CHAPTER 10

THUNDERSTORM AND TORNADOES

Thunderstorms

This chapter begins an examination of severe weather and begins with a clarification definition

- Though the term cyclone has become our generic for violent weather, and the atmosphere’s most severe weather patterns (tornadoes; hurricanes) are associated with cyclonic flows, by definition a cyclone is any air flow around a low pressure center

Thunderstorms, cont

- The most violent mid-latitude cyclones are also the smallest

- The vast majority of cyclones are not associated with violent weather

- Thunderstorms may form in conjunction with cyclonic air flow, but are not strictly cyclonic[instability and adiabatic lifting]

- Best known severe weather system

Thunderstorms, cont

- Product of warm, moist air (sufficient moisture) undergoing lifting (“forced lifting” and an unstable lapse rate) to saturation and condensation

- Significant precipitation supplier to SE U.S. (thunderstorm days – Fig. 10.2)

- Characterized by: cumulonimbus cloud formation; torrential rainfall and/or hail; thunder; lightning

Thunderstorms, cont

- Individual thunderstorms measure from less than 5 mi. in diameter to more than 30 mi.

- Cloud bases range from a few 100’ (extremely moist regions) to more than 10,000’ (arid regions)

- Cloud tops generally range from 25,000’ to 45,000’, but may occasionally top 65,000’

Thunderstorms, cont

- Updrafts vary in strength and extend from very near the ground to the cloud tops

- Growth rate of a cumulonimbus storm cloud may exceed 3000’/min

Thunderstorms, cont

At any point-in-time:- est. 2000 are in progress

… greatest concentration tropical latitudes … generally unknown above 60o latitude

[temperature; moisture; instability]

- est. 45,000 take place daily… worldwide more than 16 million

annually / 100,000 in U.S.

Air-Mass Thunderstorms

- In the U.S. frequently associated with mT air

- Inherently very warm and moist; easily made unstable by forced lifting by forced convectional and frontal lifting

… therefore peak periods are Spring and Summer; late afternoons… most frequent July / August… least frequent December / January

Air-Mass Thunderstorms, cont

Stages of Development: Cumulus- Thunderstorms begin with the

formation of cumulus clouds[instability and release of latent heat]

- Generally about 15 min. in duration- Ascending air creates violent

updrafts fueling the 3000’/min vertical cloud growth

Air-Mass Thunderstorms, cont

Stages of Development: Cumulus, contEarly cumulus stage, cloud droplets are small,

grow to raindrop size as stage develops Updraft air carries supercooled droplets above

the freezing level. As droplets grow still heavier, they fall.

Descending droplets drag air downward, friction evaporates some droplets cooling portions of cloud and creating downdrafts[assisted by entrainment surrounding cold dry, air]

… thunderstorm enters mature stage

Air-Mass Thunderstorms, cont

Stages of Development: MatureDowndrafts and falling precipitationNormal compressional heating retarded by

cold rain and downdraft air remains colder than cloud… downdraft may exceed 2,500’/min… produces a plow wind or first gust

[gusty, out-rushing surface air, bringing sharp temperature drop and pressure increase]

Air-Mass Thunderstorms, cont

Stages of Development: MatureAt the same time updrafts intensify (est. to

6,000’/min)Cumulonimbus cloud form with distinctive

anvil is formedProximity of updrafts and downdrafts create

strong vertical shear and turbulenceThunderstorm hazards reach their maximum

now (rainfall, lightning, thunder, hail, gust)

Air-Mass Thunderstorms, cont

LightningBrief, violent super-heating of the

atmosphere by electrical discharge

It is unclear how lightning actually comes to be or not to be, but it is something like this:

Air-Mass Thunderstorms, cont

Lightning, cont(1) Under some atmospheric

conditions, cloud moisture begins a separation of electrical charge…though nature attempts to “balance” charge, air is not a good conductor of electricity and strong updrafts may overcome this attempted “balancing”

Air-Mass Thunderstorms, cont

Lightning, cont(2) Lighter materials (ice crystals) that

are “+” charged are lifted to the top of the cloud

(3) Heavier ice crystals /slush / super cooled water are “-” charged and collect in the lower region of the cloud--- a pilot leader and step leaders

form

Air-Mass Thunderstorms, cont

Lightning, cont(4) The ground is “+” charged with respect

to the cloud and “+” charge begins to flow from the ground toward the cloud--- streamers… this may be “felt” by animals and people

(5) If flow-counter-flow are strong enough step leaders and streamers meet and open a channel through which they discharge a return stroke

Air-Mass Thunderstorms, cont

Or it may be that Wakinyan Tanka, the Thunder Being, is warring the water monsters, the Unktehila, again.

