Transcript NATURE OF STORMS

NATURE OF STORMS
thunderstorms = strong, rapid upward movement of warm,
unstable, moist air
1. formed of cumulonimbus clouds
2. each day approximately 44,000 occur across Earth’s surface
3. always accompanied by thunder and lightning
4. formation:
a. must be an abundant source of moisture in lower levels
of atmosphere
1) as moisture condenses, it releases latent heat
2) keeps cloud warmer that air around it; critical in
maintaining upward motion of cloud
b. mechanism must lift air so that moisture can condense
and release latent heat
c. portion of atmosphere through which cloud grows must
be unstable
1) air must continue to cool with increasing altitude for
growing cloud to stay warmer than
surrounding air
5. limits to growth:
a. process will continue until rising air meets a layer of
stable air that it cannot overcome
b. until rate of condensation, which diminishes with height,
is insufficient to generate enough latent heat to keep
cloud warmer than surrounding air
1) limits most cumulonimbus clouds to height of around
18000 m
c. typical storms only last about 30 minutes; individual
storms are only about 24 km in diameter
6. classified according to mechanism that caused air to rise
a. air-mass thunderstorms = if air rose because of unequal
heating of Earth’s surface within one air mass
1) form within a warm, moist air mass
2) widely scattered and form mostly over land
3) unequal heating of surface reaches
maximum during mid-afternoon
4) occur mostly in spring or
summer usually mid-afternoon
5) usually last less than an hour; often single storms
6) mountain thunderstorms = due to air mass rising due
to orographic lifting moving air up side of mountain
7) sea-breeze thunderstorms = local air-mass storm due
to extreme temperature differences between air
over land and air over water
b. frontal thunderstorms = produced by advancing cold fronts
and more rarely, warm fronts
1) with cold fronts, cold air pushes warm air rapidly up
steep cold front boundary
a) produce line of storms, sometimes hundreds of km
long, along edge of front
b) squall lines present
c) can persist long into night
2) with warm fronts, warm air mass
slides up and over cold air mass
a) boundary between 2 air masses is not steep, thus
air rises gradually
b) if warm air behind front is unstable & moisture
levels are high, mild storms can develop
3) occur most often in spring and summer
4) often stronger and may last for several hours
7. develop in 3 stages: classified according to direction in which
air is moving
a. cumulus stage:
1) warm, moist air starts to rise nearly vertically upward
2) creates updrafts which move moisture to upper
reaches of cloud
3) moisture condenses into visible cloud droplets &
releases latent heat
4) droplets will grown larger & eventually fall back to
surface as precipitation
b. mature stage:
1) as precipitation falls, it cools air around it
2) air now more dense, so sinks rapidly to ground with
precipitation creating downdrafts
3) updrafts and downdrafts from convection cell that
produces gusty winds associated with storms
4) equal amounts of updrafts and downdrafts exist side by
side in cumulonimbus cloud
c. dissipating stage:
1) updrafts need steady supply of warm, moist air at surface
2) once supply run out, updrafts will slow & eventually stop
3) precipitation can no longer form
4) will last until cloud runs out of previously formed
raindrops
SEVERE WEATHER
severe thunderstorms = produce some of most violent weather
conditions
1. supercells = multicell storms that are very organized and
powerful
a. self-sustaining, extremely powerful storms
b. intense, rotating updrafts
c. can last for several hours and updrafts as strong as
240 km/h
2. thunder is not usually dangerous
a. caused by a pressure wave that accompanies lightning
which produces sound wave
b. since sound waves travel slower it is heard after the
lightning has been seen
c. takes 3 seconds to travel 1 km
d. cold possibly cause hearing damage, crack plaster in
buildings
e. rumbling happens when thunder echoes from the
mountain sides
f. greatest distance it can be heard is 16 km
lightning = electricity caused by rapid rush of air in
cumulonimbus cloud
1. lightning bolt = forms when friction between updrafts and
downdrafts within cloud separates electrons from some
of their atoms either in cloud or near ground
a. travels between two regions of opposite electrical charge
b. stepped leader = invisible channel of negatively charged
air moves from cloud toward ground to relieve electrical
imbalance
c. return stroke = channel of positively charged ions that
rushes upward from ground to meet stepped leader
from cloud
d. will always follow the shortest path available between
charged surfaces
e. return stroke surges from ground to cloud, illuminating
channel with about 100 million volts
2. most will occur outside the rain band of a storm
a. single bolt can reach temperatures up to 30000oC
b. usually strikes the tallest object in an area
3. heat lightning = glow of lightning so far away that its
thunder cannot be heard
4. can produce heavy rains, high winds, hail, and tornadoes
a. lightning is leading cause of fire in the western U.S.
b. on the average, more people are killed by lightning than
any other type of severe storm
5. safety tips:
a. Go indoors, if possible. If you are traveling stay in your
car.
b. Stay off bicycles, motorcycles, scooters, golf carts, or
farm equipment.
c. If you are swimming, get out of water. Get off small
boats.
d. Avoid standing near or being the highest object in an
area.
