Severe Thunderstorms, Lightning, Tornadoes, and Tropical Cyclones: Key Concepts and Criteria

What are the three criteria that define a severe thunderstorm?

Large hail (diameter >= 1 inch), and/or wind gusts >= 50 kts (58 mph), and/or a tornado.

Why is lightning not used as a criterion for a severe thunderstorm?

Because lightning occurs in all thunderstorms.

What is a 'stepped leader' in the context of lightning formation?

A series of connected electron discharges that move toward the ground, each about 50-100 meters long.

What is a 'return stroke' in lightning formation?

A luminous discharge that surges from the ground up to the cloud after the stepped leader makes contact with the ground.

What causes the sound of thunder?

The rapid expansion of superheated air (up to 30,000°C) from a lightning strike, creating an acoustic shock wave.

What is the '5 seconds per mile' rule used for?

To estimate the distance of a lightning strike by counting the seconds between seeing the flash and hearing the thunder.

What is 'heat lightning'?

Lightning from a distant thunderstorm where the light is visible, but the thunder has dissipated before reaching the observer.

What are the three main ingredients required for any thunderstorm?

Moist air, a conditionally unstable atmosphere, and a lifting mechanism (trigger) for the updraft.

What are the three stages of an air mass thunderstorm?

Cumulus stage, mature stage, and dissipating stage.

What characterizes the cumulus stage of a thunderstorm?

Rising air only, cloud formation, and the addition of moisture to higher altitudes.

What two factors cause downdrafts to form during the mature stage of a thunderstorm?

Entrainment (mixing of dry air causing evaporation and cooling) and drag (air pulled down by falling precipitation).

What marks the beginning of the dissipating stage of a thunderstorm?

The downdraft cuts off the updraft, preventing further cloud growth.

What is the fourth ingredient required for a severe thunderstorm that is not required for an ordinary one?

Vertical wind shear (winds changing direction and increasing in speed with height).

How does vertical wind shear help a severe thunderstorm persist?

It causes precipitation to fall into the downdraft, allowing the updraft to remain separate and retain its strength.

What is a gust front?

A 'mini cold front' formed by the cold air of downdrafts spreading out at the surface.

What is a downburst?

A downdraft that spreads out horizontally from the base of a thunderstorm.

What is the definition of a microburst?

A downburst with winds extending 4 km or less.

Why are microbursts particularly dangerous for aviation?

They create sudden changes in wind direction (headwinds to tailwinds) that cause aircraft to lose lift and altitude.

What is a multi-cell thunderstorm?

A storm system where the outflow from a previous cell triggers the development of a new cell, leading to longer life and higher severity.

What is a supercell thunderstorm?

The most severe type of thunderstorm, characterized by a single, violently rotating updraft called a mesocyclone.

What is a mesocyclone?

An inner core of rotating air within a supercell thunderstorm that often spawns tornadoes.

What is an overshooting top?

A feature of a very strong thunderstorm where the updraft is so powerful it pushes through the top of the cloud (anvil).

What is an anvil cloud?

The flat, spreading top of a mature thunderstorm cloud.

What are shelf clouds or roll clouds?

Cloud formations that can develop over a gust front as warm air is lifted by the cold pool of air.

What is required for a supercell to break through a temperature inversion?

The air must be sufficiently unstable to 'erode' the inversion layer.

What process causes a thunderstorm to dissipate?

The downdraft cuts off the updraft, causing the cloud to stop growing and eventually kill itself.

What are the primary characteristics of a thunderstorm?

Generation of lightning and thunder, strong updraft and downdraft motions, and frequent gusty winds, heavy rain, and hail.

What is an Air Mass Thunderstorm?

A thunderstorm formed by the unequal heating of the Earth's surface within a maritime tropical (mT) air mass.

Define entrainment in the context of a thunderstorm.

The mixing of dry air with cloud air at the edge of the cloud.

What is the role of drag in a thunderstorm?

Air is dragged downwards as precipitation falls, contributing to the downdraft.

What criteria must be met for a thunderstorm to be classified as 'severe'?

Winds in excess of 50 knots (58 mph), hail 1 inch or greater in diameter, or a confirmed tornado.

Define a tornado.

A rapidly rotating column of air extending from the base of a thunderstorm to the ground, characterized by intense low pressure.

Why is it difficult to see a tornado?

