|
|
|
|
Cloud-to-Cloud Lighting |
|
80% of all lighting |
|
Electricity discharge happens within clouds |
|
Causes the sky to light up uniformly (sheet
lighting) |
|
Cloud-to-Ground Lighting |
|
20% of all lighting |
|
Electricity discharge happens between cloud base
and ground |
|
|
|
|
Electrification of a cloud: Charge Separation |
|
Development of a path through which the electrons can flow |
|
Discharge: Lighting |
|
|
|
|
Positive charges in the upper portions of the
cloud; Negatively charges in lower portions; Small packet of positive
charges in the cloud base. |
|
Lighting occurs in only in clouds that extend
above the freezing level č charge separation is related to ice
crystals. |
|
Lighter crystals collide with heavy hailstones
in the cloud. |
|
The lighter crystals are positively charged and
move to upper portions of the cloud. |
|
The heavy hail stones are negatively charged and
move to the lower portion of the cloud. |
|
|
|
|
The
negative charge at the bottom of the cloud causes a region of the ground
beneath it to become negatively charged. |
|
|
|
The positive charge is most dense on protruding
objects, such as trees, poles, and buildings. |
|
|
|
|
The dry air is a good electrical insulator, so a
flow of current can not occur. |
|
For
cloud-to-ground lightning to occur, a stepped-leader must emanate from the
cloud base. |
|
The leader is essentially an ionized particle
chamber about 10 cm (4 in) in diameter which forks repeatedly from a main
channel. |
|
Each section travels about 50 m in a
microsecond. |
|
The sections continue until contact is made with
an unlike charged area (the ground). |
|
|
|
|
|
|
Upon
connection, electrons flow resulting in an illuminated return stroke. |
|
Although
the electrical current is from the cloud to the ground (moves downward),
the return stroke is in the opposite direction (move upward). |
|
The upward return stroke happens so fast, our
eyes can not resolve its upward direction. |
|
|
|
|
|
|
Usually
more than one stroke is needed to neutralize all negative ions. |
|
Another leader, or dart leader, is initiated and
a return stroke follows. |
|
Dart leader moves downward faster than step
leader. |
|
The process is repeated about 4-5 times on
average. |
|
Individual strokes are almost impossible to
detect. |
|
We call a combination of all strokes a lightning
flash. |
|
|
|
|
Most of
the lighting are negatively charged cloud-to-positively charged ground
(negative lighting). |
|
But
there are also positively charged cloud-to-negatively charged ground
(positive lighting). |
|
When high-level winds are strong, thunderstorm
clouds become tilted and produce the positive lighting. |
|
|
|
|
|
The
lighting stroke can heat the air through which it travels to 30,000C
(54,000F), which is 5 times hotter than the surface of sun. |
|
This extreme heating causes the air to expand
explosively, thus initiating a shock wave that become a booming sound wave
(thunder) to travel outward. |
|
It takes
3 seconds for thunder to travel 1 km (5 seconds to travel 1 mile). |
|
|
|
|
A
thunderstorm is a storm containing lighting and thunder, and sometime
produces gust winds with heavy precipitation and hail. |
|
The storm may be a single cumulonimbus cloud, or
several thunderstorm may form into a cluster. |
|
Two types of thunderstorm: (1) air mass
thunderstorm (self-extinguishing) and (2) sever thunderstorm
(self-propagating). |
|
|
|
|
Air mass
thunderstorms are contained within uniform air masses (away from fronts)
but they are localized. |
|
Air mass
thunderstorms are self-extinguished
and are short lived phenomena (less than an hour). |
|
An air mass thunderstorm normally consists of a
number of individual cells, each undergoing a sequence of three distinct
stages: cumulus, mature, and dissipative. |
|
|
|
|
This begins with unstable air rises often as
some surfaces undergo more rapid heating than others. |
|
Only updrafts are present as air rises and
adiabatically cools. |
|
At first, the cumulus clouds grow upward only
for a short distance, then they dissipate (because of re-evaporation) |
|
Eventually, enough water vapor will be present
to sustain vertical cloud development which occurs between 5-20 m/sec
(10-45 mph) |
|
|
|
|
|
|
The mature stage is marked by precipitation and
the presence of both up and down drafts. |
|
Downdrafts are initiated through frictional drag
associated with falling precipitation. |
|
This is also a time of lightning and thunder. |
|
Cloud tops are formed where the atmosphere is
stable. |
|
An anvil head may occur as high speed winds blow
ice crystals downstream. |
|
Updrafts dominate the interior portions of the
storm while downdrafts occur toward the edges. |
|
|
|
|
|
|
The dissipative stage occurs when downdrafts
dominate airflow within the thunderstorms. |
|
This suppresses updrafts and the addition of
water vapor. |
|
