ESCI 106 – Weather and ESCI 106 – Weather and ClimateClimate

Lecture 10Lecture 10

Chapter 10:Chapter 10: “Thunderstorms and “Thunderstorms and

Tornadoes”Tornadoes”

10-27-201110-27-2011

Jennifer D. Small 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 Thunderstorms and Tornadoes!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 Mainly by the Bergeron Process

Rain begins to fall Associated with the downdraft (air going

down)

Most active stage Gusty winds, lightning, heavy rain, hail

Updrafts and downdrafts exist side by side

Stage 2: Mature Stage

DowndraftsDowndrafts 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 2 Stage 3Stage in which updrafts dominate throughout the cloud, and growth from a cumulus to a

cumulonimbus occurs.

The most intense phase, with heavy rain and possibly small hail,

in which downdrafts are found side by side

with updrafts

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 T-Storm 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 STEP-LEADER.

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

8000 K30000 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

TORNADOALLEY!

Tornado Classification

Tornados are classified as “weak,” “strong,” and “violent”

WEAKWEAK – usually rope like and narrow

STRONGSTRONG – usually the classic funnel shape

VIOLENTVIOLENT – usually have lots of debris associated with them and can be quite dark in color.

Fujita Scale Scale for Damage caused by tornadoes