Wind is invisible; a tornado is usually only visible if it forms a condensation funnel made of water droplets, dust, and debris.

How does wind shear contribute to tornado formation?

Wind shear creates horizontal spinning tubes of air, which are then tipped vertically by the thunderstorm's updraft.

What is a wall cloud?

A precursor to a tornado that often forms at the base of a mesocyclone.

When does a funnel cloud become a tornado?

As soon as the funnel cloud touches the ground.

How are tornadoes classified by appearance?

Weak (rope-like and narrow), strong (classic funnel shape), and violent (dark in color with significant debris).

What is the Enhanced Fujita (EF) Scale based on?

Wind speed and the amount and type of damage caused by the tornado.

What is the difference between a tornado watch and a tornado warning?

A watch means tornadoes are possible in the area; a warning means a tornado has been spotted or indicated by radar and immediate shelter is required.

Which country experiences the most tornadoes annually?

The United States, with more than 1,000 per year.

During which months is tornado frequency highest in the U.S.?

March to July, with the most violent ones tending to occur in April.

What atmospheric conditions favor tornado formation?

Strong jet stream, upper-level trough, cold/dry air aloft, warm/moist air at low levels, and strong vertical wind shear.

What is a 'hook echo' on radar?

A visual signature on radar where precipitation is drawn into a cyclonic spiral around a mesocyclone, indicating a potential tornado.

What is a tornadic waterspout?

A tornado that formed over land and then traveled over water.

What are 'fair weather' waterspouts?

Waterspouts not associated with supercells that form over water, typically in tropical coastal areas, and climb upwards from the surface.

How does friction contribute to the initial rotation of a tornado?

Friction and drag at the surface cause lower-level air to move more slowly than air at higher altitudes, creating horizontal wind shear.

Why do tornadoes form more frequently in certain regions?

They form where high moisture content, strong wind shear, and lifting mechanisms (like fronts and troughs) frequently collide.

What are the regional names for tropical cyclones in the Atlantic/East Pacific, Indian Ocean, and Western Pacific?

Hurricanes (Atlantic/East Pacific), Cyclones (Indian Ocean/Australia), and Typhoons (Western Pacific/China/Indonesia).

What is the minimum sustained wind speed required for a storm to be classified as a Category 1 hurricane?

74 mph (64 knots).

What is the minimum ocean temperature required for tropical cyclone formation?

Greater than 26.5°C (80°F).

Why do tropical cyclones rarely form near the equator?

The Coriolis force is too weak to initiate the necessary rotation.

What is the primary direction of movement for tropical cyclones?

They travel from west to east due to easterly trade winds.

Describe the primary circulation of a hurricane.

Air travels cyclonically around a low-pressure center, with the strongest winds located in the eyewall.

What is the secondary circulation pattern of a hurricane?

Air travels inward at low levels, upward in the eyewall, and outward at upper levels.

What process releases the latent heat that powers a hurricane?

Condensation of water vapor.

Why is the eye of a hurricane typically clear and calm?

Higher pressure aloft induces downward air motion (subsidence), which warms and dries the air through compression.

What are the four main components of the 'Hurricane Recipe'?

A pre-existing disturbance, deep warm ocean water (>80°F), moist air throughout the troposphere, and low vertical wind shear.

What is the difference between a tropical depression and a tropical storm?

A tropical depression has winds of 23-38 mph, while a tropical storm has winds of 39-73 mph and is officially named.

What is the wind speed threshold for a 'major hurricane'?

111 mph or greater.

What does the Saffir-Simpson Intensity Scale measure?

It measures hurricane intensity based solely on wind speed.

What is the 'forecast cone' (cone of uncertainty) in hurricane tracking?

A representation of the probable path of the storm center, with a 2/3 likelihood that the center remains within the cone.

Is the forecast cone an 'impacts cone'?

No, the storm is often much larger than the cone, and impacts can be felt far outside of it.

What are the three primary factors influencing storm surge?

Wind speed, shoreline shape, and timing (tide level).

Where do the strongest storm surges typically occur in relation to the hurricane?

In the eyewall and the northeast quadrant of the storm.

What percentage of landfalling hurricanes produce at least one tornado?

Nearly 70%.

List four mechanisms that cause a hurricane to dissipate.

Moving over cold water, moving over land, encountering high wind shear, or moving into a region of dry air.