1) Outdoors, the best protection is in a cave, ditch, or
canyon.
2) If you are out in open with only isolated trees nearby,
crouch in open as far away from trees as possible.
e. Indoors, stay away from open doors and windows and
metal objects such as pipes, sinks, stoves, and radiators.
f. Do not use the telephone or plugged in appliances.
downbursts = violent downdrafts concentrated in a local area
1. winds can be more than 160 km/h
2. based on size of area affected can be classified as:
a. macrobursts = path of destruction up to 5 km wide; wind
speeds more than 200 km/h; last up to 30 minutes
b. microbursts = affects areas of less than 3 km; winds
exceed 250 km/h; last average of 10 minutes; difficult
to detect and prepare for
hail = precipitation in form of balls or lumps of ice
1. causes almost $1 billion in damage; severe damage to crops
2. most frequently occurs in Central United States
3. most common during spring growing season
4. formation:
a. water droplets exist in liquid state in parts of cloud where
temperature is below freezing
b. when supercooled water droplets encounter ice pellets,
water droplets freeze on contact and cause ice pellets to
grow larger
c. strong updrafts and downdrafts will cause ice pellets of
move up and down in storm
growing in size until too
heavy for updraft to keep in
sky and will then fall to surface
floods = storms may dump rain over limited location
1. wind currents that move weather system is weak and causes
storm to move slowly
2. worsen if abundant moisture available throughout
atmosphere
3. rain falls faster than ground can absorb it or faster than
streams and rivers can transport out of an area
4. main cause of thunderstorm related deaths in U.S each year
tornadoes = narrow, funnel-shaped column of spiral winds that
extends downward from cloud base and touches ground
1. develops in low, heavy cumulonimbus clouds; usually form
from supercells
a. air made visible by dust and debri drawn into swirling
column or condensed water vapor
b. wind shear = when wind speed and direction change
suddenly with height
c. average diameter is between 150 m and 600 m
d. length of its path on ground varies from a few meters to
more than 25 km (15 miles)
e. usually only last a few minutes but can last up to an hour
or longer
f. occur most often in spring
during late afternoon or early
evenings
2. vortex = small low-pressure center of thunderstorm
a. develops down from the bottom of thunderhead as air
below is drawn in
b. rising air cools forming clouds along wall of vortex making
funnel shape visible
c. air spins in counterclockwise direction in Northern
Hemisphere
3. when low pressure area touches the ground it acts like a
giant vacuum
a. updraft in center is very strong
b. strongest winds are between 360 - 500 km/h
(224 - 311 miles/ hr)
c. destructive power due to
the speed of the winds
4. waterspouts = tornadoes over water
a. less powerful than those that form over land
b. occur with weak thunderstorms and large cumulus clouds
5. classified by Fujita tornado intensity scale
a. ranks according to path of destruction, wind speed, and
duration
b. scale runs from F0 to F5
c. F0 = winds up to 118 km/h
d. F5 = winds more than 500 km/h
e. most do not exceed F1 category
6. more common in Great Plains region of United States =
Tornado Alley (Texas, Kansas, Oklahoma)
a. can occur at any time and at any place
7. Safety tips:
a. If you are inside stay away from outside walls, windows,
and doors.
1) Go to the basement or an interior room on the lowest
level.
2) If possible, get under a heavy table or mattress.
b. In a public place, go to designated shelter area or sit
close to an interior wall on lowest level.
c. Get away from mobile homes or vehicles if possible.
d. If you are outside in an open area, look for a ravine or
ditch. Lie flat and protect your head with
your arms.
*Most injuries occur from people being trapped in
collapsed building or being hit by flying debri.
