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Meteorology
• Team of 2
• One calculator – non-programmable
• One sheet of paper with notes front and back
– May be computer generated
50 minutes of competition
Audubon Weather Guide (meteorology)
Bio/Earth CD
• WWW.wikispaces.com
• ScienceFrizzle
Competition
• Tests –from previous years
• Available on SO site
• See rules for event
http://www.nssl.noaa.gov/primer/
Tornadoes
Research has revealed that tornadoes usually form under certain types of atmospheric conditions. Those conditions can
be predicted, but not perfectly. When forecasters see those conditions, they can predict that tornadoes are likely to
occur. However, it is not yet possible to predict in advance exactly when and where they will develop, how strong they
will be, or precisely what path they will follow.
• The damage from tornadoes comes from the strong winds they
contain. It is generally believed that tornadic windspeeds can be as
high as 300 mph in the most violent tornadoes. Windspeeds that
high can cause automobiles to become airborne, rip ordinary
homes to shreds, and turn broken glass and other debris into lethal
missiles. The biggest threat to living creatures (including humans)
from tornadoes is from flying debris and from being tossed about in
the wind.
• Tornadoes are classified according to the damage they cause.
Through observational studies, T. Theodore Fujita created the
following scale in the late 1960's to classify tornadoes. The scale
correlates wind speeds with damage: F-0 is the weakest and F-5 the
strongest.
Fujita Scale
First Tornado forecast
• Tornado Basics
• What is a tornado?
• A tornado is a narrow, violently rotating column of air that extends from the base
of a thunderstorm to the ground. Because wind is invisible, you can't always see a
tornado. A visible sign of the tornado, a condensation funnel made up of water
droplets, sometimes forms and may or may not touch the ground during the
tornado lifecycle. Dust and debris in the rotating column also make a tornado
visible and confirm its presence.
• What is known?
• Tornadoes are the most violent of all atmospheric storms.
• There are two types of tornadoes: those that come from a supercell thunderstorm,
and those that do not.
• Tornadoes that form from a supercell thunderstorm are the most common, and
often the most dangerous. A supercell is a long-lived (greater than 1 hour) and
highly organized storm feeding off an updraft (a rising current of air) that is tilted
and rotating. This rotating updraft - as large as 10 miles in diameter and up to
50,000 feet tall - can be present as much as 20 to 60 minutes before a tornado
forms. Scientists call this rotation a mesocyclone when it is detected by Doppler
radar. The tornado is a very small extension of this larger rotation. Most large and
violent tornadoes come from supercells.
• CONDENSATION FUNNEL - A funnel-shaped cloud associated
with rotation and consisting of condensed water droplets (as
opposed to smoke, dust, debris, etc.)
• SUPERCELL - A thunderstorm with a persistent rotating
updraft. Supercells are rare, but are responsible for a
remarkably high percentage of severe weather events especially tornadoes , extremely large hail and damaging
straight-line winds. They frequently travel to the right of the
main environmental winds (i.e., they are right movers). Radar
characteristics often (but not always) include a hook or
pendant, bounded weak echo region (BWER), V-notch,
mesocyclone, and sometimes a TVS. Visual characteristics
often include a rain-free base (with or without a wall cloud),
tail cloud, flanking line, overshooting top, and back-sheared
anvil, all of which normally are observed in or near the right
rear or southwest part of the storm. Storms exhibiting these
characteristics often are called classic supercells; however HP
storms and LP storms also are supercell varieties.
• UPDRAFT - A small-scale current of rising air. If the
air is sufficiently moist, then the moisture
condenses to become a cumulus cloud or an
individual tower of a towering cumulus or Cb.
• MESOCYCLONE - A storm-scale region of rotation,
typically around 2-6 miles in diameter and often
found in the right rear flank of a supercell (or often
on the eastern, or front, flank of an HP storm). The
circulation of a mesocyclone covers an area much
larger than the tornado that may develop within it.
Tornado
• Non-supercell tornadoes are circulations that form
without a rotating updraft. One non-supercell tornado
is the gustnado, a whirl of dust or debris at or near the
ground with no condensation funnel, which forms
along the gust front of a storm. Another non-supercell
tornado is a landspout. A landspout is a tornado with a
narrow, rope-like condensation funnel that forms when
the thunderstorm cloud is still growing and there is no
rotating updraft - the spinning motion originates near
the ground. Waterspouts are similar to landspouts,
except they occur over water. Damage from these
types of tornadoes tends to be F2 or less.
• GUSTNADO - [Slang], gust front tornado. A small
tornado, usually weak and short-lived, that occurs
along the gust front of a thunderstorm. Often it is
visible only as a debris cloud or dust whirl near the
ground. Gustnadoes are not associated with stormscale rotation (i.e. mesocyclones ); they are more likely
to be associated visually with a shelf cloud than with a
wall cloud.
