THUNDERSTORMS

(Hailstorms and Tornadoes)

Thunderstorm Facts and FiguresThunderstorms are narrow, towering storms, 5-14 miles (8 to 22 km) high and only about 5 to 25 miles (8 to 40 km) wide. Driven by the large scale winds aloft, they seldom last long. Typically, the storm’s intensity rises to a quick peak, with cold, gusty winds, intense lightning and potentially torrential precipitation, which lasts about 10-15 minutes before gradually subsiding and usually ending within an hour.

Most thunderstorms pass by without causing significant damage or death. Violent downbursts, large hail, and tornadoes are almost exclusively confined to severe thunderstorms. Thunderstorms mete out death by three principal means--lightning, tornadoes, and floods. Lightning kills about 100 people annually in the United States. Tornadoes, despite their extreme violence, now claim an average of only 50 lives annually in the United States due to the high level of public awareness and the National Weather Service’s outstanding warning system. Flash floods, resulting from torrential downpours in normally dry or placid streams, catch more people by surprise and cause almost 200 deaths annually in the United States.

Thunderstorms are as common as popcorn. At any moment, about 2000 are exploding worldwide. A day's tally is about 45,000; a year’s total, 16 million. Their energy is immense--an average thunderstorm releases enough energy (1 trillion watts) to power 40 million homes during the half hour it lasts. Unfortunately, there is no way to tap this energy.

Thunderstorms occur most frequently in the tropics because sunlight is intense all year. They also prefer land, which heats up more rapidly than water. Thus the saying, “Convection craves continents and lightning loves land!” Africa, which straddles the equator, is the world thunderstorm capital, and their electrical effects are conducted around the world. Very few places outside the tropics get more than 100 thunderstorms a year, but at Bogor on the island of Java thunder is heard on 322 days a year.

In most of the middle latitudes, thunderstorms are most common in late spring and summer and least common in winter. In the United States the two areas most favored for thunderstorms are Florida, where humid sea breezes converge from two sides, and the high plains of Colorado and New Mexico. Thunderstorms are rare in polar latitudes and almost never form over the ice caps.

In many inland locations thunderstorms peak in late afternoon, shortly after the hottest time of the day. But as the land cools at night, thunderstorms often either move downwind from mountain ranges, following the convergence of mountain-valley breezes, or form over warm coastal waters.

All thunderstorms contain strong updrafts of buoyant, humid air and share three common features,

1. A thick layer with a conditionally unstable lapse rate (SALR < LR < DALR).

2. Adequate moisture at low levels.

3. A lifting mechanism (provided by outflow boundaries from other thunderstorms, large scale winds, sea breeze fronts, etc.).

Average Annual Number of Days with Thunderstorms

Average Annual Number of Days with Hail

http://science.nasa.gov/headlines/y2001/ast05dec_1.htm

World Map of Lightning Strike Frequency

TYPES OF THUNDERSTORMSThere are several types of thunderstorms. In order of increasing severity, they are

1. Single cell, air mass thunderstorms.2. Multicell, air mass thunderstorms.3. Squall-line thunderstorms. 4. Supercell thunderstorms.

Severe storms usually form in highly unstable atmospheres with favored settings (such as mountains, islands, or east of cold fronts and troughs in the jet stream), and significant vertical wind shear. Vertical wind shear adds to intensity by tilting or moving the updraft so that hydrometeors do not fall back into it and weigh it down or choke it on its own precipitation. Air Mass storms form when there is little vertical wind shear. Squall line storms form if there is significant shear, but the direction of the wind does not change with height. Supercell storms form when the wind speed both increases and turns with height (helicity), so that the updraft moves like a helix or corkscrew.

The next slide relates the type of thunderstorm to the shear and degree of instability.The 3 slides after that show the 3 stages in the life cycle of an air mass thunderstormcell.

