Study Notes on Thunderstorms and Tornadoes
Thunderstorms and Tornadoes
General Characteristics of Thunderstorms
Thunderstorms must contain:
Thunder
Lightning
Strong, gusty winds
Hail (in all sizes)
Precipitation (rain and snow)
Thunderstorms can also contain tornadoes.
Types of thunderstorms:
Ordinary (air mass) thunderstorms
Multicell thunderstorms
Supercell thunderstorms
Ordinary Thunderstorms
Most common type of thunderstorm.
Forms in areas with weak wind shear (variability in wind speed and direction with height).
Characteristics:
Self-extinguishing
Falls apart with the presence of colder, downdraft air
Short lifetime (typically 1 hour)
Three Stages of Development
Growth Stage
Rising air parcel cools and condenses, forming cumulus clouds that grow taller.
Latent heat release keeps moisture in the air, maintaining warmer rising air compared to surrounding air.
Involves rising and sinking of moist, dry, warm, and cold air.
Mature Stage
Cooler, drier air helps to evaporate water droplets, leading to heavy air descent.
Aerodynamic processes involve downdraft, which is the exhaling wind from a cloud or storm.
Thunderstorm begins to inhale and exhale air, feeding itself and reaching peak intensity.
Storm takes on an anvil shape.
Strong updrafts and downdrafts develop in the middle section of the storm, producing heavy rain, strong winds, and hail.
Presence of overshooting tops indicates strong updrafts pushing clouds into the stratosphere, signaling a very intense storm and possible rotation.
Decaying Stage
Updrafts weaken, and the storm begins to starve itself.
Dominance of downdrafts occurs following the development of the storm, typically around 30 minutes after initiation.
Ordinary thunderstorms are generally short-lived and often not severe.
Multicell Thunderstorms
Clusters of thunderstorms that form in various growth stages within the same area.
Comparison with ordinary thunderstorms:
Ordinary thunderstorms have weak winds, whereas multicell thunderstorms have strong winds.
Multicells exhibit continuous convection.
Severe Weather:
Multicell storms can last for several hours and produce more severe weather than ordinary thunderstorms.
Squall-Line Thunderstorms
Form an organized line of thunderstorms.
Intense rainfall occurs at the leading edge, tapering off behind the line.
Such squall lines can be positioned along or ahead of a cold front.
Individual cells are strong, featuring significant downdrafts, and can produce stratiform rain.
Bow echo phenomena characterized by leading edges jumping ahead of the main line indicate damaging winds and straight-line winds.
Supercell Thunderstorms
A distinct thunderstorm characterized by a rotating updraft (mesocyclone), differentiating it from multicell thunderstorms.
Most organized and long-lived type of thunderstorm, lasting over 2 hours and covering more than 100 miles.
Classification:
High Precipitation (HP)
Low Precipitation (LP)
Characteristics threatening severe weather include:
Wind speeds greater than 58 mph
Hail larger than 1 inch
Possible rotation indicated by the presence of tornadoes.
Can produce:
Strong downburst winds
Large hail
Heavy rainfall
Weak to violent tornadoes
Essential features:
Flanking Line (updraft/food supply for the mesocyclone)
Rear-Flank Downdraft (back part of the mesocyclone's downdraft)
Inflow ("Suck Zone"; influx of moisture-rich air into the mesocyclone)
Mesoscale Convection Systems
Large groupings of multiple storms moving together and enduring for several hours.
Two types/shapes:
Circular shape: covers multiple states; can produce severe weather and heavy rain.
Linear shape: can produce very strong winds and isolated tornadoes.
Other Parts of Severe Thunderstorms
Gust Front:
Leading edge of cold air from severe thunderstorms, effectively exhaling air from a storm.
Microbursts:
Very localized columns of sinking air, generating damaging straight-line winds at the surface.
Typically found in downdraft areas and can produce strong winds, often confused with tornado damage.
Derechos:
Widespread, long-lived violent windstorms associated with fast-moving bands of severe thunderstorms, usually taking a bow echo shape.
Most often occur in summer.
Thunder and Lightning
Lightning and thunder occur together; one cannot exist without the other.
Lightning:
A discharge of electricity within a mature thunderstorm.
Heats the air to temperatures exceeding 50,000°F, significantly hotter than the sun.
Thunder:
Caused by the heating and expansion of air, generating a shockwave.
Measuring Distance to Lightning
Distance can be estimated by counting seconds between lightning flash and thunder.
5 seconds correlates to approximately 1 mile.
Thunder sounds like a crack followed by a bang when lightning is close and rumbles when further away (due to sound bouncing off surfaces).
Mechanics of Lightning
Charge separation occurs in the cloud:
Negative charges accumulate near the cloud base.
