THUNDERSTORMS, LIGHTNING AND TORNADOES
LIGHTNING
ELECTRICITY IN THE ATMOSPHERE
Ions are most abundant in the upper atmosphere which has a positive charge
-electrical charges in the atmosphere represent potential energy and have the ability to do work
-this potential is expressed as voltage which is the energy per unit charge.
-a batter rated at 1.5 volts means that 1.5 joules are available per coulomb of charge. A coulomb is equivalent to the charge carried by about 6 times 10 (19) electrons
-higher the voltage the greater the energy release for each coulomb transferred
-voltage difference between the surface and the ionosphere is about 400,000 volts
-this voltage gradient is called the fair weather electric field (really mean electric field)
-electricity does flow in fair weather but because air is a good insulator the current is very weak and represents a transfer of electrons from the surface or positive charge transferred from the atmosphere
-for the mean electric field to be maintained it must be continuously replenished and the lightning discharges in thunderstorms are thought to be the primary recharge mechanism (cloud to ground lightning discharges transfer electrons to the surface, maintaining the voltage difference and the resulting electric field
-in the lower atmosphere, the fair weather electric field gradient is on the order of 100 V per meter (but few ions are present so total energy is low)
-only if the voltage gradient reaches millions of volts per meter will the resistance of the atmosphere be broken down
Sequence of Events
-electrification of cloud
-development of path in air through which electrons can flow
CHARGE SEPARATION
-in order to get lightning you must have charge separation in the cloud
-generally you get negative charges in lower portion and positive charge in upper portion
How does charge separation occur?
-unsure
-some characteristics indicate they occur in clouds that extend above freezing level
-they also are clouds that are precipitating
thermoelectric effect
-positive ions migrate toward the colder regions of ice crystals
-the growth of hailstones by wetting and freezing releases latent heat which makes the outer portion of hailstones slightly warmer than nearby ice crystals. If a hailstone and an ice crystal momentarily collide, positive ions migrate from the hailstone to the colder ice crystal. The ice crystal becomes positively charged and the outer edge of the hailstone takes on a negative charge. Ice crystals are easily carried to the upper portion of the cloud and bring their positive charges with them
induction
--knowing that unlike charges attract and that the upper atmosphere has a positive charge we can surmise that negative charges are more likely found at the top of falling ice pellets. Even if their is no net charge for an object. when the larger ice pellets collide with smaller cloud droplet source crystals electrons are transferred to the ice pellets. The pellets become negatively charged while the droplets or ice crystals acquire a net positive charge. the negatively charged pellets fall to the base of the cloud and smaller droplets and crystals are carried to the top of the cloud
Most lightning occurs as a result of discharge of electricity within clouds (about 80%) as compared to cloud to ground lightning
Cloud to cloud lightning occurs when the voltage gradient within or between clouds overcomes the electrical resistance of the air
-result is a spark, which partially equalizes the charge separation
-actual stroke appears as a bright uniform light and is sometimes called sheet lightning if it is visible from the ground
Cloud to ground lightning
-occurs when negative charges accumulate in the lower portions of the cloud
-positive charges are attracted to a relatively small area on the ground beneath the cloud
-this establishes a large voltage difference between the ground and cloud base
-this positive charge is established because the negative charge at the base of the cloud repels electrons on the ground below
LEADERS STROKES AND FLASHES
In cloud to ground lightning the actual lightning event is preceded by the rapid and staggered advance of a shaft from base of cloud of negatively charged air, called a stepped leader
-leader is not a single column of ionized air; it branches off from a main trunk in several places
-only about 10 cm in diameter each section of the column first surges downward about 50 m from the base of the cloud in about a microsecond.
