ATMS 120 Exam 2 Study Guide

Review the lightning myth or fact exercise.

True or False: If you touch someone who has been struck by lightning, you might get electrocuted. False True or False: Every lightning strike produces thunder. True True or False: If you are about to be struck by lightning, your hair might stand on end. True True or False: Lightning can sometimes occur on hot and humid summer evenings when there are no thunderstorms. False

Why is moisture important to thunderstorm formation?

*** Fuel for the storm is warm moist air Td at least 60°F Energy source = the production of cloud (condensation); releases latent heat Can't have instability without moisture Moisture Supply Primary: Gulf of Mexico Secondary: Evaporation from the soil and evapotranspiration from vegetation (corn sweat)

What are downbursts and microbursts? How do they form?

A downburst is strong downdraft that comes in contact with the ground Microbursts are smaller (and often more intense) downbursts Downbursts form +Sinking air is colder than the air around it +Rain falls into unsaturated air and evaporates -Cooling the air -Cold air sinks +Rain drags air downward with it +Downbursts can sink as fast as 40-50 mph Aviation hazard Microburst form +Smaller and more intense downburst

What is the definition of a tornado?

A violently rotating column of air in contact with the ground and pendant from a cumuliform cloud. +Rotation, not visible funnel defines tornado +Not in contact with the ground = funnel cloud Develop within supercell thunderstorms, but also form in thunderstorms along squall lines, near the ends of thunderstorm bow echoes, within land-filling hurricanes, and rarely within ordinary thunderstorms

What is a temperature inversion? How does it relate to stability?

An increase in air temperature with increasing altitude, which is inverse to the usual decrease in temperature with altitude in the troposphere. Layer is very stable; little to no weather occurs here

What are the three ingredients for thunderstorm formation? What additional ingredient is needed for severe thunderstorm formation?

An unstable atmosphere A trigger or lifting mechanism Adequate surface moisture Strong wind shear

What are typical lifting mechanisms that can trigger thunderstorms?

Anything that can cause the air to rise Frontal lift (Cold/warm front) Orographic lift (over mountains/valleys) Convergence Convective lift

What is the life cycle of a tornado? Why do they dissipate?

Average tornados=13 minutes (recent outbreaks = 30 min) Long lived tornadoes are over an hour Starts with funnel cloud from wall cloud or rotating dust swirl on the ground; rotation makes contact with the ground, condensation funnel cloud descends to the ground and increases intensity; intensity peaks during the mature stage (tornado is at its largest and often nearly vertically erect); some tornadoes at this stage expand to a very large size during this stage; during the shrinking stage, the vortex tilts over more becoming ropelike; during the decay stage, the tornado becomes ropelike losing contact with the ground and finally dissipate

What is a gust front or outflow boundary?

Boundary between cold outflow and warm inflow is called gust front or outflow boundary This is why the temperature drops and winds increase just before a thunderstorm begin

What is a shelf cloud? How does it form and what weather conditions might be expected as one passes?

Can be called roll clouds depending on their shape A low level, wedge-shaped arcus cloud associated with air rising over a gust front An ordinary thunderstorm will typically produce an outflow of cool air. Air rises to form gust front over the spreading cool pool (rain cooled air) Possible new thunderstorms

What is a mesocyclone? How does it form?

Cyclonic circulation within the updraft region of a supercell thunderstorm. Normally coincides with the region where tornadoes form. Typically several kilometers in diameter. Located on the southwest side of the storm Air rises from the surface to the tropopause When winds intensify, the force released can cause the updrafts to rotate The wind shear in the lowest 1.5 km of the atmosphere creates horizontally-rotating tubes of spinning air. If the updraft of a developing thunderstorm tilts this horizontally-rotating tube upward... ...the updraft begins to rotate about a vertical axis. Mesocyclones are often visually manifested by stunning structure, including cloud striations

What are the environmental, dry, and moist adiabatic lapse rates? Why are they different?

