Ch 9 - Thunderstorms

Ch 9 - ThunderstormsCh 9 - Thunderstorms

Ch 9 - ThunderstormsCh 9 - Thunderstorms• IntroductionIntroduction

– In this chapter, we continue our “scale approach” to In this chapter, we continue our “scale approach” to the understanding of atmospheric circulations.the understanding of atmospheric circulations.

– Moving to smaller scales, our consideration is now Moving to smaller scales, our consideration is now the mesoscale phenomena known as a the mesoscale phenomena known as a thunderstorm.thunderstorm.

– The thunderstorm is one of the most spectacular The thunderstorm is one of the most spectacular atmospheric circulations, and one that you must atmospheric circulations, and one that you must respect as a pilot.respect as a pilot.

– It can be bright, loud, violent, and dangerous in It can be bright, loud, violent, and dangerous in many ways (Lester, 2006).many ways (Lester, 2006).


– As with our study of macroscale As with our study of macroscale circulations, we will begin with an circulations, we will begin with an idealized model of the thunderstorm.idealized model of the thunderstorm.

– When you complete this chapter, you will When you complete this chapter, you will understand thunderstorm structure and understand thunderstorm structure and behavior as well as the wide variety of behavior as well as the wide variety of microscale phenomena that are microscale phenomena that are frequently produced by a thunderstorm frequently produced by a thunderstorm (Lester, 2006).(Lester, 2006).


– You will also become familiar with larger You will also become familiar with larger mesoscale and macroscale circulations mesoscale and macroscale circulations that provoke thunderstorms and that provoke thunderstorms and organize them into lines and clusters.organize them into lines and clusters.

– A thunderstorm is always a threat to A thunderstorm is always a threat to aircraft operations.aircraft operations.

– A wise pilot will be sure he or she A wise pilot will be sure he or she understands the what? Why? And understands the what? Why? And where? Of thunderstorms (Lester, 2006).where? Of thunderstorms (Lester, 2006).

Ch 9 - ThunderstormsCh 9 - Thunderstorms• Section A – Dry ConvectionSection A – Dry Convection• Section B – Cloudy ConvectionSection B – Cloudy Convection

– Cloud GrowthCloud Growth– Downdraft DevelopmentDowndraft Development

• Section C – Weather RadarSection C – Weather Radar– Lightning Detection EquipmentLightning Detection Equipment

Ch 9 - ThunderstormsCh 9 - Thunderstorms• Section D – Thunderstorm StructuresSection D – Thunderstorm Structures

– Thunderstorm TypesThunderstorm Types•Airmass ThunderstormAirmass Thunderstorm•Multicell ThunderstormMulticell Thunderstorm•Supercell ThunderstormsSupercell Thunderstorms

– TornadoesTornadoes– HailHail– LightningLightning

Ch 9 - ThunderstormsCh 9 - Thunderstorms• Section E – Thunderstorm EnvironmentSection E – Thunderstorm Environment

– Requirements for DevelopmentRequirements for Development– ClimatologyClimatology– Instability PatternsInstability Patterns– Thunderstorm LinesThunderstorm Lines

• Macroscale FrontsMacroscale Fronts• Squall LinesSquall Lines• Other Mesoscale LinesOther Mesoscale Lines

– Thunderstorm ClustersThunderstorm Clusters• Macroscale ClustersMacroscale Clusters• Mesoscale Convective ComplexesMesoscale Convective Complexes

Ch 9 - ThunderstormsCh 9 - Thunderstorms• Section A: Dry ConvectionSection A: Dry Convection

– Dry convection – Dry convection – air in the cumulus clouds or air in the cumulus clouds or cumulonimbus clouds originally comes from the cumulonimbus clouds originally comes from the boundary layerboundary layer• a common process within a few thousand feet of the a common process within a few thousand feet of the

groundground– Thermals Thermals – the cloudless roots of cumulus clouds– the cloudless roots of cumulus clouds– Dust devilsDust devils – the result of the spin-up of a thermal – the result of the spin-up of a thermal

is a vortex known as a whirlwind or dust devil not is a vortex known as a whirlwind or dust devil not to be confused with more violent tornadoes or to be confused with more violent tornadoes or waterspoutswaterspouts

Ch 9 - ThunderstormsCh 9 - Thunderstorms• Vortex ringVortex ring – superimposed on the overall rising – superimposed on the overall rising

motion of the thermal is a microscale circulation motion of the thermal is a microscale circulation cell that is best described as an elongated vortex cell that is best described as an elongated vortex ring.ring.– Extending upward from the ground, the vortex Extending upward from the ground, the vortex

ring has a relatively narrow core of upward ring has a relatively narrow core of upward motions surrounded by a broad region of motions surrounded by a broad region of weaker sinking motions.weaker sinking motions.