-this invisible leader pauses for about 50 microseconds then surges downward another 50 or so meters
-when leader approaches ground a spark surges upward from the ground toward the leader
-when the leader and spark connect; they create a pathway for the flow of electrons which initiates the first in a sequence of brightly illuminated return strokes
-this current flowing at about 20 kiloamps looks like it moves down but really moves upward
-there is a surge of positive charge upward toward the cloud
-current heats the air in the conducting channel to temps up to 30,000 K
-The first stroke neutralizes some but not all of the negatively charged ions near the base of the cloud
-another leader called the dart leader forms in about a tenth of a second and a subsequent stroke emerges from it. This may occur several times. The combination is called a flash and the net effect of which is to transfer electrons from cloud to ground
-total transfer of electrons is not large (about 100 watts for a minute
-electric current is brief but is about 100 times that of a household current
-voltage gradient is much larger so the energy release is much larger for each electron transferred
TYPES OF LIGHTNING
Ball lightning- appears as a round glowing mass of electrified air, up to the size of a basketball that just seems to roll through the air or along a surface for several seconds before dissipating
St. Elmos fire--ionization in the air, often just before the formation of cloud to ground lightning can cause tall objects such as church steeples or ships masts to glow as they emit a continuous barrage of sparks. This often produces a blue green tint to the air accompanied by a hissing sound
Sprites--very large but short lived electrical bursts that rise from cloud tops as lightning occurs below--looks like a giant red jellyfish extending up to 95 km above clouds with blue or green tentacles
Blue jets--ejections from tops of the most active regions of thunderstorms moving at about 100 km per second and heights of up to 50 km above surface
THUNDER
-caused by expansion of air around lightning stroke because of high temperature
-sound travels at .2 miles per second
AIR MASS THUNDERSTORMS
-most common
-least destructive
-generally last less than an hour
-contained within uniform air masses
1. Cumulus stage
-unstable air rises
-fair weather cumulus forms-
-clouds grow at 10 to 45 mph
-pressure lowers and draws in more air
2. Mature Stage
-precipitation starts to form
-drags air downward as downdraft
-most vigorous episode of thunderstorm
-anvil may be created
-updrafts dominate interior of cloud while downdrafts occur just outside it. Entrainment of unsaturated air causes the droplets along the cloud margin to shrink and cool the cloud by evaporation. Outer part of cloud becomes denser and less buoyant
3. Dissipative Stage
-downdrafts occupy more of cloud base
-water vapor is cut off
-precip diminishes and sky begins to clear
-only about 20 percent of moisture that condenses within an air mass thunderstorm actually falls as precip
SEVERE THUNDERSTORMS
-THOSE WITH WINDSPEEDS GREATER THAN 58 MPH, HAILSTONES LARGER THAN 3/4 INCH Diameter or spawn tornadoes
-updrafts and downdrafts are mutually reinforcing
-dependent upon conditions over an area from 10 to 1000 km across hence called mesoscale
-typically occur in groups and clustered together
-may be called Mesoscale convective systems
-may occur as linear bands or as oval or roughly circular clusters called MCC
-often have life spans of 12 hours to days and very common in parts of us and Canada
-stronger winds and precip than in air mass thunderstorms
Requirements for development of all severe thunderstorms
-wind shear
-high water vapor content in lower troposphere
-mechanism to trigger uplift
-potential instability
POTENTIAL INSTABILITY
-potential instability arises when a layer of dry air rests above one that is warm and humid
-if the air is potentially unstable, lifting of an entire layer of air can cause its temperature lapse rate to increase, thus making it statically unstable
MESOSCALE CONVECTIVE COMPLEXES
-GENERALLY OVAL ORGANIZED SYSTEMS CONTAINING SEVERAL Thunderstorms- can include squall lines
-not all MCC's create severe weather they are self propagating
SQUALL LINE THUNDERSTORMS
-consist of a large number of individual storm cells arranged in a linear band up to 500 km in length
-tend to form ahead cold fronts
-may last from 10 hrs to four days
-strong vertical wind shear important
-strong winds aloft push the updrafts ahead of the downdrafts and allow the rising air to feed additional moisture into the storm
SUPERCELL STORMS
-diameters from 20 to 50 m and are usually the most violent and may produce very large tornadoes
-consist of a single powerfull cell and undergo large-scale rotation
-last from 2 to 4 hours
-usually are isolated from other storms
-very complex with updraft and downdraft wrapping around each other
MICROBURSTS
-strong downdrafts may also create downbursts which can reach speeds in excess of 270 mph
-when reach the surface they can spread outward in all directions to form strong horizontal winds
-those with diameters less than 4 km are called microbursts can cause problems near airports
DISTRIBUTION OF THUNDERSTORMS
-usually occur where moist air is subject to sustained uplift
-most common in tropics
-in the united states they occur over central Florida (sea breeze) and mts of New Mexico
DOPPLER RADAR
-comes from Doppler effect (sound)
-in radar light shifts to shorter wavelengths when reflected by an object moving toward the receiver and to longer wavelengths as it bounces off an object moving away from the receiver (red shift/blue shift in astronomy)
-allows the user to observe the movement of raindrops and ice particles from the shift in wavelength
WHAT IS A TORNADO?