• LANDSPOUT - [Slang], a tornado that does not arise
from organized storm-scale rotation and therefore is
not associated with a wall cloud (visually) or a
mesocyclone (on radar). Landspouts typically are
observed beneath Cbs or towering cumulus clouds
(often as no more than a dust whirl), and essentially
are the land-based equivalents of waterspouts.
• How do tornadoes form?
• Scientists have learned a lot about tornadogenesis from theoretical
studies, field projects and physical models – but tornadogenesis – the
way tornadoes form – has vexed researchers for decades.
• SUPERCELL TORNADOGENESIS
A rotating updraft is a key to the development of a supercell, and
eventually a tornado. There are many ideas about how this rotation
begins. One way a column of air can begin to rotate is from wind
shear – when winds at two different levels above the ground blow at
different speeds or in different directions.
• An example of wind shear that can eventually create a tornado is
when winds at ground level, often slowed down by friction with the
earth's surface, come from the southwest at 5 mph. But higher up, at
5000 feet above the same location, the winds are blowing from the
southeast at 25 mph! An invisible "tube" of air begins to rotate
horizontally. Rising air within the thunderstorm tilts the rotating air
from horizontal to vertical – now the area of rotation extends through
much of the storm.
• Once the updraft is rotating and being fed by warm, moist air flowing
in at ground level, a tornado can form. There are many ideas about
this too.
• SHEAR - Variation in wind speed (speed shear)
and/or direction (directional shear) over a
short distance. Shear usually refers to vertical
wind shear, i.e., the change in wind with
height, but the term also is used in Doppler
radar to describe changes in radial velocity
over short horizontal distances.
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More Ideas About Supercell Tornadogenesis
Scientists are actively trying to prove or disprove a number of tornadogenesis hypotheses. It is
complicated science that draws on information from observations, theory, and mathematical and
physical models. These are some basic ideas (basic to a scientist, that is) about the processes that
might cause tornadoes to form from supercells:
Dynamic Pipe Effect
Development of a tornado begins when horizontal winds coming together from different directions
are strong 3-4km above the ground and weak or absent near the ground. The result is that rotation
first increases aloft. The young tornado will build downward by something called the dynamic pipe
effect (DPE): air can not enter through the sides of this belt of rotating air, but can pass through its
ends like a pipe. The partial vacuum created within the pipe draws weakly rotating air up into the
pipe's lower end. This causes the air to spin faster and eventually become part of the pipe. New
sections on the rotating pipe form at lower and lower altitudes through this same process until the
pipe (tornado) is in contact with the ground.
Another type of tornado development occurs when converging horizontal winds have the same
windspeed through all levels in the thunderstorm. Rotation increases all at once and spans several
kilometers along the vertical pipe. The tornado, in this case, forms nearly independent from how
high it is above the ground, and develops very rapidly from the ground, up.
Rear Flank Downdraft (RFD)
The Rear Flank Downdraft (RFD) may play a role in tornadogenesis. The RFD is a region of dry air
pushed towards the ground by the thunderstorm on the backside of, and wrapping around a
rotating updraft, and eventually the tornado. It is often visible as a clear slot wrapping around a
wall cloud (a persistent lowering from a rain-free base of the main thunderstorm). On radar, the
presence of a hook or a small feature hanging from the thunderstorm may indicate the presence of
an RFD. Scientists think the RFD may play a significant role in determining the development of a
tornado, how long it lasts, and how intense it is. Some scientists think that the RFD, by wrapping
around the low-level rotating updraft, forces the rotation to concentrate and lower to the ground.
• We still have many questions. Scientists know from field studies that
perhaps as few as 20 percent of all supercell thunderstorms actually
produce tornadoes. Why does one supercell thunderstorm produce a
tornado and another nearby storm does not? What are some of the
causes of winds moving at different speeds or directions that create the
rotation? What are other circulation sources for tornadoes? What is the
role of downdrafts (a sinking current of air) and the distribution of
temperature and moisture (both horizontally and vertically) in
tornadogenesis? Scientists hope to learn more about the processes that
create wind shear and rotation, tilt it vertically, and concentrate the
rotation into a tornado when they participate in a large field experiment in
2007.
• And, since not all tornadoes come from supercells, what about
tornadogenesis in non-supercell thunderstorms?
• NON-SUPERCELL TORNADOGENESIS
A non-supercell tornado does not form from organized storm-scale
rotation. These tornadoes form from a vertically spinning parcel of air
already occurring near the ground, about 1-10 km in diameter, that is
caused by wind shear from a warm, cold, or sea breeze front, or a dryline.
When an updraft moves over the spinning, and stretches it, a tornado can
form. Eastern Colorado experiences non-supercell tornadoes when cool air
rushes down off the Rocky Mountains and collides with the hot dry air of
the plains. Since these types of tornadoes happen mostly over scarcely
populated land, scientists are not sure how strong they are, but they tend
to be small. Waterspouts and gustnadoes are formed in this way too.
• DOWNDRAFT - A small-scale column of air that rapidly sinks toward
the ground, usually accompanied by precipitation as in a shower or
thunderstorm. A downburst is the result of a strong downdraft.