Environmental adiabatic (varies from location to location, day to day, and one layer of the atmosphere to another at a particular location and time) The rate at which the environmental temperature changes with the height in the atmosphere Dry adiabatic (cool and warm at the same rate) The rate (10 degrees C/km) at which an unsaturated parcel of air will change temperature if it is displaced vertically in the atmosphere Moist adiabatic The rate at which a saturated air parcel will cool as a result of expansion during its ascent if there is no exchange of heat with the surrounding environment. The rate is variable, typically 6 degree C/km at the surface and less than 6 higher in the atmosphere Rate which temperature decreases with increasing altitude

How can you tell how far away a lightning strike is form you?

Formula: # of seconds between seeing lightning and hearing thunder divided 5= # miles from you

What does a supercell updraft look like? The FFD? The RFD?

Found on the upshear side of the cloud, and is characterized by a well-defined cloud base with rapidly growing turrets above. Forward Flank Downdraft: evaporational cooling of air that spills to the earth's surface; it spreads out in all directions under a storm's downdraft but spreads out more quickly in the direction the winds are blowing (cool and moist air) Rear Flank Downdraft: represents relatively warm and dry air which is forced down form the mid-levels of the atmosphere and wraps around the back of the mesocyclone; it is visible as a clear spot in a cloud wall

Know tornado safety.

Get to the lowest floor of a substantial structure Basement preferred Don't waste time opening or closing windows Get to an interior room on the lowest floor of a sturdy structure Put as many walls between you and the tornado as possible Stay away from windows Protect yourself from flying debris +Mattress +Bike helmet +Wear shoes If you have no basement, interior bathroom or closets are best Don't wait for sirens to sound (only meant to be heard outdoors) or to see the tornado. Take shelter when you hear the warning. Car & trucks are not safe tornado shelters Mobile homes are tornado death traps Never try to escape a tornado in a car in an urban area Never seek shelter under highway overpasses--act as wind tunnels and you have little to hold on to Avoid large rooms, including auditoriums, gymnasiums, and big box stores. Safest place. Basement or interior room, stairwell or hallway on a low floor. Worst place. Avoid windows, doors, and outside walls.

Know basic lightning safety.

Go inside if possible, stay away from electrical appliances, particularly corded phones, and avoid taking a shower or otherwise coming in contact with indoor water supplies. Avoid open water outdoors. Do not go under a lone tree, or any tree (unless you are in a forest.) Do not lie on the ground. Assume a stooping position, crouch down as close to the ground as possible and minimize ground contact by staying on your toes or heels. Remove metal belts and other metal objects and move away from them. Stay in the car. Stay away from golf course or any places where you are the tallest object.

Review the main components of a squall line - Gust front and shelf cloud, convective zone, and trailing stratiform region. What are the characteristics of each?

Gust front +The leading edge of the thunderstorm's rain-cooled downdraft air. Passage is often marked by a sudden increase of wind speed Shelf cloud +Can be called roll clouds depending on their shape +A low level, wedge-shaped arcus cloud associated with air rising over a gust front +An ordinary thunderstorm will typically produce an outflow of cool air. Air rises to form gust front over the spreading cool pool (rain cooled air) +Possible new thunderstorms Convective zone +The region of the cumulonimbus cloud or thunderstorm complex with a strong updraft Trailing stratiform region +In a mesoscale convective system, the stratiform cloud region to the rear of the convective line. Most evident during the system's mature and dissipating stage. Squall line, bow echo, trailing stratiform region, convective zone, gust front, shelf cloud

How does hail form? What type of thunderstorm produces the largest hail?

Hail is frozen precipitation that falls from thunderstorms (heavy precipitation) +Hail is generally larger than 3 mm and can be up to grapefruit sized (or even larger) +Most hail is small (pea sized) +Small hail can accumulate much like snow and sleet +Severe hail = 1 inch diameter +Giant hail = 4 inch diameter Hardness of hailstones varies +Can be as soft as a snowball or rock hard! How to get big hail? +Large instability (strong updrafts) +Strong wind shear, enabling supercell formation (stronger updrafts) +Cooler temperatures/drier air (less melting on the way down)

Know hail safety.

If outdoors, get indoors If in a car, pull over to a safe place and cover your eyes/face if glass is breaking Do NOT block roadways/hide under overpasses to seek shelter from hail Indoors, stay away from windows

What is atmospheric instability?