Positive charges are positioned higher in the cloud and above ground.
Stepped Leader:
The initial “spark” of lightning that leaves the cloud base; faint and nearly invisible.
As the stepped leader nears the ground, a positive charge arises upwards to meet the negative charges below.
Return Stroke:
The upward flow of current seen as lightning; occurs rapidly, around 1/10,000 of a second.
Types of Lightning
Cloud to Ground: Most common type.
Cloud to Cloud
Cloud to Air
Other forms include:
Sheet Lightning
Ball Lightning
Heat Lightning:
Distant lightning illuminating the sky, too far for thunder to be heard, unrelated to heat.
Ribbon Lightning
Tornadoes
Definition: A rapidly rotating column of air with rotation that reaches the ground.
Characteristics:
Diameter: 50-100+ yards
Path Length: Up to 2 miles
Lifespan: 5-10 minutes
Wind Speeds: 80-300 mph
Forward Speed: Approximately 30 mph
Types of Tornadoes
Waterspout:
Similar to a tornado but occurs over water, generally featuring slower wind speeds and is associated with weak convective thunderstorms.
Landsport:
Similar to a tornado but forms from the ground up due to low-level circulation; typically weaker but can demonstrate strong characteristics on rare occasions.
Funnel Cloud:
A funnel-shaped cloud of condensed water droplets associated with rotating air.
Extends from the cloud base but does not reach the ground; not all funnel clouds develop into tornadoes.
Life Cycle of a Tornado
Dust-Whirl Stage:
Dust swirls upward from the ground toward the funnel cloud, causing minimal damage.
Organizing Stage:
Downward extension of the funnel connects with the dust whirl, causing light damage.
Mature Stage:
Tornado touches the ground with strong winds, creating significant damage.
Decaying Stage:
Tornado begins to end as rotation slows, creating a thin, rope-like funnel.
Occurrence of Tornadoes
Most tornadoes form in the late afternoon due to daytime heating and a more unstable atmosphere.
Tornadoes can occur any time of year; the most common months in some regions (e.g., Missouri) are March, April, and May.
Geographic Distribution
Primarily occurring in the United States, particularly in Tornado Alley (Midwest states).
Tornadoes occur across six continents.
Formation of Tornadoes
Essential conditions required:
Strong winds that turn with height (wind shear).
Very strong winds blowing in different directions in an unstable atmosphere create significant updrafts.
Mesocyclone:
A large rotating vortex within a supercell thunderstorm, sometimes producing tornadoes.
The turning of winds with height leads to rotational characteristics dropping from altitude to the surface.
Inside a Tornado
Very strong winds characterized by suction vortices, which are stronger winds than the tornado itself and cause considerable damage.
"Mini-tornadoes" may exist inside the main tornado, contributing to the overall damage.
Detecting Severe Weather
Doppler Radar:
Specialized weather radar capable of measuring wind velocity and precipitation/reflectivity.
Color indicators:
Red represents wind blowing away from the radar.
Green indicates wind blowing toward the radar.
Tornado Damage Assessment
Tornadic winds are not always measured directly by radar; assessment relies on evaluating the damage caused.
Meteorologists use damage assessments to estimate wind speeds, utilizing the Enhanced Fujita Scale to rank tornado destruction from EF-0 to EF-5.
Tornado Direction and Duration
Most tornadoes typically travel from southwest to northeast.
Some tornadoes may change direction and path, and some have been reported to travel backward or make complete circles.
Tornado Warning Signs
Signs of an impending tornado:
Appearance of rotating clouds is highly indicative.
Rotating wall clouds may signal a severe threat.
Circulating dust and debris visible at the ground beneath a cloud base and/or wall cloud.
Tornadoes may start without a visible funnel.
Heavy rainfall followed by rapid quietness may precede tornado formation.
A sudden and intense wind could also indicate a tornado’s presence.
Rain-wrapped tornadoes make visibility difficult due to heavy rainfall around them.
Tornadoes occurring at night are particularly dangerous as they are hard to see, though they may be illuminated by power flashes from nearby lightning.
Thunderstorms and Tornadoes
General Characteristics of Thunderstorms
Thunderstorms must contain:
Thunder
Lightning
Strong, gusty winds
Hail (in all sizes)
Precipitation (rain and snow)
Thunderstorms can also contain tornadoes.
Types of thunderstorms:
Ordinary (air mass) thunderstorms
Multicell thunderstorms
Supercell thunderstorms
Ordinary Thunderstorms
Most common type of thunderstorm.
Forms in areas where winds don't change much in speed or direction as you go higher in the sky (weak wind shear).
