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ESCI 106 – Weather and Climate
Lecture 10
Chapter 10: “Thunderstorms
and Tornadoes”
10-27-2011
Jennifer D. Small 
Weather Fact of the Day: October 27
 2006: A F1 tornado (waterspout) came ashore and
caused significant damage on the west side of
Apalachicola, FL.
 Marina boasts sank
 4 homes destroyed
 Part of a hospital roof collapsed.
 100s of trees and power lines downed.
 Fortunately, no serious injuries.
National Watches and Warnings
National Watches and Warnings
Learning Goals for Today
1. Understand how and why thunderstorms
form.
2. Understand how and why tornadoes form
3. Understand how and why hurricanes
form
Thunderstorms and
Tornadoes!
Thunderstorms - Intro
 A storm that generates
lightning and thunder.
 Are characterized by strong
“up” and “down” motions
 Frequently produces gusty
winds, heavy rain and hail.
Thunderstorms - Intro
 They can form on their own
 They can form in conjunction
with mid-latitude cyclone
 Edge of a Cold-Front
 The can form in conjunction
with hurricanes
Thunderstorms - Intro

Florida has the most thunderstorms

80-100 on average per year!

Commonly occur in the foothills of
the Rockies

Associated with Tornado Alley in
the middle of the country

Associated with drylines and air with
different humidities
Thunderstorms - Intro

Form when warm, humid air rises
in an unstable environment
 There are two categories:
1. Air Mass Thunderstorms – formed
by unequal heating of the Earth’s
surface within a maritime tropical air
mass.
2. Severe Thunderstorms – formed
by unequal heating & lifting of warm
air along a front or mountain.
Airmass Thunderstorms





Happens inside an air mass (usually mT)
Usually in spring and summer
Usually in mid-afternoon
Not associated with a front
Due to local differences in temperature
Airmass Thunderstorms
 Like mid-latitude cyclones T-Storms have
a “Life Cycle”
 Stages of Development
 Stage 1: Cumulus Stage
 Stage 2: Mature Stage
 Stage 3: Dissipating Stage
Airmass Thunderstorms
Stage 1
Stage 2
Stage 3
Stage 1: Cumulus stage

Rising air only

Makes a cloud

Cloud gets bigger and bigger

Moisture is being added to higher and
higher altitudes

Needs a continuous supply of moisture
to survive
Airmass Thunderstorms
Stage 1
Stage 2
Stage 3
Stage 2: Mature Stage

Precipitation forms


Rain begins to fall


Associated with the downdraft (air going
down)
Most active stage


Mainly by the Bergeron Process
Gusty winds, lightning, heavy rain, hail
Updrafts and downdrafts exist side
by side
Stage 2: Mature Stage

Downdrafts form for two reasons

Entrainment – mixing of dry air with cloud
air at the edge of the cloud






Causes the cloud drops to evaporate
Energy from evaporation comes from the
temperature of the air parcel
By evaporating drops, you use up the
energy and end up cooling the air parcel
Temperature drops, parcel cools
Cooler air sinks to the surface
Drag – Air is dragged downwards as
precipitation falls.
Airmass Thunderstorms
Stage 1
Stage 2
Stage 3
Stage 3: Dissipating Stage

Cooling effect of falling precipitation
and influx of colder air up top mark
it’s END!