• DRY LINE - A boundary separating moist and dry air masses, and an
important factor in severe weather frequency in the Great Plains. It
typically lies north-south across the central and southern high
Plains states during the spring and early summer, where it
separates moist air from the Gulf of Mexico (to the east) and dry
desert air from the southwestern states (to the west). The dry line
typically advances eastward during the afternoon and retreats
westward at night. However, a strong storm system can sweep the
dry line eastward into the Mississippi Valley, or even further east,
regardless of the time of day. A typical dry line passage results in a
sharp drop in humidity (hence the name), clearing skies, and a wind
shift from south or southeasterly to west or southwesterly.
(Blowing dust and rising temperatures also may follow, especially if
the dry line passes during the daytime. These changes occur in
reverse order when the dry line retreats westward. Severe and
sometimes tornadic thunderstorms often develop along a dry line
or in the moist air just to the east of it, especially when it begins
moving eastward.
• Tornado Climatology
• Where and when do tornadoes occur?
• Tornadoes occur in many parts of the world, including
Australia, Europe, Africa, Asia, and South America. Even
New Zealand reports about 20 tornadoes each year.
• Two of the highest concentrations of tornadoes outside the
U.S. are Argentina and Bangladesh. Both have similar
topography with mountains helping catch low-level
moisture from over Brazil (Argentina) or from the Indian
Ocean (Bangladesh).
• About 1,000 tornadoes hit the U.S. yearly. Since official
tornado records only date back to 1950, we do not know
the actual average number of tornadoes that occur each
year. Plus, tornado spotting and reporting methods have
changed a lot over the last several decades.
Clip on tornadoes
A recent NSSL study, using data from 1921 to 1995, estimated the daily climatological probability
of an F2 or greater tornado occurring near any location in the U.S. For this work developing highly
accurate and accessible estimates of the long-term threat from thunderstorms, winds, and large
hail as well as tornadoes, an NSSL scientist was awarded a Department of Commerce Silver
Medal.
Probability of Any Tornado:
The map shows the average number of days per year any tornado, no matter how strong or
weak, might occur within 25 miles of a point. The highest numbers indicate where at least
one tornado might occur somewhere within 25 miles as often as on 1.5 days per year.
Significant Tornado (F2 or greater):
Now we're looking at days per century. In other words, central Oklahomans can expect an F2 or
greater tornado within 25 miles about every 3 years.
Violent Tornado (F4 or greater):
Now the scale is days per millennium, meaning that southcentral Oklahoma may have a violent
tornado within 25 miles about once every 20 years.
Annual Cycle:
Residents of Norman, OK experience a distinct tornado season, beginning late February and
peaking late May. Even though we are in the heart of tornado alley and can expect one- to oneand one-half tornado days per year, our chances on any particular day peak at only about two
percent.
• Tornado season usually refers to the time of year
where the U.S. sees the most tornadoes. The
peak “tornado season” for the southern plains -often referred to as Tornado Alley -- is during May
into early June. On the Gulf coast, it is earlier
during the spring. In the northern plains and
upper Midwest, tornado season is in June or July.
But, remember, tornadoes can happen at any
time of year. Tornadoes can also happen at any
time of day, but most tornadoes occur between
4-9 p.m.
Why Tornado Alley?
• Tornado Alley is a nickname for an area that consistently experiences a
high frequency of tornadoes each year. The area that has the most strong
and violent tornadoes includes eastern SD, NE, KS, OK. Northern TX, and
eastern Colorado. The relatively flat land in the Great Plains allows cold
dry polar air from Canada to meet warm moist tropical air from the Gulf of
Mexico. A large number of tornadoes form when these two air masses
meet, along a phenomenon known as a "dryline."
• The dryline is a boundary separating hot, dry air to the west from warm,
moist air to the east. You can see it on a weather map by looking for sharp
changes in dew point temperatures. Between adjacent weather stations
the differences in dew point can vary by as much as 40 degrees or more.
The dryline is usually found along the western high plains. Air moving
down the eastern slopes of the Rockies warms and dries as it sinks onto
the plains, creating a hot, dry, cloud-free zone. During the day, it moves
eastward mixing up the warm moist air ahead of it. If there is enough
moisture and instability in the warm air, severe storms can form - because
the dryline is the "push" the air needs to start moving up! During the
evening, the dryline "retreats" and drifts back to the west. The next day
the cycle can start all over again, until a larger weather system pushes
through and washes it away.
• Tornadoes kill an average of 60 people per year, mostly from flying
or falling debris.
• The Tri-State Tornado of March 18, 1925 was the deadliest tornado
in history, killing 695 people. It is also the longest tornado track ever
known - 219 miles - across parts of Missouri, Illinois and Indiana.
• Codell, KS was struck by a tornado on May 20 three years in a row:
1916, 1917, and 1918.
