Clouds and Storm Systems

8/9/2019 Clouds and Storm Systems

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Clouds

y Clouds typically form when air rises.y Rising motion can be associated with convection in unstable air,

lifting of air over topography (mountains), or lifting of air byfronts.

y When air rises, the air's temperature cools and may reach its

dewpoint temperature, at which point it becomes saturated. y Once saturation is reached, condensation occurs and the water

vapor in the air will condense into tiny water droplets. As millionsof droplets form, a cloud will begin to take shape.



Cloud Classification

y Clouds are classified by their height (high, middle, low, or

vertically developing), physical appearance and whether theyproduce precipitation.

Here are a few Latin roots that are helpful when identifying

cloud types:

"cirro": high, 'curl of hair'

"alto": 'middle'"stratus": layer, sheet-like, low"cumulus": heap-like, puffy

"nimbus": clouds producing precipitation

Combinations of these Latin roots are used to describe the mostcommon types of clouds (i.e. a cirrostratus cloud is one that is

high and layered).

High Clouds

Cirrus, Cirrostratus, Cirrocumulus

y High level clouds form above 20,000 feet (6000

meters).y Since they form high in the atmosphere, high clouds are

composed of ice crystals, due to the cold temperaturesin this part of the atmosphere.y High-level clouds are typically thin and white in

appearance, but may display an array of colors when the

sun is low on the horizon.

y T hey get their wispy shape from the high speed winds



that occur at those altitudes.



Mid-Level Clouds

Altostratus, Altocumulus

y Mid-level clouds typically have bases between 6,500 to 20,000feet (2000 to 6000 meters).

y Since these clouds are located lower in the atmosphere, theyare primarily composed of water droplets.

y In the cold season, they can be composed of ice crystals sincethe temperatures are cold enough.



Low CloudsStratus, Stratocumulus, Nimbostratus

y Low clouds typically have bases below 6,500 feet (2000

meters).y T hese clouds are located low in the atmosphere and are

mostly composed of water droplets. On occasion, if the

temperatures are cold enough, they may contain some iceparticles and snow.



Vertically Developing Clouds

Cumulus, Cumulonimbus

Some clouds can span the depth of the troposphere and thereforecannot be classified as high, middle or low. T hese clouds are

classified as vertically developing. Cumulus clouds arecharacterized by a flat base and can grow to heights exceeding39,000 feet (12,000 meters). T hey can contain both liquid

droplets and ice particles because they cover a large depth of the

troposphere. With the right conditions, these are the clouds thatbecome powerful thunderstorms.



T hunderstorms

T hunderstorms are cumulonimbus clouds that produce thunder andlightning. T he figure below shows the average number of days that

thunderstorms occur over the United States. T he greatest occurrence

of thunderstorms occur in the southeastern United States, with asecondary maximum over the Colorado Rockies. T hese regionsfrequently have all the necessary conditions for thunderstorm

formation.





Instability

y Air is said to be unstable if it continues to rise after being given

a slight "push" upward.y Conversely, air is considered to be stable if it returns to its

original position after being "pushed" upward.

y In order for thunderstorms to develop, air needs to be unstable.

y Air is most likely to be unstable when warm, moist air is presentat the surface and cold, dry air is present aloft.



Lifting MechanismAnother ingredient that must be present is a lifting mechanism to give

the air the initial "push" upward. T here are several ways in which air

can be lifted.y Lifting primarily occurs along fronts (cold, warm, stationary, or

occluded fronts).

y Air can also be lifted as it flows over hills or mountains.Locations where these three"ingredients" come together are most

likely to experience a thunderstorm.

Stages of a

Ordinary (Non-

Severe)

T hunderstorm

Many non-severe thunderstorms go through a life cycle consisting of

three distinct stages. T his life cycle generally lasts one to two hours.

T owering Cumulus Stage

y T he first stage is the towering cumulus stage, or growthstage.