Why is deep warm water (~200m) necessary for hurricane maintenance?

It ensures that even when winds churn the ocean, the surface remains warm enough to sustain evaporation.

What is the typical diameter of a hurricane's eye?

Approximately 20 miles.

What is the typical diameter of a full-grown hurricane?

300 to 400 miles.

How does vertical wind shear affect a hurricane?

High vertical wind shear weakens or prevents the organization of a hurricane.

What happens to the size of the forecast cone as the time horizon increases?

The cone size increases because uncertainty inherently grows with time.

What is the relationship between central pressure and wind speed in a hurricane?

Lower central pressure results in stronger winds.

What is the primary difference between high and low pressure systems regarding air motion?

High pressure is associated with sinking motion (divergence), while low pressure is associated with rising motion (convergence).

What is the ENSO cycle?

The El Niño/Southern Oscillation (ENSO) is an irregular, naturally occurring cycle of warmer-than-average (El Niño) or colder-than-average (La Niña) conditions in the tropical Pacific.

What defines the Southern Oscillation?

A seesaw pattern of reversing surface air pressure at opposite ends of the Pacific Ocean that impacts rainfall.

How do trade winds behave during an El Niño event?

Trade winds weaken and can even switch direction.

How do trade winds behave during a La Niña event?

Trade winds strengthen, causing sea surface temperatures to cool in the eastern equatorial Pacific.

What is a teleconnection in climate science?

A teleconnection occurs when remote changes, such as sea surface temperature anomalies in the equatorial Pacific, impact weather patterns in distant parts of the world.

How does El Niño typically affect the Southern United States during winter?

It brings wetter and cooler weather due to a southward shift in the Subtropical Jet Stream.

How does La Niña typically affect the Southern United States during winter?

It brings drier and warmer weather.

What effect does El Niño have on Atlantic hurricane activity?

It leads to a reduction in hurricane activity due to increased vertical wind shear.

What is the difference between weather and climate?

Weather depends on initial conditions and short-term evolution, while climate is determined by the properties of the Earth system itself (boundary conditions).

What are examples of climate boundary conditions?

Land-sea distribution, solar changes, orbital variations, and greenhouse gases.

What is radiative forcing?

External factors that cause Earth to be out of radiative equilibrium, leading to climate change.

What is the concept of radiative equilibrium?

A balance between incoming and outgoing radiation that keeps Earth's average temperature relatively constant.

What are three natural forcings that affect climate?

Volcanic eruptions, solar variations, and orbital changes.

What are three anthropogenic forcings that affect climate?

Changes in atmospheric composition (greenhouse gases, aerosols) and land-use changes (deforestation, urbanization).

What is paleoclimate?

The study of prehistoric climates and their variability using indirect measures called proxies.

What is dendrochronology?

The study of tree rings to reconstruct past climate conditions.

What are the three orbital parameters known as Milankovitch cycles?

Eccentricity (shape of orbit), axial tilt (obliquity), and precession (wobble of the axis).

How does eccentricity affect Earth's orbit?

It determines how round the orbit is, with a cycle of approximately 100,000 years.

How does axial tilt (obliquity) affect the seasons?

More tilt leads to more severe seasons (warmer summers, colder winters), while less tilt leads to milder seasons.

What is the timescale of variation for Earth's axial tilt?

41,000 years.

What is precession?

The wobbling of the Earth's axis like a spinning top, which determines where in the orbit specific seasons occur.

What is the fundamental cause of the succession of Quaternary ice ages according to the Milankovitch theory?

Changes in the Earth's orbital geometry that lead to cooler summers at 65°N, reducing snowmelt and promoting ice sheet growth.

What was the Younger Dryas event?

A period between 13,000 and 10,000 years ago where Northern Hemisphere temperatures cooled abruptly to near-glacial conditions.

What characterizes the end of the Younger Dryas?

It was an abrupt climate shift, with Greenland temperatures rising 10°C in just one decade.

What characterized the Medieval Warm Period (~1000-1300)?

A period of tranquil weather that allowed for the colonization of Iceland, Greenland, and North America by the Vikings.

What factors contributed to the Medieval Climatic Optimum?

Higher than average solar radiation and reduced volcanic activity.

What was the primary impact of the Little Ice Age (~1400-1850) on Viking colonies?

The Greenland ice pack advanced, leading to the collapse and perishing of the Viking colonies.