-a tornado is the product of a thunderstorm, specifically of the
interaction of a strong thunderstorm with winds in the
troposphere.
-the process by which a tornado is formed is one in which a small
fraction of the tremendous energy of the thunderstorm is
concentrated in an area usually no more than several hundred
meters in diameter.
-a tornado is a vortex: air rotates around the axis of the tornado
about as fast as it moves toward and along the axis.
-drawn by greatly reduced atmospheric pressure in the central
core, air streams into the base of the vortex from all
directions through a shallow layer a few tens of meters deep
near the ground.
-in the base the air turns abruptly to spiral upward around the
core and finally merges at the hidden upper end of the tornado
with the airflow in the parent cloud.
-the pressure in the core may be as much as ten percent less than
that of the surrounding atmosphere, i.e. 100 mb
-winds in a tornado are almost always cyclonic
-cyclostrophic balance
-the vortex frequently--not always--becomes visible as a funnel
cloud hanging part or all of the way to the ground from the
generating storm.
-a funnel cloud forms only if the pressure drop in the core
exceeds a critical value that depends on the temperature and
humidity of the inflowing air.
-as air flows into the area of lower pressure, it expands and
cools; if it cools enough the water vapor condenses
-the warmer and drier the inflowing air is, the greater the
pressure drop must be for condensation to occur and a cloud
to form.
-sometimes no condensation funnel forms, in which case the
tornado reveals itself only through the dust and debris it
carries aloft.
-a funnel can be anywhere from tens of meters to several km
long and where it meets the parent cloud its diameter ranges
from a few meters to hundreds of meters
-usually it is cone shaped, but short, broad, cylindrical
pillars are formed by very strong tornadoes and long ropelike
tubes that trail off horizontally are also common
-over a tornado's brief lifetime (never more than a few
hours) the size and shape of the funnel may change markedly,
reflecting changes in the intensity of the winds or in the
properties of the inflowing air.
-its color varies from a dirty white to gray to dark blue gray
when it consists mostly of water droplets, but if the core fills
with dust the funnel may take on a more exotic hue, such as
the red of western Oklahoma clay
TORNADO FORMATION
A strong thunderstorm provides the concentrated, persistent updraft
needed to launch a tornado and to prevent its low pressure core
from filling from above
-when the top of such a storm is viewed from a satellite it usually
displays a characteristic sequence of rising bubbles of cloud
material that overshoot the mean cloud top by two to four km
and then subside back into the cloud mass.
-bubbles are indicators of a strong updraft with a high degree
of organization in the storm
-for a tornado to be formed, however, the air in the updraft must
begin to rotate as well
-this can happen if the updraft concentrates the spin contained
in the horizontal winds in the troposphere.
