ES 20.3 Severe Storms

Thunderstorms, Tornados, Hurricanes

ES 20.3 Severe Storms Severe weather has fascinated

people for ages. From extreme thunder and

lightning from a powerful thunderstorm, to the millions of dollars in damage racked up by major hurricanes; extreme weather can affect large areas and wreak havoc on regions of the world.

A single large storm, such as a hurricane, can cause untold damage and kill thousands of people depending on what part of the world it strikes and when it strikes.

ES 20.3 Severe Storms Thunderstorms: Have you ever seen a small

whirlwind carry dust or leaves upward on a hot day?

Have you watched a hawk drift effortlessly on a warm updraft current on a hot day?

If so, you have been observing the warm updraft currents of unstable air .

These examples are caused by a similar instability that occurs during the development of a thunderstorm.

ES 20.3 Severe Storms A thunderstorm is a storm that

generates lightning and thunder.

Thunderstorms frequently produce gusty winds, heavy rain and hail.

A thunderstorm may be produced by a single cumulonimbus cloud and influence only a small area.

Or, it may be associated with clusters of cumulonimbus clouds that stretch for kilometers along a cold front.

ES 20.3 Severe Storms How common are

thunderstorms? Consider these numbers; at

any given time there are an estimated 2000 thunderstorms in progress on Earth.

As you might expect, the greatest number of storms occur in the tropics where warmth, moisture and instability are common atmospheric conditions.

About 45,000 thunderstorms take place each day with more than 16 million occurring annually around the world.

ES 20.3 Severe Storms The United States experiences

about 100,000 thunderstorms each year, most frequently in Florida and the eastern Gulf Coast region.

Most parts of the country receive from 30 to 100 storms each year.

The western part of the United States has little thunderstorm activity because warm, moist air, unstable maritime tropical air rarely penetrates to this part of the continent.

ES 20.3 Severe Storms Development Thunderstorms:

Thunderstorms form when

warm, humid air rises in an unstable environment.

The development of a thunderstorm generally offers three stages Cumulus stage Mature stage Dissipating stage

ES 20.3 Severe Storms Cumulus stage: During the cumulus stage,

strong updrafts, or upward movements of air, supply moist air and move it aloft.

Each new surge of warm air rises higher than the last and causes the cloud to grow in height as warm, moist air rises.

ES 20.3 Severe Storms Mature stage:

Usually within an hour of the

initial updraft, the mature stage begins.

At this point in the development of a thunderstorm, the amount and size of precipitation is too great for the updrafts to support.

So, heavy precipitation is released from the cloud in the form of heavy rain. Gusty winds, lightning, heavy precipitation, and sometimes hail are produced at this stage.

The mature stage is the most active stage in the life of a storm.

ES 20.3 Severe Storms Dissipating stage:

Eventually, downdrafts (downward movements of air) dominate throughout the cloud.

This final stage is called the dissipation stage.

During this stage, the cooling effect of the falling precipitation and the flowing in of colder air from high above cause the storm to die down.

ES 20.3 Severe Storms The life span of a single

cumulonimbus cloud within a thunderstorm is only about an hour or two.

As the storm moves however, fresh supplies of warm, humid air generate new cells to replace those that are scattering.

ES 20.3 Severe Storms Tornadoes

Tornadoes are violent

windstorms that take the form of a rotating column of air called a vortex.

The vortex extends downward from a cumulonimbus cloud all the way to the ground.

ES 20.3 Severe Storms Some tornadoes consist of a

single vortex; but within many larger tornadoes, many smaller vortexes may rotate within a single larger funnel.

These smaller vortexes have diameters of only about 10 meters and rotate very rapidly.

Smaller vortexes explain occasional observations of tornado damage in which one building is totally destroyed, while another building only 20 meters away is left unharmed.

ES 20.3 Severe Storms In the United States, about 770

tornadoes are reported each year.

These severe storms can occur at any time during the year.

However, the frequency of tornadoes is greatest from April through June.

In December and January, tornadoes are much less frequent.

ES 20.3 Severe Storms Most tornadoes form in

association with severe thunderstorms.

An important process in the formation of many tornadoes is the development of the mesocyclone.

A mesocyclone is a vertical cylinder of rotating air that develops in the updraft of a thunderstorm.

The formation of this large vortex begins as strong winds high up in the atmosphere cause winds lower in the atmosphere to roll, parallel to the ground at first.

ES 20.3 Severe Storms As the updrafts associated

with the thunderstorm grow stronger, this rolling air begins to tilt upright from the updrafts.

Once the air is completely upright in a vertical position, the mesocyclone is complete.

The formation of a mesocyclone does not mean that a tornado will necessarily follow.

Few mesocyclones produce full blown tornadoes.