Ch 9 - ThunderstormsCh 9 - Thunderstorms• Section B: Cloudy ConvectionSection B: Cloudy Convection

– Cloudy convectionCloudy convection – in its most common use the – in its most common use the term cloudy convection refers to saturated air that is term cloudy convection refers to saturated air that is rising because it is warmer than its surroundingsrising because it is warmer than its surroundings

– Convective condensation levelConvective condensation level – the characteristic – the characteristic flat bases of the clouds occur at the altitudes where flat bases of the clouds occur at the altitudes where the rising unstable air first reaches saturation and is the rising unstable air first reaches saturation and is called the convective condensation level.called the convective condensation level.

– Equilibrium levelEquilibrium level – the cloudy updraft continues its – the cloudy updraft continues its upward acceleration until it reaches its equilibrium upward acceleration until it reaches its equilibrium level; that is, the altitude where the updraft level; that is, the altitude where the updraft temperature is equal to the temperature of its temperature is equal to the temperature of its surroundings.surroundings.

Ch 9 - ThunderstormsCh 9 - Thunderstorms• ****STRONG UPWARD CURRENTS IN CLOUDS ****STRONG UPWARD CURRENTS IN CLOUDS

ENHANCE THE GROWTH RATE PRECIPITATION****ENHANCE THE GROWTH RATE PRECIPITATION****

Ch 9 - ThunderstormsCh 9 - Thunderstorms• Precipitation-induced downdraftPrecipitation-induced downdraft – when the – when the

altitude of the cumulus cloud exceeds the altitude of the cumulus cloud exceeds the freezing level, there is a rapid growth of cloud freezing level, there is a rapid growth of cloud particles by the ice crystal process. particles by the ice crystal process. – At some point in this process the updraft is no At some point in this process the updraft is no

longer strong enough to support the weight of longer strong enough to support the weight of the large particles. the large particles.

– They begin to fall dragging air downward. They begin to fall dragging air downward. •This is the beginning of the precipitation-This is the beginning of the precipitation-

induced downdraft.induced downdraft.

Ch 9 - ThunderstormsCh 9 - Thunderstorms• RadarRadar – weather radar is used extensively to – weather radar is used extensively to

locate thunderstorms and to observe their locate thunderstorms and to observe their structure and behavior. structure and behavior. – Radar for radio detection and ranging is an Radar for radio detection and ranging is an

instrument that uses electromagnetic radiation instrument that uses electromagnetic radiation to detect objects and determine their distance to detect objects and determine their distance and direction from the radar site.and direction from the radar site.

Ch 9 - ThunderstormsCh 9 - Thunderstorms• Radar echoRadar echo – a small fraction of the reflected / – a small fraction of the reflected /

scattered radar signal returns to the radar antenna / scattered radar signal returns to the radar antenna / receiver where it is intercepted. receiver where it is intercepted. – The received signal constitutes a radar echo.The received signal constitutes a radar echo.

• Doppler radarDoppler radar – has the capacity to determine – has the capacity to determine velocity of a target toward or away from the radar velocity of a target toward or away from the radar by measuring the frequency difference between the by measuring the frequency difference between the transmitted and received radiation.transmitted and received radiation.

• Weather radarWeather radar – operates at specific frequencies – operates at specific frequencies or wavelengths of electromagnetic radiation that or wavelengths of electromagnetic radiation that are sensitive to scattering by ice and water are sensitive to scattering by ice and water particles.particles.

Ch 9 - ThunderstormsCh 9 - Thunderstorms• WSR-88D (NEXRAD)WSR-88D (NEXRAD) – the United States weather – the United States weather

radar network has been upgraded over the last radar network has been upgraded over the last several years with an improved Weather Surveillance several years with an improved Weather Surveillance Radar. This is a powerful Doppler Radar System.Radar. This is a powerful Doppler Radar System.