Not just any winds will do
-they must be strongly sheared vertically in both magnitude and
direction; the wind speed must increase with altitude and direction
must veer from southeast to west
-vertical shear in wind speed provides a source of rotation about
a horizontal axis
-as winds aloft are moving faster a paddlewheel effect
is set up
-shear in wind direction also provides a source of rotation
-especially effective as updraft begins
According to current models, a severe thunderstorm gives rise
to a tornado in two steps
1. first the entire thunderstorm updraft begins to rotate
-this spinning column of rising air 10 to 20 km in diameter
is called a mesocyclone
-rotation begins in the mid troposphere
2. Once rotation has begun at mid levels it builds down toward
the ground through a dynamic pipe effect
-along the rotating column the pressure field is now in balance with
the strongly curved wind field
-the inwardly directed force acting on air parcels as a result
of the reduced pressure at the center of the column is
countered by the outwardly directed centrifugal force resulting
from the parcel's rotation about the center.
-in such a condition of cyclostrophic balance the air can easily
move around and along the axis of the cyclone,but radial
motions toward or away from the axis are strongly suppressed
-almost all the air entering the column must come from
its lower end.
-it is like a vacuum cleaner hose except that instead of
being channeled by the wall of the hose the airflow in
the cyclone constrained by its own swirling motion.
The result is an intensification of the updraft and hence of the
converging
winds under the cyclone
-because of the shear in wind direction, the air converging into the
updraft has a component of spin about the center of the column
-as the air parcels distance from the center of rotation decreases
its velocity must also increase and it begins to spin faster
about the center
As air parcels converge into the base of the pipe they turn and
accelerate
upward
-this results in their being stretched vertically
-stretching narrows the diameter of the mesocyclone from two to
six km
Tilting, the dynamic pipe effect,convergence and vertical stretching
processes that feed on one another can eventually form a mesocyclone
that extends from about l km above the ground to near the top of the
thunderstorm at about l5 km.
-surface winds with speeds as high as 75 mph can blow over the large
region under the swirling column
-the rotation in the mesocyclone is still too diffuse and too
far aloft to generate truly intense surface winds
The generation of such winds comes in THE SECOND STEP by which a severe
thunderstorm gives rise to a tornado; the formation of the actual
tornado core.
-for reasons that are not yet understood, a region of enhanced
convergence and stretching, no more than l km in diameter appears
to develop inside the mesocyclone, toward one side
-Doppler radar observations suggest that the intensification of
spin begins aloft, at altitudes of several km and then quickly
builds down toward the ground.
-over such a small area the rotational motion is strong enough
for the dynamic pipe effect to reach within several tens
of meters of the ground
-close to the ground, friction prevents the establishment of
a cyclostrophic balance by slowing the rotational motion
In response to the pressure gradient between the tornado core
and the surrounding atmosphere, air streams inward through a thin
layer near the ground.
-owing to inertia, the inflow actually overshoots its equilibrium
radius, conserving its angular momentum and picking up speed
as it approaches the center of the core before turning sharply
to spiral upward
-as a result the highest wind speeds are found in a small ring shaped
region at the base of the vortex.
BASIC CHARACTERISTICS
Average path is 25 km
-May 26, l9l7 in Ill and Ind 469 km path 7 hrs 20 min
April 3 and 4, l974 148 tornadoes 315 deaths
Winds up to 400 km per hour or as less as 65 km/hr
Speed 55-70 km per hour
Move generally from southwest to northeast
-most are about 100 yards in diameter or so
-most are on ground only for a few minutes
TORNADO DISTRIBUTION
Occur in England, France, Germany, Hungary, Italy, India, Russia,
Japan, Canada and Australia
In North America are about 600 per year
Rare on east and west coasts
-May most December east
Seasonal march of maximum frequency
Time of day in Great Plains 2/3 occur between 1 and 8 at night
Tornado maxima precedes hail max. by l month and Tstorm max by 2
WHY IN THE MIDDLE WEST?
Tornadoes have occurred in every state of the U.S. as well as many
other parts of the world.