1. Cumulus (Youth) (No rain yet – all updrafts)2. Mature – the most intense and violent weather3. Dissipating (Old Age)

Thunderstorm Type as a Function of Instability and Shear

CA

PE

Single Cell Air Mass

MultiCell Air Mass

SuperCell

Squall Line

Bow Echoes

Derecho

0 40 80 SHEAR (knots) below 2.5 km AGL

(Above Ground Level)

5000

4000

3000

2000

1000

0

JET

Wind Shear

Shear = Change of wind speed with height

Helicity = Screw-like motion (shear + turning)

Air Mass Thunderstorm Life Cycle

The typical lifespan of a cell of an air mass thunderstorm lasts about half an hour. It consists of 3 stages:

1. Cumulus (Youth)

2. Mature

3. Dissipating (Old Age)

The Cumulus StageThe cumulus stage consists entirely of updrafts and all air near ground level is hot. There is no precipitation at this point in the young storm because cloud droplets and crystals are tiny but growing and accumulating. The top fringe of the cloud is distinct and closely resembles a cauliflower.

The Mature Stage

This is the stage with the most violent weather. The weight of precipitation and cooling by evaporation as the rain falls into the dry air surrounding the updraft cools and weighs the air, causing strong downdrafts of cold, rain-laden air. The downdraft splays out at the ground and shoves the warm air near the ground upward, intensifying the updraft. But the cold air outflow and shading of the ground due to the spreading anvil combine to eliminate the nearby source of hot air, so the storm produces the seeds of its own destruction.

The Dissipating Stage

This resembles old age. The nearby source of hot air has been entirely used up so that the storm consists largely of weak downdrafts and light precipitation. The only updraft is a remnant confined near the top of the cloud. The downdrafts help evaporate the cloud from bottom upward, so that the last thing left is the anvil, which has a fuzzy appearance.

The sun may come out again and start the process over if it is early enough in the afternoon.

Squall Line Thunderstorm StructureAs the storm advances toward the right, mammatus are seen first under the anvil. Wind becomes strong as temperature falls and the sky darkens as an arcus or roll cloud passes overhead and darkens. If wind shear is large enough the violent outflow forms a long line called a derecho. Heavy rain, possibly with small hail follow. Tornadoes are uncommon.

23 July 2010 near Vivian, South Dakota Record Hailstone storm ChaseTheStorms.com

See also Youtube Shelf Cloud Comes Ashore and Gust Front Videos

1900 UTC 03 May 2009

Radar echoes of an advancing squall line. Note, that the most intense precipitation and violent weather occur first. The storms often finish with a long period of gentler winds and stratiform rain and slowly rising temperature.

2000 UTC 03 May 2009

SQUALL LINETHUNDERSTORM

Thunderstorms (often in Squall Lines) form in Unstable Air ahead of Cold Fronts

The advancing cold air mass acts as a wedge, forcing the warm, unstable air aloft. The pulse of rising air propagates to the east like a wave, ahead of the cold front, producing a squall line. Sometimes, the southernmost thunderstorm becomes a Supercell.

Supercell Thunderstorms

jolsen@dutton-lainson.com Image © Jorn Olsen

When dry air surrounds thunderstorm anvils, ice crystals and droplets in the cloudy air at the base of the anvil begins to evaporate. This cools the air, which sinks in breast-like pouches called mamma or mammatus.

Mammatus

Chuck Doswell www.cimms.ou.edu/~doswell/photo.html

Supercell thunderstorms often rotate and give birth to tornadoes

Warm, moist air from S (Gulf of Mexico)

Hot, dry air from SW (Mexico)

Cold, dry air from W

http://www.nssl.noaa.gov/primer/tornado/tor_basics.html

Supercell Structure

UPDRAFT CORE

Sorting of Rain and Hail

Updrafts in severe thunderstorms are tilted up to the North because the warm, moist air blows from the South. The small, light raindrops are lifted higher than the large, heavy hailstones, so

they are carried and fall further north.

Rain Large Hail

The

tor

nado

the

n cr

osse

d th

e C

anad

ian

Riv

er,

pass

ing

into

far

sou

ther

n O

klah

oma

C

ity.