• Airborne weather radarAirborne weather radar – not as powerful as – not as powerful as surfaced based radar, airborne weather radar has the surfaced based radar, airborne weather radar has the advantage of observing the current conditions ahead advantage of observing the current conditions ahead of the aircraftof the aircraft

• AttenuationAttenuation – as a radar signal travels away from its – as a radar signal travels away from its source, it undergoes a process known as attenuation. source, it undergoes a process known as attenuation. This is a weakening of the signal that occurs as the This is a weakening of the signal that occurs as the signal absorbed, scattered, or reflected along its path.signal absorbed, scattered, or reflected along its path.

Ch 9 - ThunderstormsCh 9 - Thunderstorms• ****THE RADARSCOPE PROVIDES NO ASSURANCE ****THE RADARSCOPE PROVIDES NO ASSURANCE

OF AVOIDING INSTRUMENT WEATHER OF AVOIDING INSTRUMENT WEATHER CONDITIONS****CONDITIONS****

Ch 9 - ThunderstormsCh 9 - Thunderstorms• Radar Summary ChartRadar Summary Chart – An example of the radar – An example of the radar

echo intensity information available every hour from echo intensity information available every hour from the national radar network is shown on a weather the national radar network is shown on a weather radar summary chart. radar summary chart. – Radar Summary Charts show weather radar echo Radar Summary Charts show weather radar echo

intensity scales as a measure of precipitation rate. intensity scales as a measure of precipitation rate.

– Contours represent radar echo intensity levels1, 3, Contours represent radar echo intensity levels1, 3, and 5.and 5.

– See power point presentation on Radar See power point presentation on Radar Summary Charts for additional informationSummary Charts for additional information

Ch 9 - ThunderstormsCh 9 - Thunderstorms• Radar Summary Charts give the following Radar Summary Charts give the following

information:information:– ECHO (PRECIPITATION) TYPE ECHO (PRECIPITATION) TYPE – INTENSITY INTENSITY – ECHO CONFIGURATION AND COVERAGE ECHO CONFIGURATION AND COVERAGE – ECHO TOPS ECHO TOPS – ECHO MOVEMENT ECHO MOVEMENT – SEVERE WEATHER WATCH AREAS SEVERE WEATHER WATCH AREAS

Ch 9 - ThunderstormsCh 9 - Thunderstorms• Constant Pressure Analysis ChartsConstant Pressure Analysis Charts - Weather - Weather

information for computer generated constant information for computer generated constant pressure charts is observed primarily by balloon-pressure charts is observed primarily by balloon-ascending radiosonde packages. ascending radiosonde packages. – Each package consists of weather instruments and Each package consists of weather instruments and

a radio transmitter. a radio transmitter. – During ascent instrument data are continuously During ascent instrument data are continuously

transmitted to the observation station. transmitted to the observation station. – Radiosondes are released at selected Radiosondes are released at selected

observational sites across the USA at 00Z and observational sites across the USA at 00Z and 12Z. 12Z.

Ch 9 - ThunderstormsCh 9 - Thunderstorms– The data collected from the radiosondes are used The data collected from the radiosondes are used

to prepare constant pressure charts twice a day. to prepare constant pressure charts twice a day. – Constant pressure charts are prepared for Constant pressure charts are prepared for

selected values of pressure and present weather selected values of pressure and present weather information at various altitudes. information at various altitudes.

– The standard charts prepared are the 850 mb The standard charts prepared are the 850 mb (hPa), 700 mb (hPa), 500 mb (hPa), 300mb (hPa), (hPa), 700 mb (hPa), 500 mb (hPa), 300mb (hPa), 250 mb (hPa), and 200 mb (hPa) charts. 250 mb (hPa), and 200 mb (hPa) charts.

– Charts with higher pressures present information Charts with higher pressures present information at lower altitudes while charts with lower at lower altitudes while charts with lower pressures present information at higher altitudes. pressures present information at higher altitudes.