T he warm, moist air rises and cools, eventually condensing into acumulus cloud. As condensation occurs, it warms the air

(remember, condensation is a warming process), keeping the airinside the cloud warmer than the air around it. T his keeps the

air unstable and allows the cloud to keep growing vertically.

During this stage, updrafts keep the water droplets and icecrystals suspended in the cloud. T here is no precipitation, andgenerally no lightning, or thunder during this stage. As the cloudbuilds to altitudes where the temperature is below freezing,

large raindrops and even small hail begin to form. Eventually, the

raindrops and small hail become heavy enough that the updraft



cannot keep them suspended in the cloud and they begin to fall

as precipitation. T hese falling particles, and evaporation andcooling of air near the cloud boundaries, creates a downdraft,

which signifies the beginning of the next stage.



Mature Stage

y T he appearance of downdrafts marks the beginning of themature stage.

y During this stage, updrafts and a downdrafts are presentand the thunderstorm is at its most intense state.

T he cloud grows so high, that it reaches a stable part of the

atmosphere (possibly the stratosphere) and cannot grow any

higher. T he top of the cloud spreads out and forms an anvilshape. Lightning, thunder, heavy rain and possibly small hail areproduced during this stage.

y Sometime after the storm enters its mature stage, iteventually begins to dissipate. T his signifies the beginning

of the next stage.



Dissipating Stage

y In the final stage, the updrafts weaken and the downdraftsdominate the thunderstorm.

y Without the warm, moist air the thunderstorm usually does notlast much longer.



y Tornadoes

A tornado is a violently rotating column of air that originateswithin a thunderstorm and is in contact with the ground.

T ornadoes often only last a few minutes, but it is possible fortornadoes to last over an hour and travel many miles. Around1,000 tornadoes occur in the United States each year. More

tornadoes strike the central United States than any other place

in the world, so this region has earned the nickname "tornado





y T he term "funnel cloud" is used to describe a region of strong

rotation where the circulation has not reached the ground yet.y T he funnel becomes visible when water vapor begins to condense

into liquid droplets.y One sign that the circulation has reached the ground and has

become a tornado is that dust and debris on the ground will beginto rotate.

T he size and/or shape of the tornado is not always a measure of its

strength, although very large tornadoes are almost always quitedestructive. T ornadoes will gradually lose strength and take on a rope-like appearance.

Funnel CloudT ornado

DissipatingT ornado



Tornado Classification

T ornadoes are classified according to the damage they cause, which isrelated to their wind speed.

T he original scale, called the Fujita (F) Scale, was developed by Dr.

Fujita in the 1960s. A tornado's wind speeds are estimated based onthe damage caused by the storm, which is assessed after-the-fact.

T here are some apparent problems with the F-Scale. One problem isthat it is subjective. A different assessment may be made based on

who is assessing the damage. Another problem is the structuralintegrity of buildings may vary. One last problem is that the damage

assessment is completed after-the-fact. T his can be a problembecause the damage site might be altered before it is assessed.

y Due to these potential problems with the Fujita Scale, a new

scale, the Enhanced Fujita (EF) Scale will be implemented onFebruary 1, 2007. T his new scale uses Degree of DamageIndicators, in order to get a more realistic estimate of atornado's winds.



Enhanced Fujita (EF) Scale

Scale CategoryWindSpeed

Possible Damage

F0 Weak65-85mph

Light: tree branchesbroken, sign boardsdamaged

F1 Weak

86-

110mph

Moderate: trees snapped,mobile homes pushed off

foundations oroverturned, windowsbroken

F2 Strong111-135

mph

Significant: large treessnapped or uprooted,

weak structures

destroyed

F3 Strong

136-

165

mph

Severe: some roofs torn

off framed houses, trees

leveled

F4 Violent166-200

mph

Devastating: roofs and

some walls torn off wellconstructed houses, car

thrown or overturned

F5 Violent>200

mph

Incredible: houses may

be lifted off foundation,structures the size ofautomobiles can be

thrown over 100 meters,steel-reinforcedbuildings highly damaged





y Once a tropical depression has sustained winds of at least 39mph, it is called a "tropical storm." T his is when a tropical cyclone

is assigned a name.y A tropical storm becomes a hurricane when it reaches maximum

sustained winds of 74 mph.y Hurricanes are classified by their wind speeds using the Saffir-

Simpson Scale.