As

it pa

ssed

ove

r B

ridge

Cre

ek,

Okl

ahom

a,

arou

nd 6

:54

p.m

., a

Dop

pler

On

W

heel

s (D

OW

: W

urm

an e

t al

. 19

97,

Wur

man

20

01)

mob

ile D

oppl

er w

eat

her

rad

ar

dete

cted

win

ds o

f 30

1 m

ph (

484 

km/h

), +

/- 2

0 m

ph in

side

the

tor

nado

at

a h

eigh

t o

f 32

m A

GL

(Wu

rma

n e

t al

. 20

07)[

1] (

The

old

rec

ord

was

a 2

57-2

68

mph

win

d m

easu

rem

ent

fro

m a

Dop

pler

we

ath

er r

adar

nea

r R

ed R

ock,

Okl

ahom

a, a

s re

port

ed

in a

for

ma

l pub

licat

ion

by B

lues

tein

et

al.

(199

3)).

The

se w

ind

s, h

owev

er,

occu

rre

d ab

ove

the

gro

und,

an

d w

inds

at

the

surf

ace

ma

y no

t ha

ve b

een

quite

thi

s in

tens

e.

22 June 2007 F5 tornado Elie Manitoba

©www.extremeinstability.com

F4 Mile wide Binger OK, 22 May 1981 tornado http://www.spc.noaa.gov/faq/tornado/torscans.htm

http://www.photolib.noaa.gov/nssl/index.html

Tornadoes

Tornadoes are intense whirlwinds that form below severe thunderstorms when instability and corkscrew motion called helicity are both large. They range in width from about 50 meters to over a mile and are generally more severe the wider they are. Wind speeds range up to about 300 mph at ground level (with updrafts up to 100 mph) but such wind speeds have only been measured by radar several hundred meters above the ground. Tornadoes usually live only a matter of minutes and have a life cycle that ends in the rope stage, where the tornado looks strung out. Like hurricanes, tornadoes have a relatively calm, clear eye which only a few people have seen and lived to tell.

Eric Nguyen, 13 June 2004 Rain, tornado and hail streaks from a Kansas Thunderstorm. The tornado hangs down from a wall cloud.

World Distribution of Tornadoes

Tornado Outbreaks

This diagram shows the super outbreak of tornadoes on 3-4 April, 1964. The tornadoes, rated on the Fujita scale up to F5 (the most severe) all moved from southwest to northeast under the Supercell thunderstorms that spawned them. The most devastating tornado struck Xenia, Ohio and was videotaped by a Boy Scout who ran at the last moment. Several hundred people died that day.

The tornadoes were produced in an ideal environment, consisting of extremely unstable air with warm, humid air blowing from the south at low levels and cold, dry air blowing from the west at the jet stream level. A low pressure area with an approaching cold front helped force the warm air aloft. At the same time, the low produced snow over the Northern Great Plains, a common situation in Springtime storms.

Downdrafts from this mature thunderstorm move air and rain down to the ground and then outward (Photo © 2004 David Jenkins; Source: PhysicalGeography.net)

Downburst over an Airport Runway

When strong downbursts near the ground they spread outward and curl up at the leading edge. Approaching planes first encounter the updraft and headwinds. Both lift the plane. But then the plane encounters the downdraft and tailwinds. Both act to slam the plane into the ground.

Microburst and Arcus beneath a Thunderstorm

Jean-Francoise Millet Storm Approaching 1867-8The artist surely saw this storm

Iridescent Anvil edge from Kuching, Sarawak, Malaysia. © Randy Yit. cldappsoc

As the Storm Departs

Large Scale Environment of Supercell Thunderstorms

Supercell thunderstorms form in a large scale environment that can be predicted several days in advance even if the precise location and timing of the storms cannot be predicted more than a few hours in advance. The typical environment in the United States consists of three levels in the troposphere.

1. An advancing “tongue” of warm, moist air in the lowest 1-2 km from the South terminated by a dry line with much drier air to the west. The thunderstorm will often form along the dry line.

2. A warm, dry low level jet stream at about 2-3 km from the Southwest that caps and delays the moist air from rising until enough daytime heating or general uplift has occurred.

3. Cold, dry fast jet stream winds from the West from about 5 – 15 km.

The turning (veering) of the large scale wind with height (from South near the ground to West at Jet Stream Levels) constitutes the screw-like or helical motion that is transformed into the sometimes noticeable rotation of the thunderstorm and concentrated into the much more intense rotation of the tornado.

This situation is illustrated on the next slide

Warm, Moist Tongue

Warm, Dry Low-Level Jet

Cold, Dry Upper-Level Jet