• Understanding the threat posed by tornadoes in the United States particularly the threat of strong and violent tornadoes - is valuable
knowledge to everyone, but especially to weather forecasters and
emergency management people. Knowledge about long-term
patterns helps us be better prepared for natural disasters and could
also help scientists detect shifting patterns in severe weather
events caused by climate change.
Floods
• Flood Basics
• What is flooding?
• Flooding is an overflowing of water onto land that is normally dry. It
can happen during heavy rains, when ocean waves come onshore,
when snow melts too fast, or when dams or levees break. Flooding
may happen with only a few inches of water, or it may cover a
house to the rooftop. The most dangerous flood event, the flash
flood, happens quickly with little or no warning; other flooding
events occur over a long period and may last days, weeks, or longer.
• What is a river flood?
• A river flood occurs when water levels rise in a river due to
excessive rain from tropical systems making landfall, persistent
thunderstorms over the same area for extended periods of time,
combined rainfall and snowmelt, or an ice jam.
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What is coastal flooding?
Coastal flooding occurs when a hurricane, tropical storm, or tropical depression produces a
deadly storm surge that overwhelms coastal areas as it makes landfall. Storm surge is
water pushed on shore by the force of the winds swirling around the storm. This advancing
surge combines with the normal tides to create the hurricane storm tide, which can
increase the average water level 15 feet or more. The greatest natural disaster in the
United States, in terms of loss of life, was caused by a storm surge and associated coastal
flooding from the great Galveston, Texas, hurricane of 1900. At least 8,000 people lost their
lives.
What is inland flooding?
When tropical cyclones move inland, they are typically accompanied by torrential rain. If
the decaying storm moves slowly over land, it can produce rainfall amounts of 20 to 40
inches over several days. Widespread flash flooding and river flooding can result.
What is a flash flood?
A flash flood is a rapid rise of water along a stream or low-lying urban area. Flash flooding
occurs within six hours of a significant rain event and is usually caused by intense storms
that produce heavy rainfall in a short amount of time. Excessive rainfall that causes rivers
and streams to swell rapidly and overflow their banks is frequently associated with
hurricanes and tropical storms, large clusters of thunderstorms, supercells, or squall lines.
Other types of flash floods can occur from dam or levee failures, or a sudden release of
water held by an ice jam. Heavy rainfall in the mountains can cause downstream canyon
flooding.
Why is a flash flood so dangerous?
Flash floods can occur with little or no warning. Flash flood damage and most fatalities
tend to occur in areas immediately adjacent to a stream or arroyo. Flash floods are very
strong -- they can roll boulders, tear out trees, destroy buildings and bridges, and scour out
new channels. Rapidly rising water can reach heights of 30 feet or more. Flash floodproducing rains falling on steep terrain can weaken soil and trigger catastrophic mud slides
that damage homes, roads, and property.
• What areas are at risk from flash floods?
• Densely populated areas are at a high risk for flash floods. The
construction of buildings, highways, driveways, and parking lots
increases runoff by reducing the amount of rain absorbed by the
ground. This runoff increases the flash flood potential.
• Sometimes, streams through cities and towns are routed
underground into storm drains. During heavy rain, the storm drains
can become overwhelmed and flood roads and buildings. Low
spots, such as underpasses, underground parking garages, and
basements can become death traps.
• Areas near rivers are at risk from flash floods. Embankments,
known as levees, are often built along rivers and are used to
prevent high water from flooding bordering land. In 1993, many
levees failed along the Mississippi River, resulting in devastating
flash floods. The city of New Orleans experienced massive
devastating flooding days after Hurricane Katrina came onshore in
2005 due to the failure of levees designed to protect the city.
Hail
• Hail Basics
• What is hail?
• Hail is a form of precipitation that occurs when updrafts in thunderstorms
carry raindrops upward into extremely cold areas of the atmosphere
where they freeze into ice.
• How does hail form?
• There are two ideas about hail formation. In the past, the prevailing
thought was that hailstones grow by colliding with supercooled water
drops. Supercooled water will freeze on contact with ice crystals, frozen
rain drops, dust or some other nuclei. Thunderstorms that have a strong
updraft keep lifting the hailstones up to the top of the cloud where they
encounter more supercooled water and continue to grow. The hail falls
when the thunderstorm's updraft can no longer support the weight of the
ice or the updraft weakens. The stronger the updraft the larger the
hailstone can grow.
SUPERCOOLED WATER - Liquid water that is below 0°C, or water that stays in liquid
form if undisturbed even though it has been cooled to a temperature below its normal
freezing point. The smaller and purer the water droplets, the more likely they can
become supercooled.
• Recent studies suggest that supercooled water may accumulate on frozen
particles near the back-side of the storm as they are pushed forward
across and above the updraft by the prevailing winds near the top of the
storm. Eventually, the hailstones encounter downdraft air and fall to the
ground.
• Hailstones grow two ways: by wet growth or dry growth processes. In wet
growth, a tiny piece of ice is in an area where the air temperature is below
freezing, but not super cold. When the tiny piece of ice collides with a
supercooled drop, the water does not freeze on the ice immediately.