Saffir-Simpson Scale

CategoryWind

SpeedDamage

1 74-95mph

Damage mainly to

unanchored mobile homes,shrubbery, and trees.

296-110mph

Some damage to roofs ofbuildings, considerable

damage to shrubbery andtrees, with some treesblown down and majordamage to mobile homes.

3111-130mph

Some structural damage tosmall residences, mobile

homes destroyed, foliageblown off trees and largetrees blown down.

4131-155mph

Extensive damage todoors, windows and roofs,

shrubs, trees and all signsblown down, and complete

destruction of mobilehomes.

5 >155 mph

Severe window and door

damage, extensive roof

damage to residences andindustrial buildings, some





T he Eye

y T he eye is located in the center of the storm and is a region

of generally clear skies and light winds. T he size of the eye is

typically 20-40 miles across, but can be larger or smallerdepending on the storm.

y T he skies are generally clear in the eye because the air is

sinking in this region of the hurricane. T his sinking air actually

suppresses cloud formation. At the ground, the transition fromthe very strong winds under the eyewall to the near calmconditions in the eye can be deceiving. Some people think the

storm is over when the eye is passing over, when in fact it isonly half over and the dangerous winds on the other side of

the eye are still to come.



T he Eyewall

y T he eyewall is a wall of deep clouds that produce the torrential

rainfall that surrounds the eye of hurricanes.y T he strongest winds are found under the eyewall.



Rainbands

y T he clouds and thunderstorms that swirl in toward the storm'scenter are called spiral rainbands.

y Spiral rainbands can produce heavy downpours and wind, as wellas tornadoes.

Tropical Cyclone Environments

y Since hurricanes need warm waters for development, they only

form over warm, tropical oceans. T hey rarely form within 5°latitude of the equator, because the Coriolis Force is weak near

the equator and the thunderstorm clusters will not rotate. (T heCoriolis Force is zero at the equator and increases towards thepoles.) T here are seven regions around the world where tropical

cyclones form:



y Atlantic Basin (light

green)y Northeast Pacific

Basin (yellow)

y Northwest PacificBasin (orange)

y North Indian Basin(pink)

y Southwest Indian

Basin (purple)y Southeast

Indian/AustralianBasin (blue)

y Australian/Southwest

Pacific Basin (green)

T he following environmental conditions must be present for a tropicalcyclone to develop:

y T he ocean waters must be warm (at least 80°F / 27°C) to a depth

of approximately 150 ft.y Relatively moist air must be present throughout most of the

lower troposphere.y T he storm must form at least 5° latitude north or south of the

equator.y Winds must not change signficantly between the lower and upper

troposphere.



T ropical Cyclone Developmental Process

When a cluster of thunderstorms develops or moves into environment

described above, the disturbance can become more organized, which

leads to the formation of a tropical depression. T he warm water is one

of the most important contributors to tropical cyclone formation

because it acts as the "fuel" for the storm. As water vapor rises, it

cools and once saturation is reached, the water vapor condenses intoliquid water that we see as clouds. During the process of condensation,

heat is released. T his warms the atmosphere, making the air lighter

and causing it to rise further. As this occurs, more air must move in

near the surface to take its place. T his inflowing air will begin to



rotate under the influence of the Coriolis Force. As the pressure drops

in the center of the storm, signifying strengthening, the pressure

gradient becomes stronger. T he pressure gradient is directly related

to wind speed and the stronger the pressure gradient, the faster thewind speed.

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Clouds and Storm Systems