006).

Ch 8 – Airmasses, Fronts & Cyclones

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