Instead, liquid water spreads across tumbling hailstones and slowly
freezes. Since the process is slow, air bubbles can escape resulting in a
layer of clear ice.
• Dry growth hailstones grow when the air temperature is well below
freezing and the water droplet freezes immediately as it collides with the
ice particle. The air bubbles are "frozen" in place, leaving cloudy ice.
• Hailstones can have layers like an onion if they travel up and down in an
updraft, or they can have few or no layers if they are "balanced" in an
updraft. One can tell how many times a hailstone traveled to the top of
the storm by counting the layers. Hailstones can begin to melt and then
re-freeze together - forming large and very irregularly shaped hail.
Hail can cause significant damage
• What is the difference between hail, sleet, and graupel?
• The different ways precipitation is formed determines what
type of precipitation it becomes. Hail is larger than sleet,
and forms only in thunderstorms. Hail formation requires
air moving up (thunderstorm updraft) that keep the pieces
of ice from falling. Drops of supercooled water hit the ice
and freeze on it, causing it to grow. When the hailstone
becomes too heavy for the updraft to keep it aloft, ot it
encounters downdraft air, it falls. Sleet forms from
raindrops that freeze on their way down through a cloud.
Snow forms mainly when water vapor turns to ice without
going through the liquid stage. There is no thunderstorm
updraft involved in either of these processes.
Graupel
graupel—Heavily rimed snow particles, often called snow pellets; often indistinguishable from very small soft
hail except for the size convention that hail must have a diameter greater than 5 mm. Sometimes distinguished
by shape into conical, hexagonal, and lump (irregular) graupel.
• How does hail fall to the ground?
• Hail falls when it becomes heavy enough to overcome the strength of the
updraft and is pulled by gravity towards the earth. How it falls is
dependent on what is going on inside the thunderstorm. Hailstones bump
into other raindrops and other hailstones inside the thunderstorm, and
this bumping slows down their fall. Drag and friction also slow their fall, so
it is a complicated question! If the winds are strong enough, they can even
blow hail so that it falls at an angle. This would explain why the screens on
one side of a house can be shredded by hail and the rest are unharmed!
• How fast does hail fall?
• We really only have estimates about the speed hail falls. One estimate is
that a 1cm hailstone falls at 9 m/s, and an 8cm stone, weighing .7kg falls at
48 m/s (171 km/h). However, the hailstone is not likely to reach terminal
velocity due to friction, collisions with other hailstones or raindrops, wind,
the viscosity of the wind, and melting. Also, the formula to calculate
terminal velocity is based on the assumption that you are dealing with a
perfect sphere. Hail is generally not a perfect sphere!
Lightning
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Lightning Basics
What is lightning?
Lightning is a gigantic electrostatic discharge (the same kind of electricity that can shock you when
you touch a doorknob) between the cloud and the ground, other clouds, or within a cloud.
Scientists do not understand yet exactly how it works or how it interacts with the upper
atmosphere or the earth 's electromagnetic field.
Lightning is one of the oldest observed natural phenomena on earth. It has been seen in volcanic
eruptions, extremely intense forest fires, surface nuclear detonations, heavy snowstorms, in large
hurricanes, and obviously, thunderstorms.
What causes lightning?
The creation of lightning is a complicated process. We generally know what conditions are needed
to produce lightning, but there is still debate about exactly how lightning forms.The exact way a
cloud builds up the electrical charges that lead to lightning is not completely understood.
Precipitation and convection theories both attempt to explain the electrical structure within clouds.
Precipitation theorists suppose that different size raindrops, hail, and graupel get their positive or
negative charge as they collide, with heavier particles carrying negative charge to the cloud bottom.
Convection theorists believe that updrafts transport positive charges near the ground upward
through the cloud while downdrafts carry negative charges downward
• Thunderstorms have very turbulent environments - strong updrafts
and downdrafts occur often and close together. The updrafts carry
small liquid water droplets from the lower regions of the storm to
heights between 35,000 and 70,000 feet - miles above the freezing
level. At the same time, downdrafts are transporting hail and ice
from the frozen upper parts of the storm. When these particles
collide, the water droplets freeze and release heat. This heat keeps
the surface of the hail and ice slightly warmer than its surrounding
environment, and a soft hail, or graupel forms.
• When this graupel collides with additional water droplets and ice
particles, a key process occurs involving electrical charge: negatively
charged electrons are sheared off the rising particles and collect on
the falling particles. The result is a storm cloud that is negatively
charged at its base, and positively charged at the top.
• Opposite charges attract one another. As the positive and negative areas
grow more distinct within the cloud, an electric field is created between
the oppositely-charged thunderstorm base and its top. The farther apart
these regions are, the stronger the field and the stronger the attraction
between the charges. But we cannot forget that the atmosphere is a very
good insulator that inhibits electric flow. So, a HUGE amount of charge has
to build up before the strength of the electric field overpowers the
atmosphere's insulating properties. A current of electricity forces a path
through the air until it encounters something that makes a good
connection. The current is discharged as a stroke of lightning.