Downdraft cuts off updraft

Cloud stops growing

You’re left with weakly descending air
parcels

The cloud basically KILLS itself
Airmass Thunderstorms - Summary
Stage 1
Stage in which updrafts
dominate throughout the
cloud, and growth from a
cumulus to a
cumulonimbus occurs.
Stage 2
The most intense phase,
with heavy rain and
possibly small hail, in
which downdrafts are
found side by side with
updrafts
Stage 3
Dominated by downdrafts
and entrainment, causing
evaporation of the
structure.
Severe Thunderstorms
 Heavy downpours, flooding,
gusty straight-line winds,
large hail, lightning and
tornadoes!
 To be classified as Severe:
 Must have winds in excess of
93 km/h or 58 mph
 or hail 0.75 inches
 or a confirmed tornado
Severe Thunderstorms
 Persist for many hours
 Vertical wind sheer causes
precipitation to fall in
downdraft, allowing the
updraft to retain strength.
 Sometimes the updraft is so
strong you get overshooting
tops and anvils!
Severe Thunderstorms
Severe Thunderstorms
 Cold air of the downdrafts
spread out making
 “mini cold front”
 Called a GUST FRONT
 Called an Outflow Boundary
 Can form a Roll Cloud.
Supercell Thunderstorms
 Causes dangerous weather
 2000-3000 a year
 Large, very powerful, up to 20
km in height
 Last many hours
 20-50 km in diameter
Supercell Thunderstorms
 Vertical wind profile may
cause the updraft to ROTATE!
 MESOCYCLONE – usually
spawns Tornadoes!
 Need a lot of Latent Heat
 Requires moist troposphere
 An inversion a couple of km above
the surface (caps moisture)
 Unstable air can break through the
inversion by “eroding” it via
mixing.
Squall Lines
 Develop in the warm sector
of a Mid-Latitude Cyclone
 100-300 in ADVANCE of the
cold front.
 Linear in shape
 Can last for 10 hours or
more
Squall Lines

Sometimes preceded by
mammatus clouds

Form when the divergence and
resulting lift created by the jet
stream is aligned with strong
persistent low level flow of humid
air from the south.

Also can form on drylines
Mesoscale Convective Complexes
 Consist of many individual
T-storms organized into a
large oval or circular
cluster.
 MCCs are BIG
 At least 100,000 sq km in
area (39,000 sq miles)
 MCCs are SLOW
 Can persist for up to 12 hours
Mesoscale Convective Complexes

Associated with tornadoes

Typically form in the Great
Plains from a group of
afternoon air-mass T-Storms.

To transform from air-mass TStorm to MCC you need lots of
low level warm and moist air.
Microbursts
 Localized downdraft
 About 2.5 miles (4 km)
across
 Downdraft is accelerated by
evaporative cooling
 Typically last 2-5 minutes
Microbursts
 Outflow at the surface
can move in excess of
160 km/hr (100 mph).
 Microbursts can cause
quite a bit of destruction.
 Tress, homes, aircraft…
Lightning and Thunder
 A storm is classified as a
thunderstorm only after
thunder is heard!!
 Because thunder is
produced by lightning,
lightning must also be
present!
 We’ll talk about both
Lightning and Thunder!
Lightning
 Lightning is a discharge
of electrical energy.
 Essentially a giant “spark”
between regions of
positive (+) and negative
(-) charge.
Lightning
 May occur:
 Between cells in the same
storm (inter-cloud lightning)
 Within a cloud (intra-cloud
lightning)
 Cloud to air
 Cloud to ground (CG)
Lightning Formation
 Lightning forms when a
charge separation occurs
in a cloud
 The earth trying to equalize
the electrical difference
 Negative charges want to
flow to the ground.
Charge Separation


We don’t exactly know why it
happens
One theory:

Hail stones tend to have a warmer
surface than ice crystals

When warm hail collides with colder
ice, electrons transfer from ice to hail

Hail (-) is bigger and heavier and
settles toward the bottom of the cloud

Smaller (+) ice crystals are lofted to
the top.
Lightning Formation
 Excess electrons cause the
air to ionize
 Rip molecules apart (N2 or O2
become N+, O -…)
 Air is normally very insulating
 Ionized air is very conducting
(like a metal wire)
Lightning Formation
 Ionized air forms tubes of
~50 m in length (150 ft) and
10 cm in diameter (4 in)
 Called a LEADER
 A bunch of leaders that are
connected is called a STEPLEADER.
 NOTE: There still hasn’t been a
flash yet!!!
Lightning Formation

Each electron contains LOTS of
energy

When it reaches the surface the
energy is RELEASED

This energy is RELEASED as
HEAT!!

