Thunderstorms and LightningThunderstorms and Lightning

Review of last lectureReview of last lecture• Bergeron process: happens with coexistence of ice and super-Bergeron process: happens with coexistence of ice and super-

cooled water. cooled water. Key: Key: Saturation vapor pressure of ice < that of Saturation vapor pressure of ice < that of super-cooled water at the same temperature.super-cooled water at the same temperature.

• Further growth of ice crystals (riming and Further growth of ice crystals (riming and aggregation)Extensive riming in strong updrafts (graupel, hail)aggregation)Extensive riming in strong updrafts (graupel, hail)

• Definition of airmasses. Bergeron classification of air masses Definition of airmasses. Bergeron classification of air masses (3 letters).(3 letters).

• Fronts: 6 types (cold, warm, stationary, occluded, dry line, Fronts: 6 types (cold, warm, stationary, occluded, dry line, squall line) squall line)

• Cold front (narrow, fast, heavy precipitation), Warm front (wide, Cold front (narrow, fast, heavy precipitation), Warm front (wide, slow, light precipitation)slow, light precipitation)

• The developmental stages and vertical structure of middle The developmental stages and vertical structure of middle latitude cyclones (latitude cyclones (boundary between northern cold air and boundary between northern cold air and southern warm air,southern warm air, upper level low to the west of surface lowupper level low to the west of surface low))

• The three regions of cyclogenesis and typical tracksThe three regions of cyclogenesis and typical tracks

The most common atmospheric The most common atmospheric circulation structurecirculation structure

L

H

H

L

HeatingCoolingor No Heating

Imbalance of heating Imbalance of temperature Imbalance of pressure Wind

Radiation Convection

Latent/Sensible

Conduction

RadiationCloud/precipitation

Shallow convection Boundary layer turbulence

Mesoscale convective system ThunderstormTornado

Heat waveMidlatitude cycloneTropical cyclone

Diurnal variation

Madden-Julian Oscillation Tropical wavesAnnular modes

100,000yr100yr10yr1yr1mon1day1min1sec10-15sec

Global Climate System

Globe

Continent

State

City

Football field

1 mm

1 m

Spatial Scale

Time Scale

10-4 m Composition

Monsoon

El NinoBiennial Oscillation

Global warmingMulti-decadal Oscillation

Ice ageGlacial cycle

Convective systemsConvective systems

TornadoesTornadoes: about : about 100-600 m100-600 m, last , last 1 minute to 1 hour1 minute to 1 hour

ThunderstormsThunderstorms: about : about 10 Km10 Km, last , last 10 minutes to a 10 minutes to a couple of hours. couple of hours. 3 types: ordinary, multicell, supercell3 types: ordinary, multicell, supercell

Mesoscale convective systems (MCSs)Mesoscale convective systems (MCSs): A cloud : A cloud system that occurs in connection with an ensemble system that occurs in connection with an ensemble of thunderstorms and produces a contiguous of thunderstorms and produces a contiguous precipitation area on the order of precipitation area on the order of 100 Km100 Km or more in or more in at least one directionat least one direction, and often last for , and often last for several hours several hours to a couple of daysto a couple of days..

ThunderstormsThunderstorms

A storm containing lightning & thunderA storm containing lightning & thunder ConvectiveConvective; form when warm, humid air ; form when warm, humid air

rises in conditionally unstable environmentrises in conditionally unstable environment The warmer the rising air parcel is relative The warmer the rising air parcel is relative

to environment, the more to environment, the more buoyant forcebuoyant force is is driving it upwards (stronger convection)driving it upwards (stronger convection)

TriggerTrigger to start uplift: warming sfc, terrain to start uplift: warming sfc, terrain (orography), converging sfc winds, frontal (orography), converging sfc winds, frontal zones, divergence aloft (or combination)zones, divergence aloft (or combination)