• While all this is happening inside the storm, beneath the storm, positive
charge begins to pool within the surface of the earth. This positive charge
will shadow the storm wherever it goes, and is responsible for cloud-toground lightning. However, the electric field within the storm is much
stronger than the one between the storm base and the earth 's surface, so
about 75-80% of lightning occurs within the storm cloud.
• ELECTRICAL CHARGE - A fundamental property of matter.
Protons and the nuclei of atoms have a positive charge;
electrons have a negative charge; neutrons have no charge.
Normally, each atom has as many protons as it has
electrons and thus has no net electrical charge; in other
words, it is neutral. Charged substances have an imbalance
of positive and negative charges, a net charge that exerts a
force on other charged substances. Charges that are both
positive or both negative repel each other; charges that are
different attract.
• ELECTRIC FIELD - A field or force that exists in the space
between two different potentials, such as between
negatively and positively charged regions of a
thunderstorm
Conceptual Model of Lightning Charge Distribution Within a Thunderstorm
For many years, scientists have thought thunderstorms contain three charges, called a tripole. A
new conceptual model of electrical charge distribution inside deep convection (thunderstorms),
developed by NSSL and university scientists, could change that thinking. In the main updraft (in and
above the red arrow), there are four main charge regions. In the convective region but outside the
outdraft (in and above the blue arrow), there are more than four charge regions.
• Lightning types
• GROUND FLASHES
There are two categories of ground flashes: natural (those that
occur because of normal electrification in the environment), and
artificially initiated or triggered. Artificially initiated lightning
includes strikes to very tall structures, airplanes, rockets and towers
on mountains. Triggered lightning goes from ground to cloud, while
"natural" lightning is cloud to ground.
• Terms used to describe ground flashes include forked lightning,
which shows branching to the ground from a nearly vertical
channel; ribbon lightning, when the horizontal displacement of the
channel by the wind appears as a series of ribbons; and bead
lightning, when the decaying channel of a ground flash will
sometimes break into a series of bright and dark spots. Ball
lightning is a luminous sphere whose physics is not well understood.
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• Cloud-to-ground lightning (CG's)
A channel of negative charge, called a step leader, will zigzag
downward in roughly 50-yard segments in a forked pattern. This
step leader is invisible to the human eye, and shoots to the ground
in less time than it takes to blink. As it nears the ground, the
negatively charged step leader is attracted to a channel of positive
charge reaching up, a streamer, normally through something tall,
such as a tree, house, or telephone pole. When the oppositelycharged leader and streamer connect, a powerful electrical current
begins flowing. A return stroke of bright luminosity travels about
60,000 miles per second back towards the cloud. A flash consists of
one or perhaps as many as 20 return strokes. We see lightning
flicker when the process rapidly repeats itself several times along
the same path. The actual diameter of a lightning channel is one-to
two inches.
• STEP LEADER - A path of ionized air which extends downward from
a thunderstorm during the initial stages of a lightning strike.
Multiple branches, or steps, travel downward until the final step
leader reaches the ground, a tall object on the ground, or a positive
streamer extending upward from a ground object. The lightning
strike begins when a large negative electric current flows along the
path defined by the step leaders from the thundercloud to the
ground.
• STREAMER - A part of a lightning bolt that rises from the ground
before a lightning strike. It is a column of ionized air formed by the
flow of electrons down into the ground target. The positive
streamer extends vertically upward until it meets a descending step
leader, at which point the air's resistance is overcome and an
electric current begins to flow along the path, resulting in a
lightning strike.
• A typical cloud-to-ground flash is a negative
stepped leader that travels downward through
the cloud, followed by an upward traveling return
stroke. The net effect of this flash is to lower
negative charge from the cloud to the ground.
Less common, a downward traveling positive
leader followed by an upward return stroke will
lower positive charge to earth. These positive
ground flashes now appear to be linked to certain
severe storms and are the focus of intense
research by scientists.
In-cloud lightning
Spider lightning
• What causes thunder?
• Lightning causes thunder. Thunder is the sound caused by rapidly
expanding gases along a channel of lightning discharge. Energy from
lightning heats the air to around 18,000 degrees Fahrenheit. This
causes a rapid expansion of the air, creating a sound wave heard as
thunder. An initial tearing sound is usually caused by the stepped
leader, and the sharp click or crack heard at a very close range, just
before the main crash of thunder, is caused by the ground streamer.
• Thunder is rarely heard at points farther than 15 miles from the
lightning discharge, but occasionally can be heard up to 25 miles
away. At these distances, thunder is heard as more of a low
rumbling sound because the higher frequency pitches are more
easily absorbed by the surrounding environment, and the sound
waves set off by the lightning discharge have different arrival times.
Winter
• Winter Weather Basics
• How do winter storms form?
• Just like any other storm at other times of the year, just the right
combination of ingredients is necessary for a winter storm to develop.
• Three basic ingredients are necessary to make a winter storm.