Instability is the key to a thunderstorm's growth. When you're talking about the atmosphere, instability describes what happens to the air once you raise it. If you pushed up air from the surface several thousand feet, would it continue rising, stay level, or drop back down? If it continues rising, it's unstable.

What is the LCL? LFC? EL? LI? CAPE? Know how to find each on a sounding and how they relate to cloud structure.

LCL Cloud base = LCL (Lifting Condensation Level) The altitude to which the air must be lifted to for condensation to begin As air rises it cools. Cooling the air increases the RH. At the LCL, the T = Td and the RH = 100% and a cloud forms LFC LFC (Level of Free Convection) The point above the ground that the parcel first becomes warmer than its environment The parcel can rise on its own because it is buoyant (like a hot air balloon) This is where the unstable part of the storm forms (clouds begin to bubble) EL Equilibrium Level (EL) The point where the anvil forms. Air hits the stable (warm) air in the stratosphere and spreads out horizontally. Marks the top of the storm. LI Lifted Index Tool used by meteorologists to determine the stability of the atmosphere. Environment temp at 500 mb - Parcel temp at 500 mb CAPE (Convective Available Potential Energy) The amount of potential energy that a parcel can obtain from environmental conditions. Mathematically, it is the area between the level of free convection and the equilibrium level. In order to identify CAPE on a sounding, find the level of free convection and follow the moist adiabat through this level up to the equilibrium level. The area between this curve and the temperature curve is positive area, (shaded area below)

How to non-supercell tornadoes form? What are they called?

Landspouts and waterspouts; mesovortices and gustandoes +Preexisting circulation at surface along wind shift stretched by updraft +No downdraft or supercell required +Can occur with supercells, ordinary thunderstorms, or MCSs +Usually less intense than supercell tornadoes, but still dangerous Develop within squall line thunderstorms aligned along fronts, along outflows from mesoscale convective systems (MCS), or even in thunderstorms aligned along the seafront

How are lightning and thunder related? How is thunder created?

Lighting heats the air around the bolt to 30000C or 54000F--huge pressure surge creates sound waves +the way sound waves travel through the atmosphere depends on temperature (which decreases with altitude) +sounds waves are bent upward due to this temperature gradient and higher frequency sounds are bent upward faster, leaving the deeper sounds closer to the surface +Echo also contributes to the lasting rumble. Sound travels about 1100 ft/sec or about 750 mph or 5 miles per second

What is the supercell spectrum? Which type of supercell is the most prolific tornado producer?

Low precipitation, classic, high precipitation—determined by low level moisture and value of precipitable water Classic supercells

What is the typical life cycle of an ordinary thunderstorm?

Most common type of thunderstorm Usually not severe Last about an hour from cumulus stage Warm air rises, expands, and cools. When the air reaches its LCL, water vapor condenses and forms a cloud. Rising air reaches its LFC and continues rising on its own due to buoyancy The current of rising air is called an updraft Cannot have a thunderstorm without an updraft Latent heat release due to condensation in updraft powers thunderstorm As cloud (updraft) gets taller, precipitation begins to form mature state Updraft eventually reaches tropopause Tropopause is very stable (inversion layer) - Acts as lid on storm Air diverges outward when it reaches tropopause, forming anvil cloud at top of storm Top of updraft may penetrate into stratosphere Overshooting top Precipitation particles grow, become heavy, begin to fall into updraft Falling precipitation drags air downward with it Some rain evaporates, cooling air even more When air is cooler than its surroundings, it sinks This sinking air is called a downdraft Precipitation is required to form a downdraft dissipation stage In regimes of weak or no vertical wind shear, precipitation falls into updraft Cools updraft, drags rising air downward Updraft weakens, replaced by downdraft Surging outflow cuts off supply of warm, moist, unstable air needed to sustain updraft Updraft dissipates Thunderstorm dominated by downdrafts and precipitation and quickly dissipates Generally develop in regimes of weak vertical wind shear Most common in summer, away from fronts

How is lightning distributed by geography?

Most ideal in areas where warm, moist air rises and mixes with cold air above. Most common in Florida

What is a supercell? A hook echo?