Ch 9 - ThunderstormsCh 9 - Thunderstorms• Constant Pressure Analysis Charts give the Constant Pressure Analysis Charts give the

following information:following information:– All constant pressure charts contain analyses All constant pressure charts contain analyses

of height and temperature variations. of height and temperature variations. – Contours are drawn as solid lines on constant Contours are drawn as solid lines on constant

pressure charts and are identified by a three-pressure charts and are identified by a three-digit code located on each contour. digit code located on each contour.

Ch 9 - ThunderstormsCh 9 - Thunderstorms• Isotherms are lines of constant temperature. Isotherms are lines of constant temperature. • Isotachs are lines of constant wind speed. Isotachs are lines of constant wind speed. • Constant pressure charts are used to provide an Constant pressure charts are used to provide an

overview of selected observed en route flying overview of selected observed en route flying conditions. conditions.

• See power point presentation on Constant See power point presentation on Constant Pressure Analysis Charts for additional Pressure Analysis Charts for additional informationinformation

Ch 9 - ThunderstormsCh 9 - Thunderstorms• Lightning detection equipmentLightning detection equipment – very popular – very popular

alternative to airborne radar for light general alternative to airborne radar for light general aviation aircraft. aviation aircraft. – The equipment is designed to help the pilot The equipment is designed to help the pilot

completely avoid storm cells. completely avoid storm cells. – However, this equipment does not directly However, this equipment does not directly

indicate any areas of heavy precipitation, hail indicate any areas of heavy precipitation, hail or wind shear.or wind shear.

Ch 9 - ThunderstormsCh 9 - Thunderstorms• ThunderstormThunderstorm – based on surface observations, a – based on surface observations, a

thunderstorm is defined as a local storm produced thunderstorm is defined as a local storm produced by a cumulonimbus cloud and always accompanied by a cumulonimbus cloud and always accompanied by lightning and thunder.by lightning and thunder.

• Airmass thunderstormAirmass thunderstorm – an ordinary – an ordinary thunderstormthunderstorm

• Severe thunderstormSevere thunderstorm – a severe thunderstorm – a severe thunderstorm has a greater intensity than an airmass has a greater intensity than an airmass thunderstorm as defined by the severity of the thunderstorm as defined by the severity of the weather it produces: wind gusts of 50 knots or weather it produces: wind gusts of 50 knots or more and/or hail three-quarters of an inch or more more and/or hail three-quarters of an inch or more in diameter and/or strong tornadoes.in diameter and/or strong tornadoes.

Ch 9 - ThunderstormsCh 9 - Thunderstorms• Single-cell airmass thunderstormSingle-cell airmass thunderstorm – lasts less – lasts less

than one hourthan one hour• Supercell severe thunderstormSupercell severe thunderstorm – may last two – may last two

hours or longerhours or longer• Multicell thunderstormMulticell thunderstorm – a multicell storm is a – a multicell storm is a

compact cluster of thunderstormscompact cluster of thunderstorms

Ch 9 - ThunderstormsCh 9 - Thunderstorms• Cumulus stageCumulus stage – when atmospheric moisture – when atmospheric moisture

and instability are sufficient the evolution of the and instability are sufficient the evolution of the airmass thunderstorm begins. airmass thunderstorm begins. – In the cumulus stage an important change In the cumulus stage an important change

occurs in the nature of convection. occurs in the nature of convection. – There is a marked increase in the scale of the There is a marked increase in the scale of the

circulation. circulation. – The size of the updraft region becomes larger The size of the updraft region becomes larger

than the size of any of the individual thermals than the size of any of the individual thermals that are feeding the region. that are feeding the region.

Ch 9 - ThunderstormsCh 9 - Thunderstorms• Towering cumulus (TCU)Towering cumulus (TCU) – during the cumulus – during the cumulus

stage, the convective circulation grows rapidly stage, the convective circulation grows rapidly into a towering cumulus (TCU) cloud which into a towering cumulus (TCU) cloud which typically grows to 20,000 feet in height and three typically grows to 20,000 feet in height and three to five miles in diameter. to five miles in diameter. – The cloud reaches the next stage of The cloud reaches the next stage of

development in about 15 minutes.development in about 15 minutes.