• Cold air – below freezing temperatures in the clouds and near the ground
are necessary to make snow and/or ice.
• Lift – something to raise the moist air to form the clouds and cause
precipitation. An example of lift is warm air colliding with cold air and
being forced to rise over the cold dome. The boundary between the warm
and cold air masses is called a front. Another example of lift is air flowing
up a mountainside.
• Moisture – to form clouds and precipitation. Air blowing across a body of
water, such as a large lake or the ocean, is an excellent source of moisture.
• Snow – Most precipitation that forms in
wintertime clouds starts out as snow because
the top layer of the storm is usually cold
enough to create snowflakes. Snowflakes are
just collections of ice crystals that cling to
each other as they fall toward the ground.
Precipitation continues to fall as snow when
the temperature remains at or below 0
degrees Celsius from the cloud base to the
ground.
• Snow Flurries – Light snow falling for short durations. No
accumulation or light dusting is all that is expected.
• Snow Showers – Snow falling at varying intensities for brief
periods of time. Some accumulation is possible.
• Snow Squalls – Brief, intense snow showers accompanied
by strong, gusty winds. Accumulation may be significant.
Snow squalls are best known in the Great Lakes Region.
• Blowing Snow – Wind-driven snow that reduces visibility
and causes significant drifting. Blowing snow may be snow
that is falling and/or loose snow on the ground picked up
by the wind.
• Blizzard – Winds over 35mph with snow and blowing snow,
reducing visibility to 1/4 mile or less for at least 3 hours.
• Sleet occurs when snowflakes only partially melt when they fall
through a shallow layer of warm air. These slushy drops refreeze as
they next fall through a deep layer of freezing air above the surface,
and eventually reach the ground as frozen rain drops that bounce
on impact.
• Freezing Rain occurs when snowflakes descend into a warmer layer
of air and melt completely. When these liquid water drops fall
through another thin layer of freezing air just above the surface,
they don't have enough time to refreeze before reaching the
ground. Because they are "supercooled," they instantly refreeze
upon contact with anything that that is at or below O degrees C,
creating a glaze of ice on the ground, trees, power lines, or other
objects. A significant accumulation of freezing rain lasting several
hours or more is called an ice storm
• Freezing Rain occurs when snowflakes descend
into a warmer layer of air and melt completely.
When these liquid water drops fall through
another thin layer of freezing air just above the
surface, they don't have enough time to refreeze
before reaching the ground. Because they are
"supercooled," they instantly refreeze upon
contact with anything that that is at or below O
degrees C, creating a glaze of ice on the ground,
trees, power lines, or other objects. A significant
accumulation of freezing rain lasting several
hours or more is called an ice storm
Crystals
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Extreme cold often accompanies a winter storm or is left in its wake. Prolonged exposure to the
cold can cause frostbite or hypothermia and become life threatening. Infants and elderly people are
most susceptible. What constitutes extreme cold and its effect varies across different areas of the
United States. In areas unaccustomed to winter weather, near freezing temperatures are
considered "extreme cold." Freezing temperatures can cause severe damage to citrus fruit crops
and other vegetation. Pipes may freeze and burst in homes that are poorly insulated or without
heat. In the north, below zero temperatures may be considered as extreme cold. Long cold spells
can cause rivers to freeze, disrupting shipping. Ice dams may form and lead to flooding.
Ice Storms
Heavy accumulations of ice can bring down trees, electrical wires, telephone poles and lines, and
communication towers. Communications and power can be disrupted for days while utility
companies work to repair the extensive damage. Even small accumulations of ice may cause
extreme hazards to motorists and pedestrians. Bridges and overpasses are particularly dangerous
because they freeze before other surfaces.
Heavy Snow Storms
Heavy snow can immobilize a region and paralyze a city, stranding commuters, stopping the flow of
supplies, and disrupting emergency and medical services. Accumulations of snow can collapse
buildings and knock down trees and power lines. In rural areas, homes and farms may be isolated
for days, and unprotected livestock may be lost. In the mountains, heavy snow can lead to
avalanches. The cost of snow removal, repairing damages, and loss of business can have large
economic impacts on cities and towns.
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Winter storms are considered deceptive killers because most deaths are indirectly related to the
storm. People can die in traffic accidents on icy roads, heart attacks while shoveling snow, or of
hypothermia from prolonged exposure to cold. Wind Chill is not the actual temperature but rather
how wind and cold feel on exposed skin. As the wind increases, heat is carried away from the body
at an accelerated rate, driving down body temperature. Animals are also affected by wind chill;
however, cars, plants and other objects are not.
Frostbite – Frostbite is damage to body tissue caused by extreme cold. A wind chill of -20 degrees F
will cause frostbite in just 30 minutes. Frostbite causes a loss of feeling and a white or pale
appearance in extremities, such as fingers, toes, ear lobes, or the tip of the nose. If symptoms are
detected, get medical help immediately. If you must wait for help, slowly re-warm affected areas.
