Chapter 10 - Test Four Flashcards
Storms comprised of cumulonimbus clouds that contain:
Lightning and thunder
Thunderstorms
Can produce gusty winds, torrential rains, hail and tornadoes.
They form when warm, moist air (mT) rises in an unstable environment.
Various mechanisms can trigger the upward movement of thunderstorms:
- Convection (unequal heating of earth’s surface)
- Cold Fronts
- Topographic lifting
- Sea-breeze induced lifting
The globe experiences 45,000 thunderstorms / day (Fig. 10.2), most in:
- Lower (more tropical) latitudes
- Over land masses
The US experiences 100,000 thunderstorms every year (Fig 10.3)
- More in southeast (mT air)
- max. in FL (sea breeze convergence Fig. 4.21)
- less on west coast, New England (mP air)
- North Carolina averages 40-50
- Raleigh: 42
Air-Mass Thunderstorms
Scattered, individual storms that develop within unstable mT air masses.
Air-Mass Thunderstorms characteristics:
- Small (10s of kms in diameter)
- short lived (1- 2 hours)
- Non- severe
- Most occur in spring and summer
- Most occur in the afternoon
Thunderstorm Stages of Development
A three-stage model was developed in the 1940’s after numerous thunderstorm related airplane crashes prompted field studies in Florida.
Cumulus, Mature, and Dissipating Stage.
Cumulus Stage
Warm, moist air rises, expands, cools and condenses, forming thencumulus cloud. Condensation releases Latent Heat Energy, warming the cloud making it less dense than surrounding air
- Allows continue rising and growth
Cumulus Stage Characteristics:
- 5-10 km height
- updrafts only
- no precipitation
- no lightning/thunder
Mature Stage
Cumulonimbus cloud grows well above freezing level, allowing cloud droplets to grow larger thru both Bergeron and Collision/Coalescence processes, until too large to be supported by updrafts.
- Rain starts to fall- creating a cool dense downdraft
Mature Stage Characteristics:
- 10-15 km height
- Anvil cloud spreads out at tropopause
- Updrafts and downdrafts
- Intense precipitation
- Frequent lightning/thunder
Dissipating Stage
Begins when downdrafts begin to dominate updrafts, cutting off the release of Latent Heat Energy, which is the “fuel” of the thunderstorm.
- The storm literally collapses on itself
Dissipating Stage Characteristics:
- Still 15 km height
- Downdrafts only
- Precipitation decreases, stops
- Lightning/thunder decreases, stops
A single Air Mass Thunderstorm generally goes thru all ____ stages within several hours.
three
Surprisingly, ____ of the water vapor that is condensed in a thunderstorm actually reaches the ground as rain. The remaining 80% Is carried off as cloud that eventually evaporates.
less than 20 percent
Hail
Produced by very strong up/down drafts (up to 100 mph) within Cb clouds
Hail stones grow by colliding and coalescing with:
Super cooled liquid water droplets in the upper portions of the Cb cloud that freeze instantaneously
&
Regular liquid water droplets
in the lower portions of the Cb cloud
that freeze more slowly only after the
hail ascends back into the upper portions
of the Cb
A cross section of a hailstone reveals its life-cycle much like the cross section of a tree:
Clear sections result from slow freezing (air escapes: no bubbles): occurs in lower part of Cb
&
Opaque sections result from instant freezing (air can’t escape: bubbles): occurs in upper part of Cb
Hail Stone Facts
A golf ball sized hailstone requires: - 1 billion collisions! - 1 hour “hang time” Largest ever found fell in 2003: - Weight: 1.7 ibs - Diameter: 7 inches Causes billions of $ damage yearly
NC ranks #____ nationally in damage
2
Severe Thunderstorms are defined by National Weather Service as a thunderstorm with:
Hail ≥ 0.75” in diameter (nickel sized), Wind gusts ≥ 50 knots (58 mph), Tornadoes
Only 10% thunderstorms become severe, so annually, the US experiences roughly:
10,000 severe thunderstorms.
Severe thunderstorms can also produce:
Flash Flooding (Box 10.1)
Flash Flooding
Localized floods of great volume, but short duration.
- Number 1 (storm related) killer in the United States
Severe thunderstorms also form in warm, moist, unstable air, but need an additional “ingredient”:
Strong Vertical Wind Sheer
Strong Vertical Wind Sheer
Defined as a rapid change in wind speed and/or direction wrt height.
Wind Sheer allows the thunderstorm to tilt vertically, which prevents the downdrafts from “____” the updrafts
killing
Strong Vertical Wind Sheer tilt allows the updrafts to continue to grow…
resulting in additional Latent Heat energy release - allowing the storm to become stronger hence severe characteristics
Characteristics as a result of Strong Vertical Wind Sheer:
- 15-20 km height
- Intense rain, large hail
- Continuous lightning/thunder
- Gust fronts, roll clouds (fig 10.7)
- Overshooting tops
Supercell Thunderstorms
Only 2-3% of thunderstorms reach “Supercell” status
Characteristics:
- > 20 km height
- Mesocyclone/ Tornadoes
Squall Line
Relatively narrow, long (up to 500 kms) line of thunderstorms that form along or ahead of cold fronts in mT air
Mesoscale Convective Complexes (MCCs)
Extremely large (1000 km diameter), long lived (12-36 hours) complex of many thunderstorms working in concert to maintain favorable environment
Microbursts
An intense concentration of downdraft winds that results in:
- Winds exceeding 100 mph
- Extensive straight line wind damage
The combination of downburst and gust fronts has been responsible for numerous ____ disasters.