Most intense thunderstorms in the Earth's atmosphere Always rotate Circulation so dominate its immediate area that the entire storm behaves as a single entity rather than a group of cells Produce the most tornadoes, all severe tornadoes, most large hail, and damaging straight-line winds Needs +Trigger mechanism +Gulf moisture +Unstable atmosphere +Wind shear Hook echo originates from precipitation particles rotating around the updraft The characteristic curved shape signature of a tornadic supercell on a radar reflectivity image. A tornado is often located near the end of the hook.

Why are downdrafts important in supercell tornadogenesis?

Need a downdraft to bring rotation from aloft to the ground Rear-flank downdraft Rear-flank downdraft descends and creates another horizontal vortex along the surface. It also generates vertical rotation and brings rotation from aloft to the surface.

Do mountains, cities, lakes, or bodies of water prevent tornadoes?

No

What are the three thunderstorm types?

Ordinary (single-cell) thunderstorms Usually not severe Form in environments with weak vertical wind shear Multicellular thunderstorms (MCSs) Thunderstorm complexes Form in environments with moderate to strong vertical wind shear Supercell thunderstorms Almost always severe Produce nearly all intense tornadoes and hail larger than golf-ball sized Form in environments with strong vertical wind shear *Vertical wind shear most important factor in determining thunderstorm type

What is a bow echo and what is the main weather hazard associated with them?

Radar reflectivity feature of severe thunderstorms having an arc or bow shape. Often associated with strong straight-line winds and derechos and even tornadoes.

How does the relative humidity of air change as it rises and sinks? Why?

Relative Humidity: Ratio of water vapor in air to that required for saturation. Increase Relative Humidity: Add moisture to air. Cool the air. Decrease Relative Humidity: Remove moisture from air. Warm the air. Depends on temperature. Higher RH means air is closer to saturation

What is a wall cloud? How does it form?

Sometimes rotates Defined as a lowered cloud base beneath the updraft Tornadoes can develop from rotating wall clouds inflow of warm moist air Wall cloud forms as a storm ingests humid, rotating air from forward-flank downdraft

Review the sequence of events of a lightning strike

Stepped ladder, return stroke, dart leader, second return stroke

When (during a year and during a day) are tornadoes most likely? Why?

Summer months & late afternoon Offer the best combination of vertical wind shear and instability After the warmest hours of the day Tornado outbreaks occur when a storm system propelled by a strong, southward dip in the jet stream punches into the Plains, Midwest or South. This is accompanied by warm and humid air flowing northward out of the Gulf of Mexico.

On basis are tornadoes rated?

Ted Fujita - Studied 70 years worth of tornado, damage (over 30,000 tornados) Basis - Damage. Tornado winds usually not measured directly. Assigning value - F0-F5 (EF0 -EF5) (Look at image on study guide)

Why is lightning from the anvil more dangerous than that form the base of a storm?

The most dangerous lightning strikes are those that occur when our guard is down; lightning strikes from a sky that appears to be clear and the nearest thunderstorm on radar is many miles away. This is called anvil lightning and has been referred to as "lightning from the blue." Technically, that type of lighting isn't "from the blue" at all (assuming "blue" is meant to suggest that the sky was cloud-free when the strike occurred). Rather, all lightning emanates from the clouds of thunderstorms. Most lightning strikes occur within or in very close proximity to the thunderstorm producing it; however, anvil lighting can occur dozens — even a hundred or more — miles away from the parent thunderstorm.

What is the typical electric change separation inside of and near a thunderstorm? What causes this charge separation?

The upper part of the storm including the anvil has an excess of positive ions and is positively charged, while the lower part of the storm has an excess of negative ions is negatively charged. The ground beneath the main part of the storm is positively charged, while the ground beneath the anvil is negatively charged. Cloud needs ice Interface charging +The transfer of electrical charge from one conducting object to another when the conductors are brought in contact. Electrons flow across the interface between the two conductors (graupel/hail & ice crystals) Induction charging +A feedback mechanism in which the atmosphere's background electric field causes positive and negative charge to migrate to opposite ends of a solid or liquid particles, enabling the transfer of charge during collisions of particles (positively charge rise to the top (smaller) and negatively charge (larger) fall creating strong electric field)

Why do ordinary thunderstorms typically only last for about an hour?