ES 20.3 Severe Storms Tornado Intensity:

Pressures within some

tornadoes have been estimated to be as much as 10 percent lower than pressures immediately outside the storm.

The low pressure within a tornado causes air near the ground to rush into a tornado from all directions.

As the air streams inward, it spirals upward around the core.

Eventually, the air merges with the airflow of the cumulonimbus cloud that formed the cloud.

ES 20.3 Severe Storms Because of the

tremendous amount of pressure change associated with a strong tornado, maximum winds can sometimes approach 480 kilometers per hour.

One scale used to estimate tornado intensity is the Fujita Tornado Intensity Scale.

Enhanced Fujita Scale is the replacement

Because tornadoes’ winds are difficult to measure directly, a rating on this scale is usually determined by assessing the worst damage produced by a storm.

Enhanced Fujita ScaleFUJITA SCALE DERIVED EF SCALE OPERATIONAL EF SCALE

F Number Fastest 1/4-mile (mph)

3 Second Gust (mph) EF Number 3 Second Gust

(mph) EF Number 3 Second Gust (mph)

0 40-72 45-78 0 65-85 0 65-85

1 73-112 79-117 1 86-109 1 86-110

2 113-157 118-161 2 110-137 2 111-135

3 158-207 162-209 3 138-167 3 136-165

4 208-260 210-261 4 168-199 4 166-200

5 261-318 262-317 5 200-234 5 Over 200

http://www.spc.noaa.gov/faq/tornado/ef-scale.html

Tornadoes By Year More tornadoes VS. Better Reporting (Which is it?)

April 26, 2011 Weather Map More tornadoes VS. Better Reporting (Which is it?)

April 27, 2011 Weather Map More tornadoes VS. Better Reporting (Which is it?)

April 28, 2011 Weather Map More tornadoes VS. Better Reporting (Which is it?)

April 27, 2011 Weather Map The Breakdown:

AS PART OF A SYSTEM WHICH WREAKED HAVOC ON THE EASTERN HALF OF THEUNITED STATES FROM APRIL 14TH TO APRIL 16TH, WIDESPREAD SUPERCELLSBROUGHT A RECORD NUMBER OF TORNADOES TO ALABAMA ON APRIL 15TH. ONTHIS DAY ALONE, ALABAMA EXPERIENCED 45 TORNADOES, ALL OF WHICH WEREEF-3 OR WEAKER.

A LITTLE MORE THAN A WEEK LATER, FROM APRIL 25TH TO APRIL 28TH, MUCHOF THE EASTERN HALF OF THE UNITED STATES EXPERIENCED ONE OF THE MOSTEXTENSIVE TORNADO OUTBREAKS THIS COUNTRY HAS EVER SEEN. CENTRALALABAMA TOOK THE BRUNT OF ITS DAMAGE ON APRIL 27TH. FIRST, A QUASI-LINEAR CONVECTIVE SYSTEM MOVED THROUGH DURING THE EARLY MORNINGHOURS, FOLLOWED BY THE OUTBREAK OF VIOLENT TORNADIC SUPERCELLS INTHE AFTERNOON. THE MORNING ACTIVITY PRODUCED WIDESPREAD WIND DAMAGEAND SEVERAL TORNADOES. THE AFTERNOON ACTIVITY PRODUCED THE MAJORITYOF THE MOST INTENSE DAMAGE. ON THIS DAY, ALABAMA EXPERIENCED62 TORNADOES.

http://www.alabamawx.com/?p=52138

April 27, 2011 Weather Map

http://www.cbs42.com/Photo.aspx?content_id=842027db-7eb4-478b-8a79-3838e97c530f

The Tuscaloosa-Birmingham tornado during the April 2011 event caused at least 65 fatalities. This tornado had a maximum width of 1.5 miles and a track 80 miles long.

http://www.srh.noaa.gov/bmx/?n=event_04272011

April 27, 2011 Weather Map The Breakdown:

http://www.alabamawx.com/?p=52138

CENTRAL ALABAMA FORAPRIL 15TH:EF-5: 0EF-4: 0EF-3: 4EF-2: 10EF-1: 10EF-0: 5THE ACTUAL TORNADO STRENGTH BREAKDOWN IN CENTRAL ALABAMA FORAPRIL 27TH LOOKS LIKE THIS:EF-5: 1EF-4: 4EF-3: 8EF-2: 5EF-1: 10EF-0: 1

April 27, 2011 Weather Map The Breakdown:

http://www.alabamawx.com/?p=52138

THERE WERE A TOTAL OF 4 DEATHS DIRECTLY RELATED TO THE TORNADOES INCENTRAL ALABAMA ON APRIL 15TH.