airline
Lightning is an electrical discharge that occurs within thunderstorms:
- 80%: cloud-to-cloud/ within cloud (Sheet Lightning)
- 20%: Ground-to-cloud (most dangerous)
In order for lightning to occur a charge separation must occur within / near the cumulonimbus (Cb) cloud; results in:
separate regions of opposite electrical charge (+, - ions)
Although the source of charge separation within lightning is debatable; it results in:
+ Positive ions accumulating in the upper part of the Cb
- Negative ions accumulating in the lower portion of the Cb
Because “opposites attract” the negative ions at the base of the Cb attract positive ions on the ground under the cloud
These positive ions accumulate around tall objects (trees, building, etc.)
When the charge separation gets too large, the insulating property of the atmosphere is surpassed, resulting in the electrical discharge called:
The Lightning Stroke
The Lightning Stroke
When the negative charge at the base of the Cb becomes excessive, a flow of negative ions rushes toward the ground, ionizing the air
- This flow, which is invisible, is called the:
Stepped Leader
As the stepped leader approaches the ground, a flow of positive ions rushes up to meet it, when they meet a tremendous visible electrical
discharge occurs, called the:
Return stroke
This Return Stroke neutralizes the charge separation.
- It is typically several miles long
- But, only about 1 inch in diameter!
Thunder
Lightning causes the air to instantly heat to 33,000ºC:
- Results in an explosive expansion of air creating a shockwave resulting in thunder
Because light travels at 3 x 10 8 m/s we see lightning immediately; sound; however, travels much slower (3 x 10 2 m/s) so the thunder lags.
We can use this difference in speed to estimate the distance that
lightning occurs from a location.
Sound takes ~ 5 seconds to travel one mile
Example: 20 seconds / 5 = 4 miles away
Typically, thunder can only be heard if lightning is within 10-12 miles. This leads to the term “____ lightning”, which is no different than normal lightning.
heat
Lightning is the # 2 (Storm related) killer in the U.S. Annually, lightning:
- Kills roughly 100 people (NC ranks#3)
- Starts 10,000 fires
- Causes $5 Billion in damage (homes, forests)
Tornadoes
An intense rotating column (or vortex) of air extending from the base of a Cb whose circulation has reached the ground
- Extremely low barometric pressure (900 mb)
- Extremely strong winds
- Also called twisters, cyclones
- some are single vortex, others multiple vortices (Fig.10.19)
- Only 1 % of thunderstorms produce tornadoes
Tornado Development
Doppler Rader cannot dictate a tornado
Mesocyclone:
vertical cylinder of rotating air within a thunderstorm
- Detectable only by Doppler Rader
Wall Cloud:
Portion of the mesocyclone that is observed beneath the Cb as an area of rotating clouds
- This is where the funnel will appear
Funnel Cloud:
Cone shape cloud rotating beneath wall cloud
- Comprised of condensation only
Tornado:
Circulation has reached the surface
- Debris gets pulled into the circulation
- Roughly ½ of mesocycones will produce a tornado
- A tornadic thunderstorm can go through all four stages within 30 minutes
Tornado Climatology
The vast majority of the world’s tornadoes occurs in the US
- 1200 annually (figs. 10.21)
- Many occur in “Tornado alley” , which stretches from Texas to Nebraska
North Carolina averages ____ tornadoes per year
10-20
Most US tornadoes occur during spring and early summer (Fig. 10.21)
- Following the clash between cP air and mT air
Most US tornadoes occur during the afternoon and early evening
- Diurnal heating increases instability, likelihood of thunderstorms, hence tornadoes.
Profile of a Tornado
General Characteristics Average Extreme
Diameter: 500 - 2000 feet 1 mile
Movement Speed: 30 mph 70 mph
Movement Direction: from sw to ne any
Lifetime: Minutes hours
Distance: 10-15 miles 100 miles
Wind speed: 100-200 mph 300 mph
Tornado Destruction is measured by the
Fujita Intensity Scale
- intensity scale developed in the 1960’s
- based on observed damage
Fujita Intensity Scale
Fujita Scale Description %
F0, F1 weak tornadoes 63 F2, F3 Strong tornadoes 35
F4, F5 Violent Tornadoes less than 2
70% of tornado deaths are associated with the rare ____
F4/F5s
Noted Tornado Outbreaks
1925 Tri-State Tornado (Fig. 10.23)
1974 Super outbreak (Box 10.4, Fig. 10.E)
2011 Worst Tornado year in US history
Noted NC Tornadoes
1984 Over 40 fatalities, 800 injuries in eastern NC
1988 Several fatalities , 160 injuries in Raleigh
2011 Over 20 fatalities (4 in Raleigh), 30 tornadoes
Tornado Forecasting
The Storm Prediction Center in Oklahoma issues:
Tornado Watches for the entire US, which are designed to alert the public to the fact that conditions are favorable for the development of tornadoes.
Watch “Boxes” are typically issued for:
- Large areas (25,000 square miles)
- Relatively long time period (4-6 hours).
The local National Weather Service Offices (ours is at Centennial Campus) issue:
Tornado Warnings only after a tornado has been spotted or indicated on Doppler RADAR.
Warnings are typically issued for:
- Small areas (i.e. counties)
- Short time period (30-60 minutes)
