Download Squall line

Part 4. Disturbances
Chapter 11
Lightning, Thunder, and Tornadoes
Annual distribution of lightning strikes
There are about 40,000 thunderstorms daily on the Earth that
produce lightning. A few produce tornadoes.
Development of lightning and thunder
(a) Positive and negative
charges separate in the
cloud.
(b) The step leader is a flow of
negative charges
(electrons) toward positive
charges.
(c) A flow of positive charges
moves toward the step
leader.
(d) As the positive and
negative charges combine,
the lightning stroke is seen.
(e) A dart leader is a
secondary stroke just after
the first.
Thunder arises from the rapid heating of the air by the lightning stroke, which
sends out a sound wave at 5 miles/sec.
Charge separation in a cloud
Lightning only forms in clouds that extend into air that
is below freezing
• Collisions between ice crystals and graupel may help transfer
charge from solid ice to liquid films on some ice crystals
Positive charges tend to accumulate at the top of a
cloud, negative charges in the lower part of a cloud
Runaway discharge -- electrons accelerated to a very
high speed, colliding with air molecules and creating
more free electrons. High-speed moving electrons
radiate light as lightning.
Strong electrical fields occur prior to lightning
Safest areas from lightning -- indoors or in an automobile. Outdoors, do
not stand under tall objects. Do not touch telephones or electrical
appliances. Lightning can strike in the same place twice!
A positive stroke can occur when thunderstorms become tilted
Types of lightning
• Forked
• Sheet (heat)
• Ball
• St. Elmo’s fire
• Sprites
• Blue jets
A blue jet
Types of Thunderstorms
Air Mass
Frontal
Squall Line
Mesoscale Convective Complexes (MCC)
Air Mass thunderstorm lifecycle
Air Mass thunderstorms form in humid, unstable air. Each cell
lasts no more than a few hours from development to
dissipation.
Severe Thunderstorms
• Winds exceed 93 km/hr (58 mph), have large
hailstones (1.9 cm; 0.75 in) or produce
tornadoes
Mesoscale convective complex’s (MCCs)
• Self-propagating thunderstorm systems
Mesoscale convective systems (MCSs) include
MCCs and squall lines
• Squall line -- linear band of thunderstorms
usually out ahead of a cold front (a type of
MCS)
An MCC over South Dakota
A radar image of outflow boundaries
Outflow
boundaries -front edge of
cold air
flowing out
away from a
thunderstorm;
a gust front
occurs at an
outflow
boundary
Thunderstorm movement in an MCC
Cells dissipating
Movement
and lifecycle
of individual
thunderstorm
cells (A, B, C,
D, E, F, and
G) and how
they change
with time
Direction of
movement of the
individual
thunderstorm cells
Direction of
movement of
the line of
thunderstorms
Cells forming
A squall line (MCS)
This squall
line is
probably along
or ahead of an
advancing dry
line
A radar image of a squall line
Wind shear and vertical motions in a squall line thunderstorm
Vertical wind profile in (a) here
Gust front-induced shelf and roll cloud
Internal structure of a supercell
A supercell
thunderstorm
is an
extremely
powerful
thunderstorm
cell.
Supercell
thunderstorms
can spawn
tornadoes.
Organization of a supercell and actual radar signature
Hook echo
Outflow boundary
The above are map views of a supercell thunderstorm.
Downbursts, Derechos, and Microbursts
Downbursts -- strong downdrafts in a mature
thunderstorm
Derecho -- MCS-induced strong downdraft that
can last for hours
Microburst -- small diameter downburst that
usually lasts only a few minutes
Microbursts create aviation hazards
Tornadoes
• Tornado characteristics and dimensions
– 100-yard average diameter
– Movement = 50km/hr (30 mph) over 3-4
km (2-2.5 mi)
– Winds = 65 km/hr (40 mph) to 450 km/hr
(280 mph)
• Tornado formation
– Squall lines, MCCs, supercells, tropical
cyclones
Tornado-producing supercell
Tornadoes
typically
drop out of
the wall
cloud on the
southwest
side of a
supercell
A possible
mechanism of
tornado
formation
Tornado development along a convergence boundary
A non-supercell tornado development along outflow zone
Circular areas
shows places
where
thunderstorm
inflow and
outflow circulation
resulted in
vorticity and
tornado
development
Global tornado frequency
Most of Earth’s tornadoes occur in the lower elevation areas of
North America
“Tornado Alley”
Most tornadoes
occur in the
springtime, when
the contrast between
warm and cold air in
the atmosphere is
the greatest
-- Most injuries and
deaths in tornadoes
are in automobiles
and mobile homes
and are caused by
flying debris
-- US averages 91
tornado deaths each
year
-- Safest area in a
tornado is in a
basement or an
interior room, away
from windows
Multiple suction vortices greatly increase damage
Linear tornado damage path
• Tornado outbreaks
– A single weather system producing a
large number of tornadoes
• Waterspouts
– Similar to tornadoes
– Develop over warm waters
– Smaller and weaker than tornadoes
End of Chapter 11
Understanding Weather and
Climate
4th Edition
Edward Aguado and James E. Burt