Ch 9 - ThunderstormsCh 9 - Thunderstorms• ****A CONTINUOUS UPDRAFT IS NORMALLY ****A CONTINUOUS UPDRAFT IS NORMALLY

ASSOCIATED WITH THE CUMULUS STAGE OF A ASSOCIATED WITH THE CUMULUS STAGE OF A THUNDERSTORM****THUNDERSTORM****

• ****AN INDICATION THAT DOWNDRAFTS HAVE ****AN INDICATION THAT DOWNDRAFTS HAVE DEVELOPED AND THAT THE THUNDERSTORM CELL DEVELOPED AND THAT THE THUNDERSTORM CELL HAS ENTERED THE MATURE STAGE IS WHEN HAS ENTERED THE MATURE STAGE IS WHEN PRECIPITATION BEGINS TO FALL FROM THE CLOUD PRECIPITATION BEGINS TO FALL FROM THE CLOUD BASE****BASE****

• ****THUNDERSTORMS REACH THEIR GREATEST ****THUNDERSTORMS REACH THEIR GREATEST INTENSITY DURING THE MATURE STAGE****INTENSITY DURING THE MATURE STAGE****

• ****IN THE LIFE CYCLE OF A THUNDERSTORM, THE ****IN THE LIFE CYCLE OF A THUNDERSTORM, THE DISSIPATING STAGE IS DOMINATED BY DISSIPATING STAGE IS DOMINATED BY DOWNDRAFTS****DOWNDRAFTS****

Ch 9 - ThunderstormsCh 9 - Thunderstorms• Shelf CloudShelf Cloud – a shelf cloud often indicates the – a shelf cloud often indicates the

rising air over the gust frontrising air over the gust front• Outflow boundaryOutflow boundary – an outflow boundary is the – an outflow boundary is the

remnant of a gust front that continues to exist remnant of a gust front that continues to exist long after the thunderstorm that created it have long after the thunderstorm that created it have dissipated.dissipated.

• Supercell Supercell – this thunderstorm type almost – this thunderstorm type almost always produces one or more of the extremes of always produces one or more of the extremes of convective weather: very strong horizontal wind convective weather: very strong horizontal wind gusts and/or large hail and/or strong tornadoes. gusts and/or large hail and/or strong tornadoes. This is due to thunderstorm structureThis is due to thunderstorm structure

Ch 9 - ThunderstormsCh 9 - Thunderstorms• Wall cloudWall cloud – a portion of the rain-free cloud base – a portion of the rain-free cloud base

may appear lower in what is called a wall cloudmay appear lower in what is called a wall cloud• MammatusMammatus – the bulges that appear under the – the bulges that appear under the

anvil of a thunderstorm anvil of a thunderstorm • TornadoTornado – a violently rotating column of air – a violently rotating column of air

which is found below cumulonimbus cloudswhich is found below cumulonimbus clouds• Funnel cloudFunnel cloud – a tornado which does not reach – a tornado which does not reach

the surfacethe surface

Ch 9 - ThunderstormsCh 9 - Thunderstorms• Water spoutWater spout – a tornado that occurs over water – a tornado that occurs over water• GustnadoesGustnadoes – near gust fronts and the edges of – near gust fronts and the edges of

downbursts, tornado-like vortices known as gustnadoes downbursts, tornado-like vortices known as gustnadoes sometimes occursometimes occur

• Cold air funnelCold air funnel – a cold air funnel is a weak vortex that – a cold air funnel is a weak vortex that occasionally develops after a cold front passage in occasionally develops after a cold front passage in association with rain shower and / or thunderstorm association with rain shower and / or thunderstorm activity.activity.

• HailHail – another product of strong upward motions; as snow – another product of strong upward motions; as snow collides with water droplets the droplets freeze in the collides with water droplets the droplets freeze in the process known as accretion. In a thunderstorm, accretion process known as accretion. In a thunderstorm, accretion may produce a larger particle that becomes the nucleus of may produce a larger particle that becomes the nucleus of a hailstone.a hailstone.