Their short lifetime is directly related to the fact that the environment winds in the thunderstorm's vicinity do not change substantially with height. Winds aloft are typically weak and vertical shear minimal.

Where are thunderstorms most common in the United States? Why?

They tend to form during hot afternoons--typical conditions found for example during the warm, humid summer in the southern and eastern United States. They typically form around weak boundaries. East of the Rockies

Where in the United States are tornadoes most likely?

Tornado Alley +Texas, Oklahoma, Kansas, Nebraska, Iowa, Illinois, Indiana, Maryland, Louisiana, Mississippi, and Florida +Most frequent over the Midwest & Great Plains +Southern states have longer tornado seasons because winters are short. +Florida has the most tornadoes because the sea breeze triggers thunderstorms nearly every day during the summer.

What is a multiple vortex tornado? How are they different than regular tornadoes?

Tornadoes that contain smaller, rapidly spinning whirls known as subvortices, or suction vortices The combination of the rotation of the suction vortices, the tornado vortex, and the movement (translation) of the tornado can cause a chaotic wind field and damage pattern where some houses are destroyed while others nearby nearby escape significant damage.

What is the updraft? Downdraft? Overshooting top? Anvil? How do they form?

Updraft +Cloud consists of a warm, buoyant plume of rising air...cauliflower appearance +Triggered by lifting or heating of the surface Downdraft +Air is dragged downward +Enhanced by evaporation as precipitation encounters dry air mixing in from the sides and top of the cloud Overshooting top +Intrusion of a thunderstorm's updraft and its condensed ice particles into the stratosphere as a result of the air's upward momentum Anvil +Cloud formation found at high elevation of a thunderstorm; most extensive on the forward position of a moving thunderstorm; air form the updraft exhausts horizontally at the tropopause

What is the difference between a watch and warning?

Watches = Conditions Are Favorable for Severe Weather +When a severe thunderstorm or tornado watch is issued, it means conditions are favorable for severe thunderstorms or tornadoes. Warnings = Severe Weather is Happening Now! +Tornado Warnings +A tornado has been sighted -by an official (i.e., a police officer) -by a trained storm spotter +Doppler radar has detected rotation within a storm or tornado debris. Severe Thunderstorm Warning +Hail 1 inch or greater +Doppler radar detects winds > 50 kts +Weather station observes winds > 50 kts +Storm spotter or official reports hail > 1 inch or wind damage

How does the temperature of air change as it raises and sinks? Why?

What causes the air to continue rising? Density. Colder air is more dense than warm air. And if you had a bubble of warm air surrounded by colder air, it would float up, just like a hot air balloon. Hot (less dense) air rises, expands, and cools As an air parcel sinks, the pressure around it increases, so the air parcel is compressed Pressure always increases as you go down toward the surface Compression transfers energy to the parcel Sinking air compresses and warms

What is outflow? How does it form?

When the cold downdraft reaches ground, it spreads outward +Forms pool of cold air at surface beneath storm +Known as a cold pool or outflow Boundary between cold outflow and warm inflow is called gust front or outflow boundary This is why the temperature drops and winds increase just before a thunderstorm begins +Also why it is cooler just after a thunderstorm Convergence at outflow boundary may trigger new storms

What is a derecho?

Widespread windstorm created by severe thunderstorms. Also called straight-line winds. Result when downbursts reach the ground Intense winds blowing in the same direction +Different from tornadoes because winds in tornadoes rotate and blow from many different directions +Can reach 100 mph +Debris falls in the same direction

What are the criteria used by the National Weather Service to determine if a storm is severe? How often does each type of severe weather occur in the US each year?

Winds: At least 50 kts +15000 Hail: 1+" in diameter +10000 Tornado +1200

What are mammatus clouds? Are they dangerous?

cellular pattern of pouches hanging from the anvil clouds often associated with severe thunderstorms; yes

Why do supercells typically persist for much longer than ordinary thunderstorms?

organized internal structures make storm able to maintain itself strong vertical wind shear and strong winds aloft allow the precipitation to fall away from the updraft aloft, which allows the storm to feed off of the warm, moist air that is flowing into it at the surface, hence it will last longer; updraft is sustained