Characteristics:
Self-extinguishing (falls apart on its own)
Falls apart when colder air from its own downdraft (air pushing down) spreads out.
Short lifetime (typically 1 hour)
Three Stages of Development
Growth Stage
Rising air cools down and its moisture turns into clouds, making cumulus clouds grow taller.
Hidden heat, released when water vapor turns into liquid (latent heat release), keeps the rising air warmer and helps it keep going up.
Involves air moving up and down, both moist and dry, warm and cold.
Mature Stage
Cooler, drier air helps evaporate water droplets, making this air heavy and causing it to rush downward.
This downward rush of air is called a downdraft, like the storm 'exhaling' wind.
The thunderstorm starts to 'breathe in' (updrafts) and 'breathe out' (downdrafts) air, which feeds the storm and makes it strongest.
The storm takes on an anvil shape at the top.
Strong updrafts (air moving up) and downdrafts (air moving down) happen in the middle, causing heavy rain, strong winds, and hail.
If you see clouds poking really high into the very top layer of the atmosphere (overshooting tops), it means there are extremely strong updrafts, signaling a very intense storm and possible spinning.
Decaying Stage
The updrafts (air moving up) weaken, and the storm starts to 'starve' itself.
Downdrafts (air moving down) become the main force, usually about 30 minutes after the storm started.
Ordinary thunderstorms are generally short-lived and usually not severe.
Multicell Thunderstorms
Clusters of thunderstorms that are at different stages of growth all in the same area.
Comparison with ordinary thunderstorms:
Ordinary thunderstorms have weak winds, while multicell thunderstorms have strong winds.
Multicells keep producing new updrafts, leading to continuous storm activity.
Severe Weather:
Multicell storms can last for several hours and produce more severe weather than ordinary thunderstorms.
Squall-Line Thunderstorms
Form a long, organized line of thunderstorms.
Very heavy rain falls at the front of the line, getting lighter behind it.
These lines can form along or ahead of a cold front.
Individual storms within the line are strong, with significant downdrafts (air pushing down), and can produce widespread, lighter rain (stratiform rain) behind the intense front.
If the leading edge of the line bulges forward like a bow (a bow echo), it often means very damaging straight-line winds are happening.
Supercell Thunderstorms
A special type of thunderstorm known for having a rotating updraft (called a mesocyclone), which makes it different from multicell storms.
This is the most organized and longest-lasting type of thunderstorm, often lasting over 2 hours and covering more than 100 miles.
Classification:
High Precipitation (HP) - lots of rain with the rotation
Low Precipitation (LP) - less rain, rotation might be more visible
Characteristics that indicate severe weather include:
Wind speeds greater than 58 \text{ mph}
Hail larger than 1 \text{ inch}
Possible rotation, often leading to tornadoes.
Can produce:
Strong downburst winds (fast sinking air hitting the ground)
Large hail
Heavy rainfall
Weak to violent tornadoes
Essential features:
Flanking Line (a line of rising clouds that feeds the storm's spinning part/mesocyclone)
Rear-Flank Downdraft (a strong rush of air moving down from the back part of the mesocyclone's spinning action)
Inflow (the "Suck Zone"; moisture-rich air being pulled into the spinning part of the storm)
Mesoscale Convection Systems (MCS)
Large groups of many storms moving together and lasting for several hours.
Two types/shapes:
Circular shape: can cover multiple states; can produce severe weather and heavy rain.
Linear shape: can produce very strong winds and isolated tornadoes.
Other Parts of Severe Thunderstorms
Gust Front:
The leading edge of cold air that rushes out from a severe thunderstorm, like the storm's cold breath spreading out.
Microbursts:
Very small, focused columns of air sinking extremely fast, creating damaging straight-line winds when they hit the surface.
Usually found in areas where air is rushing downwards (downdraft) and can cause strong winds that are often confused with tornado damage.
Derechos:
Widespread, long-lasting, violent windstorms connected to fast-moving lines of severe thunderstorms, often shaped like a bow.
Happen most often in the summer.
Thunder and Lightning
Lightning and thunder always happen together; you can't have one without the other.
Lightning:
A huge burst of electricity inside a strong thunderstorm.
Heats the air to temperatures hotter than 50,000^\circ \text{F}, which is much hotter than the sun.
Thunder:
Caused by the sudden super-heating and expansion of air from lightning, which creates a loud shockwave.
Measuring Distance to Lightning
You can guess how far away lightning is by counting the seconds between seeing the flash and hearing the thunder.
5 seconds means it's about 1 \text{ mile} away.
Thunder sounds like a sharp crack followed by a bang when lightning is very close, and a rumbling sound when it's farther away (because the sound bounces off things).
