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, cont
Early 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: Mature
Downdrafts and falling precipitation
Normal 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: Mature
At the same time updrafts intensify (est. to
6,000’/min)
Cumulonimbus cloud form with distinctive
anvil is formed
Proximity of updrafts and downdrafts create
strong vertical shear and turbulence
Thunderstorm hazards reach their maximum
now (rainfall, lightning, thunder, hail, gust)
Air-Mass Thunderstorms, cont
Lightning
Brief, 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 selfdestructive *
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 electricity
2. Low ceiling and visibility
3. Hail and icing
4. Turbulence
5. Squall Lines
6. 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 hopand-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-Scale
F0 - light damage
F1 – moderate damage
F2 – considerable damage
F3 – severe damage
F4 – devastating damage
F5 – 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 accuracy
tornado watch
tornado warning