-more however occur in the central and southeastern U.S. and
esp. in north Texas and Oklahoma and eastern Kansas.
-especially in the spring
Everything needed for the formation of tornadoes is here
-highly unstable distribution of temperature and humidity
in the atmosphere
-strong cold fronts to provide lift
-favorable upper atmospheric winds
TORNADIC THUNDERSTORMS
-tornadoes form in the updrafts of thunderstorms which form in
unstable air masses
-free convection results in which the potential energy stored
in the unstable atmosphere is converted into kinetic energy
- it is especially helpful if there is a low lifting
condensation level
-the typical unstable atmosphere consists of a warm humid
surface layer which is overlain by cool drier air
-the two may be separated by a capping inversion which
is a thin layer in which the temp increases with height
- a parcel of air rising into the inversion layer will
be cooler than the air around it and it will tend to
be pushed back down.
-as the day progresses and the ground is heated by the sun
the air under such a capping inversion warms further and
may become more humid
- if enough heating occurs the inversion may be locally eroded
and the highly unstable surface air erupts explosively at a
few points.
-surface air flows horizontally into these points of eruption,
forming tall thunderstorm clouds.
TORNADO FORMING CONDITIONS
1.Warm moist air at low levels
-air is usually maritime tropical and about 5000 feet thick
2. Dry tongue in middle levels
-with dry air blowing in from w or sw
-at 10 to 12 km this air ascends over low and cools thus
steepening lapse rate
3. inversion at about 7000 meters
-increasing heating and instability potential in lower air mass
-explosive convective overturning
-vest with conditional instability
4. Inversion disappears
5. triggering mechanism
-hurricane, squall lines or mid latitude cyclones
In many cases tornadoes are visible in front of thunderstorms in mature
state
Probability of tornado is proportional to severity of thunderstorm
and inversely proportional to number of thunderstorms.
EVOLUTION AND STRUCTURE
A typical tornado producing thunderstorm has a lifetime of two to
three hours and usually engenders only one relatively short lived
tornado.
-most of the storm's lifetime consists of initial organizational
and final dissipative phases.
-the period of maturity during which a tornado is most likely
to form may last for only a few tens of minutes
-in this phase the storm moves across the land, sweeping up
a continuous supply of humid unstable air.
-on rare occasions the updraft and the attendant tornado cyclone
can approach a quasi steady state in which case the storm is
called a supercell
-in some supercells the strength of the tornado cyclone pulsates
creating a sequence of tornadoes
-in rare instances a tornado cyclone will remain active for several
hours, forming a single long lasting tornado with a
continuous damage path
-Tri State tornado of March l8, l925 689 dead 352km path
Tornado vortexes vary a great deal in shape and size
-tornadoes that are classified as weak according to the Fujita
scale typically have a single nonturbulent funnel often in the
form of a long narrow inverted cone with a smooth surface
-funnel may not reach all the way to the ground.
-vertical wind velocities appear to be greatest along the
central axis
-Vortex of tornadoes classified as strong with wind speeds
from 113 to 200 mph is generally turbulent and its funnel
cloud, a broad column that almost always extends to the ground
has a rolling, boiling appearance
-Vortex of tornadoes classified as violent (over 206 mph)
have a central core that appears to be calm, a clear eye
that appears to be surrounded by two or more subsidiary
vortexes.
-the nonswirling, descending air in the eye is drawn down
from aloft by the extremely low pressure at the ground;
the eye is clear because water droplets in the air evaporate
as the air descends
-as the inner flow reaches the ground it joins the primary
flow and as it moves upward subsidiary vortexes are formed
-the subsidiary vortexes spin rapidly about their own
helical axis and at the same time rotate about the cent
line of the tornado
-helps explain weird damage paths.
