Thunderstorms -Basics

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THUNDERSTORM -BASICS
By
Prof A. Balasubramanian
CENTRE FOR ADVANCED STUDIES IN
EARTH SCIENCE
UNIVERSITY OF MYSORE
MYSORE-6

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Introduction:
A thunderstorm is a rain shower during which
you hear thunder.
Since, the thunder comes from lightning, all
thunderstorms have lightning.
A thunderstorm is the result of convection.
A thunderstorm is classified as “severe” when it
contains one or more of the following: hail one
inch or greater, winds gusting in excess of 50
knots (57.5 mph), or a tornado.

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Occurrence:
Thunderstorms are most likely to occur in the
spring and summer months and during the
afternoon and evening hours, but they can occur
year-round and at all hours.
Thunderstorms frequently occur in the late
afternoon and at night in the Plains states.
Locations of severe thunderstorms :
The greatest severe weather threat in the U.S.
extends from Texas to southern Minnesota.

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Origin of a thunderstorm form:
Three basic ingredients are required for a
thunderstorm to form:
moisture,
rising unstable air (air that keeps rising when
given a nudge), and
a lifting mechanism to provide the “nudge.”
The sun heats the surface of the earth, which
warms the air above it.

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If this warm surface air is forced to rise , it will
continue to rise as long as it weighs less and
stays warmer than the air around it.
As the air rises, it transfers heat from the
surface of the earth to the upper levels of the
atmosphere (the process of convection).
The water vapor it contains begins to cool,
releases the heat, condenses and forms a cloud.
The cloud eventually grows upward into areas
where the temperature is below freezing.

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As a storm rises into freezing air, different types
of ice particles can be created from freezing
liquid drops.
The ice particles can grow by condensing vapor
(like frost) and by collecting smaller liquid
drops that haven't frozen yet (a state called
"supercooled"). When two ice particles
collide, they usually bounce off each other, but
one particle can rip off a little bit of ice from the
other one and grab some electric charge.

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Lots of these collisions build up big regions of
electric charges to cause a bolt of lightning,
which creates the sound waves we hear as
thunder.
Thunderstorms are a great way for the
atmosphere to release energy.
When warm moist air meets colder drier air, the
warm air rises, the water vapor condenses in the
air, and forms a cloud.

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As the water vapor condenses it releases heat,
which is a form of energy.
A large amount of the thunderstorm's energy
comes from the condensation process that forms
the thunderstorm clouds.
As the thunderstorm progresses, eventually the
rain cools the entire process down and the
energy is gone. Thunderstorms also help keep
the Earth in electrical balance.

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The Earth's surface and the atmosphere conduct
electricity easily – the Earth is charged
negatively and the atmosphere, positively.
There is always a steady current of electrons
flowing upwards from the entire surface of the
earth. Thunderstorms help transfer the negative
charges back to earth (lightning is generally
negatively charged).

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Without thunderstorms and lightning, the earth-
atmosphere electrical balance would disappear
in five minutes.
Sources of moisture:
Typical source of moisture for thunderstorms
are the oceans.
Instability:
Air is considered unstable if it continues to rise
when given a nudge upward (or continues to
sink if given a nudge downward).

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An unstable air mass is characterized by
warm moistair near the surface and cold dry air
aloft.
In these situations, if a bubble or parcel of air is
forced upward it will continue to rise on its
own.
As this parcel rises it cools and some of the
water vapor will condense forming the familiar
tall cumulonimbus cloud that is the
thunderstorm.

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Sources of Lift (upward):
Typically, for a thunderstorm to develop, there
needs to be a mechanism which initiates the
upward motion, something that will give the air
a nudge upward.
This upward nudge is a direct result of air
density.
Some of the sun's heating of the earth's surface
is transferred to the air which, in turn, creates
different air densities.

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The propensity for air to rise increases with
decreasing density.
This is difference in air density is the main
source for lift and is accomplished by several
methods.
Differential Heating:
The sun's heating of the earth's surface is not
uniform. For example, a grassy field will heat at
a slower rate than a paved street.

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A body of water will heat slower than the
nearby landmass.
This will create two adjacent areas where the air
is of different densities.
The cooler air sinks, pulled toward the surface
by gravity, forcing up the warmer, less dense
air, creating thermals.

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Fronts, Drylines and Outflow Boundaries:
Fronts are the boundary between two air
masses of different temperatures and therefore
different air densities.
The colder, more dense air behind the front lift
warmer, less dense air abruptly.
If the air is moist thunderstorms will often form
along the cold front.