Electrons are drained at the
surface first so the FLASH
starts at the ground!!!
Lightning Formation

AGAIN: Electrons are
drained at the surface first
so the FLASH starts at
the ground!!!

This is why lightning is said
to GO UP, rather than
down

The movement is STILL
from CLOUD to GROUND

But the “heating” happens at
the surface FIRST.
Lightning Formation

The electrons closest to the
Earth are “discharged” first

Heat that is released causes
air around a step-leader to
reach 8,000-30,000 K!!!!!

The warmer, the shorter the
shorter the wavelength
30000 K
8000 K
Lightning Formation

The emission of radiation from very
hot air leads to the VISIBLE flash of
lightning.

Sometimes after the 1st flash you
have a left over channel that can be
recharged very quickly.

Can have several (3-4) in rapid
succession (1/10 of a second
apart). (lead by a DART LEADER)
Lightning
Lightning Occurrence in US
Thunder

When air is heated quickly a shockwave forms 
causes thunder

Similar to planes crossing the sound barrier, firecrackers
and gun shots

Sound travels at 330 m/s or 1000ft/s

If thunder takes 3 seconds to happen after observing
lightning then the storm is:

3 s * 1000 ft/s = 3000 ft away (6/10 of a mile away).
Thunder

A good “rule of thumb”:

The storm is 1 mile away for very 5 seconds in between lightning
and thunder.

NOTE: Thunder actually happens at the same time the lightning
strikes, but you will hear a delay because light travels much
faster than sound!!
Tornadoes - Introduction

A tornado is a rapidly rotating
narrow region of low pressure

Wind speeds from 70-300 mph

Pressure can be as low as 900 mb

Tornadoes form during intense
thunderstorms.
Tornado Development
 Step 1: The first object that forms is a
rotating body of air at the ground
 This occurs because of vertical wind sheer
a) Wind speeds are higher as
you increase in altitude
b) Drag and Friction: Friction
and drag at the surface cause
the air at the ground to move
more slowly.
Tornado Development
 Step 2: Horizontal rotating air is lifted off the
ground by the Updraft of a Thunderstorm
Once the horizontal
rotating air mass is
lifted nearly vertically
it is considered a
MESOCYCLONE!
Tornado Development
 Step 3: Mesocyclone is fully developed in
the updraft of a thunderstorm
 If a tornado develops it descends from the
slowly rotating “wall cloud” in the lower part of
the cloud.
Mesocyclone Review
 Remember it is a vertical
cylinder of rotating air,
typically 3-10 km across
(2-6 miles)
 Develops in the updrafts of
severe T-Storms
 Usually precedes tornadoes
by 30 min
Mesocyclone Review
 Stretching of the
mesocyclone column causes
faster rotation
 Just like a figure skater!!
 Something that begins with a
larger diameter rotating at a slow
speed begins to rotate faster as
the tube is elongated and the
diameter decreases
 No one knows why this happens!
Thunderstorm + Tornado
 From the wall cloud a very
narrow, fast rotating
structure emerges.
 This forms a funnel cloud
(as long as the cloud does
not touch the ground)
 As soon as the funnel cloud
touches the ground it is
called a tornado.
Thunderstorm + Tornado
 Some tornadoes have multiple suction vortices
 Intense areas of high winds that are part of
ONE tornado
There can be 4-6
suction vortices.
The stronger the
tornado the more
vortices you’re likely to
get.
Weak tornadoes usually
don’t have them
Tornado Occurrence in US
TORNADO
ALLEY!
Tornado Classification
 Tornados are classified as
“weak,” “strong,” and “violent”
 WEAK – usually rope like and
narrow
 STRONG – usually the classic
funnel shape
 VIOLENT – usually have lots of
debris associated with them and
can be quite dark in color.
Fujita Scale
 Scale for Damage caused by tornadoes