-called suction spots or suction vortices
TORNADOES IN SUPERCELL STORMS
-STEP ONE IS SLOW HORIZONTAL ROTATION OF A LARGE SEGENT OF THE CLOUD UP TO 20 KM IN DIAMETER
-ROTATIion begins in cloud interior several km aove surface
-resulting large votice is called a mesocyclone and often precedes the formation of the actual tornado by up to 30 minutes
-formation of mesocyclone depends on vertical wind shear
=moving upward from surface wind direction shifts from south to west as speed increases
-this wind shear causes a rolling motion about a horizontal axis and under the right conditions strong updrafts in the storm tilt the horizontally rotating air so that the axis of rotation becomes approximately vertical and this provides the initial rotation within the cloud interior
-intensification of the mesocyclone requires that the area of rotation decrease which leads to an increase of wind speed--column stretches downward and may go below the cloud as a wall cloud--then a funnel cloud--then maybe a tornado on the ground
-Doppler can see the rotation within the cloud---but not all mesocyclones can produce tornadoes
NONSUPERCELL TORNADO DEVELOPMENT
-in these cases the tornadoes probably have sources nearer the surface than supercell tornadoes
-one mechanism is where outflows from two or more thunderstorms converge near the surface and some rotation occurs
-another mechanism is where strong rotation develops in a rapidly growing cumulonimbus cloud and it stretches down to the surface
TORNADO DAMAGE
-GENERALLY RESULTS FROM STRONG WINDS AND not actually pressure drop
-flying debris also is a cause of much damage
-suction vortices
-almost impossible to get direct readings
FUJITA SCALE
F0--UP TO 72 MPH -shallow trees, chimneys, some siding and signs
F1--up to 112- damage to roofs--cars swept off roads--damage to mobile homes
F2--up to 157 roofs torn off homes, mobile homes destroyed, large trees toppled
F3- up to 206--trains overturned roofs torn off and walls destroyed
F4--up to 260--frame houses totally destroyed, cars picked up
F5--up to 318--steel reinforced concrete structures badly damaged
F6--above 319--theoretical only though may occur in small part of F5 tornado or F4
DUST DEVILS
Spawned at ground level and may turn in either direction
Surface becomes very warm and small shallow whirls of upflowing
and inflowing air start at the surface. These whirlwinds mix the
air and keep the surface cool.
Source of energy is the layer of unstable air near the ground.
May move about freely and can be advected many kilometers from
the region of formation.
Source of rotation
-probably interaction between the field of motion of the
dust devil and local convective or shear processes
-considerable amounts of vertical vorticity may be present
in the convection itself
-it is possible that background rotation is supplied by local
convective motions which are advected along with the dust devil
WATERSPOUTS
Waterspout is an intense columnar vortex (not necessarily containing
a funnel shaped cloud) o small horizontal extent, over water
In South Florida, only rarely does the visible funnel extend from the
cloud base to the sea surface
Like the tornado most of the visible funnel is condensate
Funnel cloud extension therefore depends on the distribution of
ambient temperature, ambient water vapor and pressure drop due to
vortex circulation.
In the lower keys of Florida all waterspouts undergo a regular life cycle
composed of five discrete but overlapping states.
1. dark spot stage--characterized by a prominent light colored
disk on the sea surface, surrounded by a dark patch diffuse on its
outer edges--the dark spot may or may not have a small funnel cloud above
it initially, but signifies a complete vortex column extending from
cloud base to sea surface.
2. spiral pattern stage--primary growth phase, characterized by the
development of alternating dark and light colored bands spiraling
around the dark spot on the sea surface.
3. spray ring (incipient spray vortex) stage--concentrated spray ring
around the dark spot with a lengthening funnel cloud above
4. mature waterspout stage--spray vortex of minimum intensity and
organization, gradual weakening of the spiral pattern and maximum
funnel cloud length and diameter
5. decay stage--when the waterspout dissipates(often abruptly)
as it is intercepted by the cool downdrafts from a nearby rain
shower
Average life for funnel is about 15 minutes
Areas such as China and east coast of US where cool dry air may
overrun warm moist air