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Drylines are the boundary between two air
masses of different moisture content and divides
warm, moist air from hot, dry air.
Moist air is less dense than dry air.
Drylines therefore act similarly to fronts in that
the moist, less dense air is lifted up and over the
drier, more dense air.
The air temperature behind a dryline is often
much higher due to the lack of moisture.

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That alone will make the air less dense but the
moist air ahead of the dryline has an even lower
density making it more buoyant. T
he end result is air lifted along the dryline
forming thunderstorms.
This is common over the plains in the spring
and early summer.
Outflow boundaries are a result of the rush of
cold air as a thunderstorm moves overhead.

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The rain-cooled, more dense, air acts as a "mini
cold front", called an outflow boundary.
Like fronts, this boundary lifts warm moist air
and can cause new thunderstorms to form.
Terrain: As air encounters a mountain it is
forced up because of the terrain.
Upslope thunderstorms are common in the
Rocky Mountain west during the summer.

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The Thunderstorm Life Cycle:
Thunderstorms have three stages in their life
cycle: The developing stage, the mature stage,
and the dissipating stage.
The developing stage of a thunderstorm is
marked by a cumulus cloud that is being pushed
upward by a rising column of air (updraft). The
cumulus cloud soon looks like a tower (called
towering cumulus) as the updraft continues to
develop.

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There is little to no rain during this stage but
occasional lightning.
The thunderstorm enters the mature stage when
the updraft continues to feed the storm, but
precipitation begins to fall out of the storm,
creating a downdraft (a column of air pushing
downward).
When the downdraft and rain-cooled air spreads
out along the ground it forms a gust front, or a
line of gusty winds.

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The mature stage is the most likely time for
hail, heavy rain, frequent lightning, strong
winds, and tornadoes.
Eventually, a large amount of precipitation is
produced and the updraft is overcome by the
downdraft beginning the dissipating stage.
Rainfall decreases in intensity, but lightning
remains a danger.

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Shape of thunderstorms:
Thunderstorms may look like tall heads of
cauliflower or they can have “anvils.”
An anvil is the flat cloud formation at the top of
the storm.
An anvil forms when the updraft (warm air
rising) has reached a point where the
surrounding air is about the same temperature
or even warmer.

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The cloud growth abruptly stops and flattens
out to take the shape of an anvil.
Thunderstorm Types :
1. Single-Cell Thunderstorms
2. Multi-Cell Storm
3. Squall Line
4. Supercell
5. Bow Echo
6. Derecho

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Often called “popcorn” convection, single-cell
thunderstorms are small, brief, weak storms
that grow and die within an hour or so.
They are typically driven by heating on a
summer afternoon. Single-cell storms may
produce brief heavy rain and lightning.
A multi-cell storm is a common, garden-
variety thunderstorm in which new updrafts
form along the leading edge of rain-cooled air
(the gust front).

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Individual cells usually last 30 to 60 minutes,
while the system as a whole may last for many
hours.
Multicell storms may produce hail, strong
winds, brief tornadoes, and/or flooding.
A squall line is a group of storms arranged in a
line, often accompanied by “squalls” of high
wind and heavy rain.
Squall lines tend to pass quickly and are less
prone to produce tornadoes than are supercells.

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They can be hundreds of miles long but are
typically only 10 or 20 miles wide.
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.

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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.
A supercell is an often-dangerous thunderstorm
with a very organized internal structure
including a rotating updraft that allows it to
keep going for up to several hours.

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Supercells are capable of producing severe
weather including high winds, large hail, and
strong tornadoes.
They are most frequently isolated and often
develop in the warm air ahead of a squall line.
A supercell also usually forms in an
environment with strong vertical wind shear
that causes the updraft to begin rotating.
A “bow echo” is a radar signature of a squall
line.

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How many thunderstorms are there?
Worldwide, there are an estimated 16 million
thunderstorms each year, and at any given
moment, there are roughly 2,000 thunderstorms
in progress.
There are about 100,000 thunderstorms each
year in the U.S. alone. About 10% of these
reach severe levels.

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SCALE OF THUNDERSTORMS:
A Mesoscale Convective System (MCS) is a
collection of thunderstorms that act as a system.
An MCS can spread across an entire state and
last more than 12 hours.
On radar one of these monsters might appear as
a solid line, a broken line, or a cluster of cells.