• LightningLightning – the visible electric discharge produced by a – the visible electric discharge produced by a thunderstormthunderstorm

Ch 9 - ThunderstormsCh 9 - Thunderstorms• ****LIGHTNING IS ALWAYS PRESENT IN (AND ****LIGHTNING IS ALWAYS PRESENT IN (AND

NEAR) A THUNDERSTORMNEAR) A THUNDERSTORM

Ch 9 - ThunderstormsCh 9 - Thunderstorms• Stepped leaderStepped leader – the lightning stroke is actually – the lightning stroke is actually

a series of events which begins with a nearly a series of events which begins with a nearly invisible stepped leader that carries electrons from invisible stepped leader that carries electrons from the base of the cloud to the ground, creating an the base of the cloud to the ground, creating an ionized channel for the subsequent dischargeionized channel for the subsequent discharge

• Return strokeReturn stroke – a bright return stroke occurs, – a bright return stroke occurs, marking the route of the positive charge along the marking the route of the positive charge along the original path of the stepped leader, back up into original path of the stepped leader, back up into the cloudthe cloud

• Dart leadersDart leaders – the initial discharge is often – the initial discharge is often followed by several so-called dart leaders and followed by several so-called dart leaders and more return strokesmore return strokes

Ch 9 - ThunderstormsCh 9 - Thunderstorms• Initial liftInitial lift – when the surface layer is lifted a – when the surface layer is lifted a

sufficient distance, strong convection occurs. sufficient distance, strong convection occurs. – Initial lifting is the minimum amount of vertical Initial lifting is the minimum amount of vertical

displacement necessary to release the displacement necessary to release the instability. instability.

Ch 9 - ThunderstormsCh 9 - Thunderstorms• ****THE CONDITIONS NECESSARY FOR THE ****THE CONDITIONS NECESSARY FOR THE

FORMATION OF CUMULONIMBUS CLOUDS ARE FORMATION OF CUMULONIMBUS CLOUDS ARE MOIST, UNSTABLE AIR AND A LIFTING ACTION****MOIST, UNSTABLE AIR AND A LIFTING ACTION****

Ch 9 - ThunderstormsCh 9 - Thunderstorms• Lifted indexLifted index – a practical approach to the evaluation – a practical approach to the evaluation

of the potential instability requirement for of the potential instability requirement for thunderstorms is to use a stability index. thunderstorms is to use a stability index. – The lifted index is the difference between the The lifted index is the difference between the

observed 500 mb (~18,000 feet) temperature and observed 500 mb (~18,000 feet) temperature and the temperature that a parcel of air would have if the temperature that a parcel of air would have if lifted from near the earth’s surface to the 500 mb lifted from near the earth’s surface to the 500 mb level.level.•Lifted index (0 – 2) = weak chance of severe Lifted index (0 – 2) = weak chance of severe

thunderstormthunderstorm•Lifted index (-3 – -5) = moderate chance of Lifted index (-3 – -5) = moderate chance of

severe thunderstormsevere thunderstorm•Lifted index (< = -6) = strong chance of severe Lifted index (< = -6) = strong chance of severe

thunderstormthunderstorm

Ch 9 - ThunderstormsCh 9 - Thunderstorms• ****THE LIFTED INDEX IS THE TEMPERATURE ****THE LIFTED INDEX IS THE TEMPERATURE

DIFFERENCE FOUND BY SUBTRACTING THE DIFFERENCE FOUND BY SUBTRACTING THE TEMPERATURE OF A PARCEL OF AIR TEMPERATURE OF A PARCEL OF AIR THEORETICALLY LIFTED FROM THE SURFACE TO THEORETICALLY LIFTED FROM THE SURFACE TO 500 MILLIBARS FROM THE EXISTING 500 MILLIBARS FROM THE EXISTING TEMPERATURE AT 500 MILLIBARS****TEMPERATURE AT 500 MILLIBARS****

Ch 9 - ThunderstormsCh 9 - Thunderstorms• Squall lineSquall line – a squall line or instability line is a – a squall line or instability line is a

broken or continuous line of thunderstorms not broken or continuous line of thunderstorms not necessarily associated with a front.necessarily associated with a front.