Precipitation then ceases and the cloud
eventually evaporates. |
|
|
|
|
|
|
|
Occur when winds exceed 93 km/hr (58 mph), have
large hailstones (1.9 cm; 0.75 in) or produce tornadoes. |
|
These systems differ from air mass thunderstorms
in that the up and downdrafts support each other to intensify the storm. |
|
Particular atmospheric conditions must persist
across the mesoscale (10-1000 km) for severe thunderstorms to develop. |
|
|
|
|
|
|
Clusters of severe thunderstorms are called mesoscale
convective systems (MCSs). |
|
MCSs
occur as squall lines, or as circular clusters called mesoscale convective
complex’s (MCCs). |
|
Individual storms develop in concert in a
situation which propagates additional thunderstorms. |
|
Many MCSs have life spans from up to 12 hrs to
several days. |
|
Severe thunderstorms may also form from
individual supercells which contain only one updraft (supercells may also
be a part of an MCS). |
|
|
|
|
MCCs account for the greatest amount of severe
weather in the U.S. and Canada. |
|
Circular clusters of thunderstorms which are
self propagating in that individual cells create downdrafts which interact
to form new cells. |
|
Colder, denser downdrafts spread across the
surface and help force warm, moist surface air aloft. |
|
This outflow boundary initiates a new cell. |
|
The entire system typically propagates eastward. |
|
|
|
|
|
|
|
Bands may be as long as 500 km (300 mi) usually
about 300-500 km (180-300 mi) in advance of cold fronts. |
|
Strong vertical wind shear is essential to the
development of these prefrontal waves as it ensures that updrafts will be
positioned ahead of the downdrafts. |
|
This feeds moisture into the system which is
also aided by gust front propagation ahead of the situation. |
|
|
|
|
|
|
|
Although
supercells consist of a single cell they are typically more violent than
MCCs or squall lines. |
|
Strong wind shear is responsible for wrapping up
and downdrafts around each other in these tornado producers. |
|
This creates large-scale rotation which is
typically absent from MCCs and squall lines. |
|
|
|
|
Strong downdrafts can create deadly gusts of
winds, called downbursts. |
|
Downbursts can be mistakenly considered as tornadoes. |
|
When
downbursts have diameters of less than 4 km, they are called microbursts. |
|
Microbursts are dangerous to airplanes. |
|
|
|
|
Thunderstorms develop where moist air is forced
aloft. |
|
Occurs
frequently in the tropics, nearly daily in some locations. |
|
In the
U.S., most frequent region is the Gulf South. |
|
|
|
|
Tornadoes are zones of extremely rapid, rotating winds beneath
the base of cumulonimbus clouds. |
|
Strong
counterclockwise (in N.H.) winds originate in relation to large pressure
gradients over small spatial scales. |
|
Pressure
differences may be as much as 100 mb over a few tenths of km. |
|
|
|
|
Typically have diameters of about 100 yards but
may be much larger. |
|
Usually
a short lived phenomena lasting only a few minutes, but some have lasted
for hours. |
|
Movement is generally about 50km/hr (30 mph)
over an areas about 3-4 km (2-2.5 mi) long. |
|
Winds may be as low as 65 km/hr (40 mph) or as
high as 450 km/hr (280 mph). |
|
Come in wide range of shape and size. |
|
|
|
|
|
|
|
|
|
Common
to frontal boundaries, squall lines, MCCs, supercells and tropical
cyclones. |
|
Most
violent tornadoes are associated with supercells |
|
|
|
|
|
|
|
|
Vertical wind shear creates a horizontal vortex. |
|
The vortex is tilted vertically by strong
updrafts and forms a mesocyclone. |
|
The vortex stretches downward when the
mesocyclone intensified. |
|
A wall cloud id formed under the cloud base,
which then develops into a tornadoes. |
|
Only about 1/2 of all mesocyclones actually
spawn a tornado |
|
|
|
|
|
|
|
|
|
|
|
|
The U.S. is the world leader in tornado
production. |
|
This results from the regular interaction
between the air mass from the Gulf of Mexico and the air mass from the
polar continent. |
|
The absence of topographic barriers ensures
regular mixing and the production of violent storm systems. |
|
|
|
|
|
|
The vast majority occur in Tornado Alley, a
region from the southern Plains to the lower Great Lakes. |
|
Texas has the highest tornado frequency of any
state. |
|
|
|
|
May is
the month of highest frequency while June is a close second. |
|
Many states show tornado peaks during different
months, however, late spring is the time of greatest overall activity. |
|
It is the season when air mass contrasts are
especially strong. |
|
|
|
|
|
|
Winds, not pressure change, cause the greatest
amount of damage. |
|
Flying debris causes the greatest amount of
injuries. |
|
Some tornadoes have multiple suction vortices
which may account for rather selective damage patterns. |
|
Tornadoes are classified using the Fujita scale
which ranks tornadoes based on damage. |
|
|
|
|
|
|