- So say the Lakota Sioux

Air-Mass Thunderstorms, cont

Dissipation- With a shift to predominately

downdraft air the thunderstorm has moved to the final stage – dissipation--- end of precipitation, etc--- cold air influx--- cloud begins to break up

* Air-mass thunderstorms are self-destructive *

Severe Thunderstorms

- For NWS classification of severe: (1) must have winds in excess of 58 mph (50 knots)

or(2) hailstones with diameters exceeding 0.75 in (1.9 cm)

- Capable of producing heavy downpours and flash flooding

- Strong gusty straight-line winds, large hail, frequent lightning; occasional tornado

- The more severe, the more lightning

Severe Thunderstorms, cont

- Differ from air-mass thunderstorm in both duration and structure--- hours vs minutes in duration--- length of thunderstorm duration gives them the name steady-state thunderstorms--- associated with weather systems--- fronts, converging winds, and low pressure troughs aloft force upward motion spawning severe thunderstorms

Severe Thunderstorms, cont

- Presence of strong vertical shear (differing wind speeds and direction) that “tilt” updrafts so that falling precipitation falls into downdrafts rather than updrafts allowing updrafts to remain strong

- Sometimes updrafts are sufficient that the cloud top is pushed into the stable (no lapse rate) Stratosphere… termed overshooting (Fig 10-6)

- Edge of the well-developed downdraft acts to wedge moist, unstable air maintaining updraft… mature stage maintained

Severe Thunderstorms, cont

- This downdraft acts as a “mini-cold front” – a gust front - advancing into the surrounding warm air--- frequently a roll cloud is formed by gust front lifting of warm air (Fig 10-7)

- The advance of the gust front may provide necessary lifting for spawning new thunderstorms further away

Supercell Thunderstorms

- Particularly violent thunderstorm type- Est. 2000-3000 annually in the U.S.- A small proportion of the 100,000 that

occur annually; source of a disproportionate amount of thunderstorm death and damage

- Less than ½ of these storms produce tornadoes… virtually all strongest and most violent (F4+) are supercell spawned

Supercell Thunderstorms, cont

- A supercell is a single, very powerful cell reaching as high as 65,000’ (20 km)

- Diameter between 12-30 mi (20-50 km)

- May persist for several hours - Frequently characterized by mesocyclonic

rotation- Requirement of huge amounts of unstable

air appears to be aided by inversion

Thunderstorm Hazard

Thunderstorms pack about every weather hazard known (esp. if you are airborne) into a vicious bundle

1. Lightning and static electricity2. Low ceiling and visibility3. Hail and icing4. Turbulence5. Squall Lines6. Tornadoes

Tornadoes

The tornado is the most violent weather form that we experience.

It’s seeming blind, random destruction makes it a dark, boding, demonic, uncontrollable force; One that defies our planning, our understanding and our logic

It stokes our greatest fears and fascinations

Tornadoes, cont

Interesting: - During the 20th Century, Montgomery

County experienced 25 tornadoes… a tornado on average every 4 years… 6 touched down in Clarksville

- Clarksville’s January 22,1999 tornado was one of 55 that day… a National Weather Service one day record

Tornadoes

The most violent thunderstorms have patterns of strong air intake into their base

Where this air initially has rotation, it can form an concentrated spinning vortex

Pressure in this vortex (a “straw” of condensation)will be low (may be significant) and draw in dust and debris

This low pressure generates a funnel shaped cloud downward from cloud base… funnel cloud; water spout; tornado

Tornadoes, cont

May occur in any class thunderstorm… most common in severe storms associated with cold fronts or squall lines

A particular cloud, cumulonimbus mamma, has been associated with violent thunderstorms and tornadoes

Tornadoes, cont

- From the Spanish tornar – “to turn”- Intense centers of low pressure- Intensity of low pressure creates a rotating

vortex around its center… downward extending from a cumulonimbus cloud drawing air from ground level

- Low creates an intense pressure gradient where winds may exceed 300 mph

Tornadoes, cont

Development and Occurrence- Often produced in association with

mid-latitude cyclones, their cold fronts and squall lines… typically conflict zone cP and mT air

- <1% of thunderstorms produce tornadoes

- ¾ of world annual total occur in North America

Tornadoes, cont

- May occur in any state or region- About 800 are reported annually and

may occur in any month… season and time of preference are

Spring in the late afternoon[made 1/22/99, 4:15 am, atypical]

Tornadoes, cont

“Small, Brief, Violent, Erratic”- Millions of tales of destruction- Tornado destruction linked to:

(1) very high winds (horizontal pressure gradient)

(2) strong updrafts (vertical pressure gradient)

(3) subsidiary vortices(4) ancillary projectiles

Stages in Tornado “Life”

During its life cycle a tornado will pass through a number of stages:

(1) Funnel Cloud – rotating funnel of air forming in the cumulonimbus cloud and “descending” to the ground

(2) Tornado – column touches ground; collects debris cloud at ground level

Stages in Tornado “Life”, cont

(3) Mature Tornado – tornado at widest extent; period of most extensive cloud base-to-ground contact (but will hop-and-skip; period of greatest damage

(4) Shrinking Tornado – narrows; increasingly loses vertical structure, becomes more “serpentine”; damage is reduced

(5) Decaying Tornado – continues No. 4, very elongated (some call rope stage) from friction and cloud movement; “breaks” from cloud and ground and is finished

Tornadoes, cont

No direct way to measure wind velocity- Measure indirectly by storm damage

with F-ScaleF0 - light damageF1 – moderate damageF2 – considerable damageF3 – severe damageF4 – devastating damageF5 – incredible damage

Tornadoes, cont

- 63% of all tornadoes are classed as “weak” (F0, F1)

- 35% are “strong” (F2, F3)- 2% are “violent” (F4, F5)- The 2% that are “violent” account for

about 70% of the fatalities by tornado - Because tornadoes are short-lived and

erratic in behavior they are extremely difficult to forecast with any accuracytornado watch tornado warning