THERE WERE A TOTAL OF 139 DEATHS DIRECTLY RELATED TO THE TORNADOESIN CENTRAL ALABAMA ON APRIL 27TH.OF THOSE 139 DEATHS ON APRIL 27TH, 86 PEOPLE WERE KILLED INPERMANENT STRUCTURES, SUCH AS A HOME, FACTORY OR CHURCH. 46 WEREKILLED WHILE IN MANUFACTURED HOMES. 2 FATALITIES OCCURRED WHILEPEOPLE WERE STILL IN THEIR VEHICLES AND 2 OTHERS WHILE OUTDOORS.THESE NUMBERS ARE COURTESY OF FEMA, LOCAL EMA AND THE RED CROSS.

ES 20.3 Severe Storms Tornado safety

The Storm Prediction Center located in Norman, Oklahoma monitors different kinds of severe weather.

The SPC’s mission is to provide timely and accurate forecasts and watches for severe thunderstorms and tornadoes.

Tornado watches alert people to the possibility of tornadoes in a specified area for a particular time period.

A tornado warning is issued when a tornado has actually been sighted in an area or as indicated by weather radar,

ES 20.3 Severe Storms Hurricanes

If you’ve ever been to the

tropics or seen photographs of these regions, you know that warm breezes, steady temperatures, and heavy but brief tropical showers are the norm.

It is ironic that these tranquil regions sometimes produce the most violent storms on Earth.

Whirling tropical cyclones that produce sustained winds of at least 119 kilometers per hour are known as hurricanes in the United States.

ES 20.3 Severe Storms In other parts of the world,

these severe tropical storms are called typhoons, cyclones and tropical cyclones.

Regardless of the name used to describe them, hurricanes are the most powerful storms on Earth.

At sea, they can generate 15 meter waves capable of destruction hundreds of miles away.

If a hurricane hits land, strong winds and extensive flooding can cause billions of dollars in damage and great losses of life.

ES 20.3 Severe Storms Hurricane Katrina was one

such storm, flooding New Orleans as seen at right.

In August 2005, Katrina brought flooding rains and high winds to New Orleans.

The storm surge from Katrina damaged the levees that separated New Orleans from Lake Ponchetrain., flooding much of the city in one of the worst disasters that has ever hit the United States.

ES 20.3 Severe Storms Hurricanes are becoming a

growing threat because more and more people are living and working near coastal areas.

At the start of the 21st century, more than 50% of the U.S. population lived within 75 kilometers of the coast.

This number is expected to increase even more within the coming decade.

ES 20.3 Severe Storms Occurrence of Hurricanes

Most hurricanes form between 5 and 20 degrees north and south latitude in the tropics.

The North pacific has the greatest number of hurricanes, averaging 20 per year.

The coastal regions of the United States experience fewer than, on average, 5 per year.

Although many tropical storms develop each year, few reach full hurricane status. A storm is a hurricane only if the wind has sustained speeds of at least 119 kilometers per hour.

2004: 4 simultaneous hurricanes

ES 20.3 Severe Storms Development of Hurricanes:

A hurricane is a heat engine

that is fueled by the energy given off when huge quantities of water vapor condense.

Hurricanes develop most often in the late summer when water temperatures are warm enough to provide the necessary moisture and heat for the atmosphere.

A hurricane begins as a tropical storm that consists of disorganized clouds and thunderstorms. Low pressure and little or no rotation are characteristic of these storms.

2004: 4 simultaneous hurricanes

ES 20.3 Severe Storms Occasionally, tropical

disturbances become full hurricanes.

At right we see a cross-section of a hurricane. An inward rush of warm

moist air moves toward the core of the storm.

The air then turns upward and rises in a ring of cumulonimbus clouds.

This donut shaped wall that surrounds the center of the storm is called the eye-wall.

ES 20.3 Severe Storms Here, near the eye-wall,

the heaviest wind speeds and greatest rainfall occur.

Surrounding the eye-wall are curved bands of clouds that trail away from the center of the storm.

Notice that near the top of the hurricane, the rising air is carried away from the storm center by an outflow.

This outflow provides room for more inward flow of air at the surface.

ES 20.3 Severe Storms At the very center of the

storm is the eye of the hurricane.

This well known feature is a zone where precipitation ceases and winds subside.

The air within the eye gradually descends and heats by compression, making it the warmest part of the storm.

ES 20.3 Severe Storms The intensity of a hurricane

is described using the Saffir-Simpson Scale.

The most devastating damage from a hurricane is caused by storm surges.

A storm surge is a dome of water about 65 to 100 kilometers wide that sweeps across the coast where a hurricane’s eye moves onto the land.

ES 20.3 Severe Storms A hurricane weakens when

it moves over cool ocean waters that cannot supply adequate heat and moisture.

Intensity also drops when storms move over land because there is not sufficient moisture.

In addition, friction within the rough land surface causes winds to subside.

Finally, if a hurricane reaches a location where the airflow aloft is unfavorable, it will die out as well.