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A derecho (pronounced similar to “deh-REY-
cho” in English) is a widespread, long-lived
wind storm that is associated with a band of
rapidly moving showers or thunderstorms.
Although a derecho can produce destruction
similar to that of tornadoes, the damage
typically is directed in one direction along a
relatively straight swath.

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As a result, the term “straight-line wind
damage” sometimes is used to describe derecho
damage.
By definition, if the wind damage swath extends
more than 240 miles (about 400 kilometers) and
includes wind gusts of at least 58 mph (93
km/h) or greater along most of its length, then
the event may be classified as a derecho.

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Thunderstorm Detection-Weather Satellites:
We can see thunderstorms with a variety of
tools. Most areas of Earth can be seen by
weather satellites.
Satellites take pictures of Earth at regular
intervals from space, telling us where clouds are
located. Meteorologists watch these pictures
over time to watch for rapidly growing clouds, a
clue to a possible thunderstorm.

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Satellites also can tell us the temperature of the
clouds.
Clouds with cold tops are usually very high up
in the atmosphere, and could mean the cloud is
tall enough to be a thunderstorm.
Meteorologists also track how these clouds
move to see what areas will be affected by the
storm next.

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RADARS:
Weather radar is very important to
meteorologists because it can detect rain and
severe weather even when it is cloudy or dark.
Doppler radar sends out electromagnetic wave
fields that can be reflected back to the radar by
things in the air like precipitation.
The amount of energy that is reflected back can
tell us how heavy the rain might be or tell us
there is hail.

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Doppler radar can also show us how the wind is
blowing near and inside the storm.
This is helpful in understanding what kinds of
hazards the thunderstorm might have (tornado,
microburst, gust fronts, etc.) associated with it.
It also helps us understand how the
thunderstorm is feeding itself.

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Thunderstorm Forecasting:
The National meteorology Department
monitors and forecasts the potential for severe
weather over the nation and will give critical
information concerning the threat of severe
weather at your location.

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COMPUTER FORECAST MODELS
Meteorologists often rely on massive computer
programs called numerical weather prediction
models to help them decide if conditions will be
right for the development of thunderstorms.
These models are designed to calculate what the
atmosphere will do at certain points over a large
area, from the Earth's surface to the top of the
atmosphere.

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Data is gathered from weather balloons
launched around the globe twice each day, in
addition to measurements from satellites,
aircraft, ships, temperature profilers and surface
weather stations.
The models start with these current weather
observations and attempt to predict future
weather using physics and dynamics to
mathematically describe the atmosphere's
behavior.

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The predictions are usually output in text and
graphics (mostly maps).
ENSEMBLE FORECASTING:
Another technique is to run the same model
several times with varying starting weather
conditions. This approach results in a number of
predictions that produce a range of possible
future weather conditions.

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Forecasters use their experience, knowledge,
persistence (what makes us think the weather is
going to change from what it is now?) and eyes
(looking out the window!) to fine-tune their
forecasts.
SATELLITES:
Satellites are critical in short-term forecasting.
Satellite images can give an early indication of
a developing thunderstorm by showing where
cumulus clouds are forming.

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Cumulus clouds grow rapidly into
cumulonimbus clouds if conditions are right,
and you can track their growth using satellite
images. Satellites also give clues about what
type of thunderstorm may be developing.
What direction does the wind come from
during a thunderstorm?
There is no one direction the wind comes from
when thunderstorms or tornadoes occur.

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Kinds of damage thunderstorms may cause:
Many hazardous weather events are associated
with thunderstorms.
Under the right conditions, rainfall from
thunderstorms causes flash flooding, killing
more people each year than hurricanes,
tornadoes or lightning.
Lightning is responsible for many fires around
the world each year, and causes fatalities.

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Hail up to the size of softballs damages cars and
windows, and kills livestock caught out in the
open.
Strong (up to more than 120 mph) straight-line
winds associated with thunderstorms knock
down trees, power lines and mobile homes.
Tornadoes (with winds up to about 300 mph)
can destroy all but the best-built man-made
structures.

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Why are severe thunderstorms so
dangerous?
Rainfall from thunderstorms causes flash
flooding, killing more people each year than
hurricanes, tornadoes or lightning.
Lightning is responsible for many fires around
the world each year, and cause fatalities.
Hail up to the size of softballs damages cars and
windows, and kills wildlife caught out in the
open.

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Strong (up to more than 120 mph) straight-line
winds associated with thunderstorms knock
down trees, power lines and mobile homes.
Tornadoes (with winds up to about 300 mph)
can destroy all but the best-built man-made
structures.

Thunderstorms -Basics

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