Three stages have been identified in ordinary thunderstorms:Three stages have been identified in ordinary thunderstorms:a)a) an unstable atmosphere and vertical updrafts keep precipitation suspendedan unstable atmosphere and vertical updrafts keep precipitation suspendedb)b) MATUREMATURE:: entrainment of dry air that causes cooler air from evaporation, entrainment of dry air that causes cooler air from evaporation,

triggering triggering downdraftsdowndrafts and falling precipitation and and falling precipitation and gust frontsgust frontsc)c) DISSIPATINGDISSIPATING:: weakening updrafts and loss of the fuel source after 15 to 30 weakening updrafts and loss of the fuel source after 15 to 30

minutes.minutes.

Thunderstorms I. Ordinary StormsThunderstorms I. Ordinary Storms

Cool downdrafts leaving a Cool downdrafts leaving a mature and dissipating storm mature and dissipating storm may offer relief from summer may offer relief from summer heat, but they may also force heat, but they may also force surrounding, low-level moist air surrounding, low-level moist air upward.upward.

Hence, dying storms often Hence, dying storms often trigger new storms, and the trigger new storms, and the successive stages may be successive stages may be viewed in the sky. viewed in the sky.

Thunderstorms II. Multicell StormThunderstorms II. Multicell Storm

A Multicell ThunderstormA Multicell Thunderstorm

Storms producing a minimum of Storms producing a minimum of a) 3/4 inch hail and/or a) 3/4 inch hail and/or b) wind gusts of 50 knots and/or b) wind gusts of 50 knots and/or c) tornado winds, classify as severe.c) tornado winds, classify as severe.

Thunderstorm III. Supercell StormThunderstorm III. Supercell Storm

Formation of supercell thunderstormsFormation of supercell thunderstorms

1. 1. Before thunderstorms develop, a Before thunderstorms develop, a change in wind direction and an increase change in wind direction and an increase in wind speed with increasing height in wind speed with increasing height creates an invisible, horizontal spinning creates an invisible, horizontal spinning effect in the lower atmosphere. effect in the lower atmosphere.

2. Spinning horizontal vortex tubes created 2. Spinning horizontal vortex tubes created by surface by surface wind shearwind shear may be may be tiltedtilted and and forced in a vertical path by updrafts. This forced in a vertical path by updrafts. This rising, spinning, and often stretching rising, spinning, and often stretching rotating air may then turn into a rotating air may then turn into a mesocyclonemesocyclone..

3. Most strong and violent tornadoes form 3. Most strong and violent tornadoes form within this area of strong rotation.within this area of strong rotation.

Vertical structure of a supercell thunderstormVertical structure of a supercell thunderstorm

In ordinary storms, the In ordinary storms, the downdraftdowndraft and falling precipitation cut and falling precipitation cut off the off the updraftupdraft. But in supercell storms, winds aloft push the . But in supercell storms, winds aloft push the rain away and the updraft is not weakened and the storm rain away and the updraft is not weakened and the storm can continue maturing and maintain its structure for hours.can continue maturing and maintain its structure for hours.

Cloud structure of a supercell thunderstormCloud structure of a supercell thunderstorm

Video: Video: All Alone in the Night - All Alone in the Night - Time-lapse footage of the Earth Time-lapse footage of the Earth

as seen from the ISSas seen from the ISS

http://www.youtube.com/watch?http://www.youtube.com/watch?v=FG0fTKAqZ5g v=FG0fTKAqZ5g

Lightning: IntroductionLightning: Introduction• Lightning is a discharge of

electricity, a giant spark• 80% of lightning occurs when

clouds discharge electricity. This is referred to as cloud-to-cloud lightning; occurring when voltage gradient overcomes the electrical resistance of the air.

• 20% of lightning occurs when electrical discharge travels between the base of the cloud and the surface. This is referred to as cloud-to-ground lightning.