blizzard = snowstorm with high winds and low temperatures
1. brought in by strong mid - latitude lows
2. must have two conditions
a. must be enough moisture
b. temperature must be cold enough for snow
3. lows form and grow along the polar front
4. usually have stronger winds in winter
5. moisture supply for winter storms differ in different parts of
the country
Ex: Midwestern United States - Gulf of Mexico
East Coast - Atlantic Ocean
6. need temperatures of -6oC (-21oF) or lower
7. winds often range from 50 - 75 km/h (30 - 45 mph)
8. blowing, powdery snow can
cause visibility near zero
9. people and animals should
avoid exposure
TROPICAL STORMS:
hurricanes = large, swirling, low pressure system that forms
over tropical oceans
1. also called tropical cyclone = low pressure area that
contains rising warm air
a. air circulates toward center in counterclockwise motion
b. in Western Pacific Ocean called typhoons
c. greatest number occur in western North Pacific Ocean
Hurricane Fran
2. usually form in regions of trade winds between 20oN and
20oS latitudes where water is very warm; originate along
ITCZ
a. as long as it remains over water, warm moist air will rise
providing energy
b. as water evaporates from ocean surface, latent heat is
stored
c. latent heat is released when air begins to rise and water
vapor condenses into clouds & rain
d. rising air creates area of low pressure at ocean surface
e. wind speed increases & air pressure decreases as storm
develops
f. move according to wind currents that steer them
g. last until it can no longer produce enough energy to
sustain itself
1) usually happens when moves over land
3. stages:
a. tropical wave = disturbance from ITCZ or as weak, low
pressure system
b. tropical depression = acquires cyclonic circulation round
center of low pressure
c. tropical storm = when winds speeds around low pressure
center of tropical depression
d. hurricane = air pressure continues to fall, winds around
center reach at least 120 km/h (75 mph)
4. characteristics:
a. eye forms at center
1) central area of sinking air
2) usually 15 to 50 km in
diameter (9-30 miles)
3) no rain, very calm - almost no wind
b. eye wall = area where highest/strongest winds are
located
1) completely surrounds eye
2) area of intense thunderstorms
3) when it hits land locations to right side of storm
receives worst damage
c. storms average diameters
of 300- 600 km (180- 360 miles)
Hurricane Katrina
eyewall
5. damage caused
a. high winds, tornadoes, heavy rains, high waves, and
flooding
b. storm surge = quickly rising ocean water levels that
cause widespread flooding
1) piles up water along shore and then blows it inland
2) causes the greatest damage of storm
3) more severe during high tide
4) can be from 4 to 20 feet or more
5) in Northern Hemisphere, occurs primarily on right side
of storm; strongest onshore winds occur
6. rated on Saffir-Simpson hurricane scale
a. uses barometric pressure in inches, wind speeds in mph,
and potential for property damage
b. gives an estimate of storm surge in feet
c. Category 1: minimum wind speeds of 74 mph
d. Category 5: winds in excess of 155 mph
e. Category 3: considered to be a major hurricane
7. storm is named when its winds reach 39 mph
(tropical storm)
a. women’s names were used from 1953 to 1978
b. in 1979 men’s names were added as well as names from
other languages
c. countries affected by storms suggest names
d. names are on a six year cycle
e. when one causes great damage its name is retired from
the six - year cycle
Ex: Alicia, 1983; Hugo, 1989
wave cyclones = large storms that develops along cold or
stationary fronts
1. low-pressure centers
2. wind spiral in toward central region
3. usually lasts several days
4. in North America move in easterly
direction about 32 to 64 km/hr
5. spin counterclockwise
anticyclone = storm that spirals outward from high
pressure center
1. spins clockwise in Northern Hemisphere
2. bring dry weather
3. if stalls over area can cause
droughts and air pollution problems
RECURRING WEATHER:
drought = extended periods of well-below-normal rainfall
1. result of shifts in global wind patterns that allow large high
pressure systems to last for weeks or
months over continental areas
2. under dome of high pressure, air sinks and will resist any
attempt to lift moisture through it
3. will last until global patterns shift enough to move high
pressure system out of way
heat wave = extended periods of above-normal temperatures
1. formed by same high pressure systems that cause droughts
2. air warms by compression and causes above normal
temperatures
3. high pressure system also blocks cooler air masses from
moving into an area
4. can lead to serious health problems
5. heat index = assesses effect of body’s increasing difficulty
in regulating its internal temperature as relative
humidity rises
a. routinely reported in oF
b. issued by National Weather Service
cold wave = extended period of below-normal temperatures
1. formed by large, high pressure systems
2. caused bys systems of continental polar or arctic origin
3. movement influenced by jet streams
4. wind-chill factor = estimates heat loss from human skin
caused by combination of cold air and wind
5. wind chill index = estimates how cold air actually feels to
human body
National Weather Service = issue advisories when severe
weather is observed or conditions exist
1. watch = when expect severe weather over area for next few
hours
a. issued for severe thunderstorms, tornadoes, floods,
blizzards, and hurricanes
b. covers area of 100 km by 200 km or larger (60-120 miles)
c. during:
1) stay updated on weather reports
2) be ready to take shelter
3) plan where to go when warning is issued
2. warning = severe weather conditions exist
a. tornado spotted, severe thunderstorm reported,
hurricane approaching an area
b. will give location of storm when detected, area into
which it is likely to move, and period of time during
which storm could hit
c. during:
1) take immediate action - evacuate if necessary
2) always have flashlight, batteries, and blankets ready if
an emergency should happen
3. type of severe weather determines where advisories are
issued from
a. National Severe Storms Forecast Center
1) located in Kansas City, Missouri
2) severe thunderstorms and tornadoes
b. National Hurricane Center
1) located in Miami, Florida
2) issue hurricane warnings when conditions are
expected within 24 hours
c. local weather stations can issue severe thunderstorm or
tornado warnings whenever necessary