Ch 9 - ThunderstormsCh 9 - Thunderstorms• ****EMBEDDED THUNDERSTORMS ARE ****EMBEDDED THUNDERSTORMS ARE

THUNDERSTORMS THAT ARE OBSCURRED BY THUNDERSTORMS THAT ARE OBSCURRED BY MASSIVE CLOUD LAYERS AND CANNOT BE MASSIVE CLOUD LAYERS AND CANNOT BE SEEN****SEEN****

• ****THE MOST SEVERE WEATHER CONDITIONS, ****THE MOST SEVERE WEATHER CONDITIONS, SUCH AS DESTRUCTIVE WINDS, HEAVY HAIL, AND SUCH AS DESTRUCTIVE WINDS, HEAVY HAIL, AND TORNADOES ARE GENERALLY ASSOCIATED WITH TORNADOES ARE GENERALLY ASSOCIATED WITH SQUALL LINES****SQUALL LINES****

Ch 9 - ThunderstormsCh 9 - Thunderstorms• ****SQUALL LINES ARE NOT NECESSARILY ****SQUALL LINES ARE NOT NECESSARILY

ASSOCIATED WITH FRONTS AND MAY CONTAIN ASSOCIATED WITH FRONTS AND MAY CONTAIN EITHER OR BOTH AIRMASS AND SEVERE EITHER OR BOTH AIRMASS AND SEVERE THUNDERSTORMS****THUNDERSTORMS****

• ****SQUALL LINES MOST OFTEN DEVELOP AHEAD ****SQUALL LINES MOST OFTEN DEVELOP AHEAD OF A COLD FRONT****OF A COLD FRONT****

Ch 9 - ThunderstormsCh 9 - Thunderstorms• Dry lineDry line – the moisture boundary which is called – the moisture boundary which is called

a dry line is apparent in the distribution of surface a dry line is apparent in the distribution of surface dew point temperaturesdew point temperatures

• Mesoscale convective complexesMesoscale convective complexes – These are – These are nearly circular clusters of thunderstorms that nearly circular clusters of thunderstorms that develop primarily between the Rockies and the develop primarily between the Rockies and the Appalachians during the warmer part of the year.Appalachians during the warmer part of the year.

SummarySummary• The thunderstorm, by itself, is a distinct The thunderstorm, by itself, is a distinct

mesoscale atmospheric circulation that begins mesoscale atmospheric circulation that begins as cloudless convection in the boundary layer as cloudless convection in the boundary layer and develops through a great depth of the and develops through a great depth of the atmosphere in a very short period of time.atmosphere in a very short period of time.

• Thunderstorms have a range of structures.Thunderstorms have a range of structures.• Some of these support the generation of new Some of these support the generation of new

thunderstorms as well as the development of thunderstorms as well as the development of very intense and long-lived severe very intense and long-lived severe thunderstorms (Lester, 2006).thunderstorms (Lester, 2006).

SummarySummary• It has become clear that in order to really It has become clear that in order to really

understand thunderstorms, it is necessary understand thunderstorms, it is necessary to know about a variety of smaller and to know about a variety of smaller and larger scale circulations that influence larger scale circulations that influence thunderstorm development and behavior.thunderstorm development and behavior.

• These include extratropical cyclones, fronts, These include extratropical cyclones, fronts, squall lines, and individual thermals, as well squall lines, and individual thermals, as well as downdrafts, gust fronts, tornadoes, and as downdrafts, gust fronts, tornadoes, and suction vortices (Lester, 2006).suction vortices (Lester, 2006).

SummarySummary• Considering that thousands of Considering that thousands of

thunderstorms occur over the earth’s thunderstorms occur over the earth’s surface every day, and that a single surface every day, and that a single thunderstorm may produce lightning, thunderstorm may produce lightning, very heavy rainshowers, hail, strong very heavy rainshowers, hail, strong winds, low visibilities, wind shear, winds, low visibilities, wind shear, turbulence, and icing, it is not surprising turbulence, and icing, it is not surprising that thunderstorms have a substatial that thunderstorms have a substatial impact on aircraft operations (Lester, impact on aircraft operations (Lester, 2006).2006).

SummarySummary• Considering that thousands of Considering that thousands of

thunderstorms occur over the earth’s thunderstorms occur over the earth’s surface every day, and that a single surface every day, and that a single thunderstorm may produce lightning, thunderstorm may produce lightning, very heavy rain showers, hail, strong very heavy rain showers, hail, strong winds, low visibilities, wind shear, winds, low visibilities, wind shear, turbulence, and icing, it is not surprising turbulence, and icing, it is not surprising that thunderstorms have a substantial that thunderstorms have a substantial impact on aircraft operations (Lester, impact on aircraft operations (Lester, 2006).2006).

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Ch 9 - Thunderstorms