Video: Lightning in super slow Video: Lightning in super slow motionmotion

http://www.youtube.com/watch?http://www.youtube.com/watch?v=RLWIBrweSU8 v=RLWIBrweSU8

1. Charge separation1. Charge separation. Charge layers in the . Charge layers in the cloud are formed by the transfer of positive cloud are formed by the transfer of positive ions from warmer graupel to colder ice crystal ions from warmer graupel to colder ice crystal when they collide with each other.when they collide with each other.

2. 2. Stepped leader. Stepped leader. When the negative charge When the negative charge near the bottom of the cloud is large enough to near the bottom of the cloud is large enough to overcome the air's resistance, a stepped overcome the air's resistance, a stepped leader forms.leader forms.

3. 3. Return stroke.Return stroke. A region of positive ions A region of positive ions move from the ground toward this charge, move from the ground toward this charge, which then forms a which then forms a return stroke return stroke into the cloud.into the cloud.

4. 4. Dart leaderDart leader. Not all of the first stroke . Not all of the first stroke neutralizes the negatively charged ions and neutralizes the negatively charged ions and results in another leader in 1/10 of a secondresults in another leader in 1/10 of a second

Processes of Lightning Formation

ThunderThunder• Charge differences between the thunderstorm and ground can Charge differences between the thunderstorm and ground can

cause lightning strokes of 30,000°C, and this rapid heating of air cause lightning strokes of 30,000°C, and this rapid heating of air will creates an explosive shock wave called will creates an explosive shock wave called thunderthunder..

• It takes about 3 seconds for thunder to travel 1 kilometer (It takes about 3 seconds for thunder to travel 1 kilometer (5 sec 5 sec per mileper mile).). A lag in lightning strike and thunder occurs due to sound traveling slower than light.

• When thunder is farther away, the echoing of sound waves off of objects (like buildings and hills) causes thunder to sound rumbling.

Global distribution of lightning strikesGlobal distribution of lightning strikes

Significant difference between continent and ocean

Lightning SafetyLightning Safety

Practice the following when lightning is present:Practice the following when lightning is present:• Always take cover in a building. Always take cover in a building. • Do not stand under a tree or other tall object that might Do not stand under a tree or other tall object that might

serve as a lightning rod.serve as a lightning rod.• Avoid standing on mountain summits, ridges, rooftops, Avoid standing on mountain summits, ridges, rooftops,

or other high areas. or other high areas. • Avoid caves.Avoid caves.• Avoid open water (pool, lake, or hot tub).Avoid open water (pool, lake, or hot tub).

SummarySummary1.1. The general size and lifetime of mesoscale convective The general size and lifetime of mesoscale convective

systems, thunderstorms and tornadoes. 3 types of systems, thunderstorms and tornadoes. 3 types of thunderstorms. thunderstorms.

2.2. 3 stages of the ordinary thunderstorms. 3 stages of the ordinary thunderstorms. Downdraft and falling precipitation cut off the updraft.

3.3. Formation of multi-cell thunderstorms. Downdrafts initiate Formation of multi-cell thunderstorms. Downdrafts initiate new thunderstorms in nearby regions.new thunderstorms in nearby regions.

4.4. 3 stages of the supercell thunderstorms. 3 stages of the supercell thunderstorms. Winds aloft push downdraft/precipitation away and the updraft is not weakened.

5.5. Two types of lightning (cloud-to-cloud 80%, cloud-to-Two types of lightning (cloud-to-cloud 80%, cloud-to-ground 20%)ground 20%)

6.6. 4 steps of lightning development. How fast does thunder 4 steps of lightning development. How fast does thunder travel? travel?

Works citedWorks cited

http://www.nps.gov/romo/index.htm http://www.nps.gov/romo/index.htm http://www.rockymountainhikingtrails.com/lightning-http://www.rockymountainhikingtrails.com/lightning-

hiking-rocky-mountains.htm hiking-rocky-mountains.htm http://en.wikibooks.org/wiki/Engineering_Acoustics/http://en.wikibooks.org/wiki/Engineering_Acoustics/

Thunder_acoustics Thunder_acoustics