However, if the person is also showing signs of hypothermia, warm the body core before the
extremities.
Hypothermia (low body temperature) – Warning signs of hypothermia include uncontrollable
shivering, memory loss, disorientation, incoherence, slurred speech, drowsiness, and apparent
exhaustion. Take the person's temperature, and if it is below 95 degrees F, immediately seek
medical care. If medical care is not available, begin warming the person slowly. Warm the body core
first, and use your own body heat to help, if necessary. Get the person into dry clothing and wrap
them in a warm blanket covering the head and neck. Do not give the person alcohol, drugs, coffee,
or any hot beverage or food; warm broth is better. Do not warm extremities (arms and legs) first!
This drives the cold blood toward the heart and can lead to heart failure.
Winds
• Types of damaging winds
• Straight-line winds – a term used to define any thunderstorm wind
that is not associated with rotation, and is used mainly to
differentiate from tornadic winds.
• Downdrafts – A small-scale column of air that rapidly sinks toward
the ground. A downburst is a result of a strong downdraft.
• Downbursts – A strong downdraft with horizontal dimensions larger
than 4 km (2.5 mi) resulting in an outward burst or damaging winds
on or near the ground. (Imagine the way water comes out of a
faucet and hits the bottom of the sink.) Downburst winds may
begin as a microburst and spread out over a wider area, sometimes
producing damage similar to a strong tornado. Although usually
associated with thunderstorms, downbursts can occur with showers
too weak to produce thunder.
• Microbursts – A small concentrated downburst that produces an
outward burst of damaging winds at the surface. Microbursts are
generally small (less than 4km across) and short-lived, lasting only
5-10 minutes, with maximum windspeeds up to 168 mph. There are
two kinds of microbursts: wet and dry. A wet microburst is
accompanied by heavy precipitation at the surface. Dry
microbursts, common in places like the high plains and the
intermountain west, occur with little or no precipitation reaching
the ground.
• Gust front – A gust front is the leading edge of rain-cooled air that
clashes with warmer thunderstorm inflow. Gust fronts are
characterized by a wind shift, temperature drop, and gusty winds
out ahead of a thunderstorm. Sometimes the winds push up air
above them, forming a shelf cloud or detached roll cloud.
Microburst
Gustfront
• Derecho – A derecho is a widespread thunderstorm wind event caused
when new thunderstorms form along the leading edge of an outflow
boundary (a surface boundary formed by the horizontal spreading of
thunderstorm-cooled air). The thunderstorms feed on this boundary and
continue to reproduce themselves. Derechos typically occur in the
summer months when complexes of thunderstorms form over the plains
and northern plains states. Usually these thunderstorms produce heavy
rain and severe wind reports as they rumble across several states during
the night. The word "derecho" is of Spanish origin and means "straight
ahead". They are particularly dangerous because the damaging winds can
last a long time and can cover such a large area.
• Bow Echo – A radar echo which is linear but bent outward in a bow shape.
Damaging straight-line winds often occur near the "crest" or center of a
bow echo. Bow echoes can be over 300km in length, last for several hours,
and produce extensive swaths of wind damage at the ground.
Doppler Radar
• The type of damaging winds most dangerous to aviation, especially
during landing and take-off, is the type spawned by a microburst in
an isolated rain shower or thunderstorm. Downbursts or
microbursts are mainly known for their ability to produce wind
shears which can slow airspeed and cause aircraft to lose altitude at
a very critical time for flight near the ground. A plane will encounter
strong headwinds followed by strong tailwinds as it enters and flies
through a microburst. Only 27 people survived the Delta Flight 191
crash caused by a microburst that occurred in 1986 in Dallas-Fort
Worth. However, great strides have been made in understanding
and avoiding the risk from low altitude wind shear. Part of this
success has been due to the progress made in detecting and
distributing real-time information regarding these hazards.
http://wxmaps.org/
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A. Why Do They Exist?
Infrared satellite image of North America
Our spinning planet is covered with moving air. Some of this air is over the ocean and some is over
the land. The ground gets warm faster than water, but also gets cold faster than water. That is why
it is normally colder over land in the winter, and warmer over land in the summer. When the ocean
is warmed by the sun, the water absorbs heat and warms the air above it.
At the same time, the land can become cold and cool the air above it. Because of heating and
cooling, air can move up and down. When a pocket of air gets too warm, it rises until it is the same
temperature as the surrounding air. Just the opposite happens when a pocket of air gets cooled, it
sinks. The combination of warm and cold areas, and rising and sinking air results in what we call
weather systems.
B. How Do They Move?
Areas of high pressure, called Highs, have air that sinks. This air tends to be cold and produces clear
skies and fair weather. Areas of low pressure, called Lows, have warm air that rises. They can cause
clouds to form, rain to fall, and storms to occur. Weather systems in the United States and within
the mid-latitudes, between 30° N and 60° N and between 30° S and 60° S, usually move from West
to East. Weather systems in other latitudes move from East to West
Hurricanes
Hurricane Strike