Mechanics of Lightning
Inside the cloud, different electrical charges build up: negative charges usually gather at the bottom of the cloud, and positive charges are higher up and on the ground below.
Stepped Leader:
This is the first, almost invisible 'spark' of lightning that comes down from the cloud base.
As this 'spark' gets close to the ground, a positive charge from the ground reaches up to meet it.
Return Stroke:
This is the super-fast upward rush of electricity that you see as the bright visual flash of lightning; it happens incredibly quickly, in about 1/10,000 \text{ of a second}.
Types of Lightning
Cloud to Ground: The most common type.
Cloud to Cloud
Cloud to Air
Other forms include:
Sheet Lightning (distant lightning lighting up a large area)
Ball Lightning (a rare, glowing sphere of electricity)
Heat Lightning:
Distant lightning that makes the sky light up but is too far away for you to hear the thunder; it has nothing to do with heat.
Ribbon Lightning (appears as parallel streaks, usually due to strong winds spreading out multiple return strokes)
Tornadoes
Definition: A rapidly spinning column of air that touches the ground.
Characteristics:
Diameter: 50-100+ \text{ yards} (about the length of a football field)
Path Length: Up to 2 \text{ miles}
Lifespan: 5-10 \text{ minutes}
Wind Speeds: 80-300 \text{ mph}
Forward Speed: Approximately 30 \text{ mph}
Types of Tornadoes
Waterspout:
Like a tornado but forms over water, generally has weaker winds, and is connected to weaker thunderstorms.
Landspout:
Similar to a tornado but forms from the ground up due to spinning air near the surface; typically weaker but can sometimes be strong.
Funnel Cloud:
A funnel-shaped cloud of condensed water droplets that is spinning.
It extends from the cloud but doesn't touch the ground; not all funnel clouds become tornadoes.
Life Cycle of a Tornado
Dust-Whirl Stage:
Dust swirls up from the ground towards the funnel cloud, causing very little damage.
Organizing Stage:
The funnel cloud extends downward and connects with the dust whirl, causing light damage.
Mature Stage:
The tornado touches the ground with strong winds, causing a lot of damage.
Decaying Stage:
The tornado starts to weaken as its spinning slows down, becoming a thin, rope-like funnel.
Occurrence of Tornadoes
Most tornadoes form in the late afternoon because the sun has warmed the ground all day, making the atmosphere more unstable.
Tornadoes can happen any time of year; the most common months in some regions (like Missouri) are March, April, and May.
Geographic Distribution
Mainly occur in the United States, especially in 'Tornado Alley' (Midwest states).
Tornadoes happen on all six continents.
Formation of Tornadoes
Key conditions needed:
Strong winds that change direction or speed as you go higher (wind shear).
Very strong winds blowing in different directions within an unstable atmosphere create powerful updrafts (air rushing up).
Mesocyclone:
A large spinning column of air inside a supercell thunderstorm; this spinning can sometimes lead to tornadoes.
When winds turn with height, it causes this spinning motion to drop from high in the sky down to the ground.
Inside a Tornado
Contains very strong winds, including even stronger, smaller mini-swirls called suction vortices within the main tornado, which cause even more damage.
These "mini-tornadoes" can exist inside the main tornado, adding to the overall destruction.
Detecting Severe Weather
Doppler Radar:
A special weather radar that can measure how fast rain, hail, or dust are moving, as well as where precipitation is (reflectivity).
Color indicators:
Red means wind is blowing away from the radar.
Green means wind is blowing toward the radar.
Tornado Damage Assessment
The actual wind speeds of tornadoes aren't always measured directly by radar; instead, scientists estimate wind speeds by looking at the damage they cause.
Meteorologists use damage assessments to estimate wind speeds, using the Enhanced Fujita Scale to rank how destructive a tornado was, from EF-0 (light damage) to EF-5 (incredible damage).
Tornado Direction and Duration
Most tornadoes usually travel from southwest to northeast.
Some tornadoes might change direction and path, and some have even been seen traveling backward or making full circles.
Tornado Warning Signs
Signs that a tornado might be coming:
Seeing clouds that are visibly rotating is a very strong sign.
Rotating 'wall clouds' (a lowered, rotating part of the thunderstorm base) can signal a serious threat.
Spinning dust and debris visible on the ground directly under a cloud base or wall cloud.
Tornadoes can sometimes start without a visible funnel.
Heavy rain followed by a sudden, eerie quietness might come right before a tornado forms.
A sudden and intense gust of wind could also mean a tornado is present.
Rain-wrapped tornadoes are very dangerous because heavy rain makes them hard to see.
Tornadoes at night are especially dangerous because they are difficult to spot, though flashes from power lines breaking nearby (power flashes) might light them up.