Lightning Types and Strike Sequence

• Lecture 1 is about different types of lightning and the sequence of events in a lightning strike

Common Types of Lightning

• Intracloud lightning (IC) happens within a single cloud and is the most common type that's often invisible from the ground.
• Cloud-to-ground lightning involves an anvil shape at the top with positively charged voltage.
• Positive strikes travel a longer distance and happen only if it has more volts.
• Positive strikes are less frequent, come from the anvil, often much stronger, cause natural wildfires, and account for 10-25% of Canadian CG lightning being positive.
• Negative strikes are more numerous and come from the cloud base.

Sequence of Events in a Lightning Strike

• Charge Separation refers to friction in the storm cloud that causes positive charges at the top and negative charges at the bottom.
• Stepped Leader's negative charge forms an ionized path (stepped leader) moving downward toward the ground.
• Return Stroke involves positive charge rising from the ground, meeting the stepped leader, resulting in a bright lightning flash
• Dart Leader's follow-up leader may travel along the same path, causing additional, less intense strikes.
• Thunder is rapid heating and air expansion from the lightning causing a shockwave.
• Thunder delay helps estimate strike distance at 5 seconds per mile.
• Africa has the highest density of lightning worldwide
• Florida is "lightning alley" in America
• Lightning in Canada results in 2.4 million CG strikes per year with 6-12 deaths
• Lightning detection networks include a Canadian lightning detection network and lightning detection from space.
• Sound travels slower than light, so divide the seconds between seeing lightning and hearing thunder by 3 to estimate range in kilometers.
• Monitor weather conditions
• Use the 30/30 rule: If less than 30 seconds between a lightning flash and thunder, move indoors and stay there 30 minutes after the last lightning or thunder.
• Safe places include a fully enclosed metal vehicle with windows up or a substantial permanent building but avoid hard-wired telephones
• When outdoors, avoid unsafe areas with small structures, huts, rain shelters, nearby metallic objects (pole, fence), trees, water, open fields, and hill tops
• Use lightning safety crouch feet together, hands over ears
• Start CPR on nearby people struck by lightning

Thunderstorm Clouds

• Thunderstorms are thick clouds with lightning and thunder under
• Thunderstorms have a cloud top near the top of the troposphere (10-15km) and a cloud base near the ground
• Main updraft (stem of mushroom) is about 15km in diameter
• Anvil can be 100s km in diameter
• Strong updrafts and downdrafts are typical
• With very strong updrafts, the dome of clouds overshoot above the anvil
• Storm energy is derived from temperature and humidity

Thunderstorm Cells

• Cumulonimbus are thunderstorms and are composed of large cells that evolve during 15-45 minutes
• Most thunderstorms contain 2+ cells, each in different stages of evolution

Multicell Thunderstorms

• Typically lasts longer than an individual cell
• A squall line is a line of thunderstorms consisting of many thunderstorms in a line
• Squall lines often form along a cold front
• Supercells are very large, rotating single-cell thunderstorms
• Supercells can cause tornadoes, large hail, frequent lightning, heavy rain, strong winds, and can last for hours
• Includes supercell types that are low precipitation, classical, and high precipitation
• Rotating thunderstorms are also called a mesocyclone
• A thunderstorm cell's life cycle has the cumulus stage with updraft, mature stage, and dissipating stage
• Multi-cell thunderstorms are made up of various stages of multiple cells that may last several hours

Storm Energy

• The sun is the source of atmospheric heat, with solar energy being absorbed at the top thermosphere, middle stratosphere), and bottom (earth surface).
• The absorbed ground warms the air in the troposphere and powers the storm
• Solar energy is absorbed by the ground, making it warmer and affecting the air via sensible heat and latent heat
• Latent heat increases humidity in air and sensible heat increases temperature in air

Storm Locations

• Favorable locations for thunderstorms have the greatest supply of heat and moisture
• Areas closer to the equator have warm ocean currents that lead to warm, humid air, such as Florida, Gulf States of USA, Prairies and central Canada

Lecture 2: Weather Radars

• Remote sensors measure the storm from the outside
• Identify thunderstorms from satellite images that may have an oval shape of anvil cloud and shadow the lumpy region of updraft overshoot.
• Visible images show clouds during daytime only, whereas IR images can show clouds both day and night
• Weather radars with environment and climate change Canada have been deployed
• Radar images of thunderstorm cells show the precipitation inside the storm, namely up and downdraft stem of mushroom cloud
• Radar microwaves reflect on everything in the atmosphere, including bird migration
• dBZ is a disaster density scale for radar-echo strength and an indicator of rainfall-rate

Supercells

• Types of supercells include low precipitation that produces lots of hail potentially, classic rainy downdraft combined with rain-free updraft, and high precipitation with updraft mostly surrounded by rain
• Hybrid storms/mixed mode supercells may be in-between or can feature 2+ types

Mammatus Clouds

• Bulbous clouds are on the underside of an anvil
• Rounded, pouch-like structures hang from the underside of clouds, often associated with thunderstorms.
• Indicate severe weather potential, such as thunderstorms or tornadoes because they are caused by sinking air and often appear after the storm has passed

Supercell Striations

• Rotating supercells typically have striations, or layers, around the mesocyclone
• Parallel, streaky lines of clouds often form in layers
• Striations indicate a stable atmosphere or a weather front
• Turbulence at higher altitudes is common as well as cloud types like altostratus or cirrostratus

Virga

• Wispy streaks of precipitation fall from the clouds but evaporate before reaching the ground
• Indicate dry below the cloud level and a potential for rain or thunderstorms if conditions change

Haboobs

• Intense dust or sand storms are often characterized by a wall of dust that moves quickly across the landscape
• Occurs in desert regions, storms are caused by the downdrafts of thunderstorms, which lifts sand or dust into the air

Arc Clouds

• Arc-shaped cloud formations are often a part of a larger storm system or weather front
• Associated with warm or cold fronts that indicate changes such as the approach of a storm system

Thunderstorm Hazards

• Rain downpours have extremely large rainfall rates that can causes flash floods - do not go into fast flowing water, even in a car because 30 cm of flowing will sweep it away.
• More than half of the US flooding deaths occur in vehicles
• Downbursts lead to a gust front creating a haboob (dust storm)
• The haboob leads to an arc cloud of warm air
• Downdraft speeds are between 20-90km/h
• Horizontal wind speeds near ground can reach 250km/h
• Micro bursts are small in diameter which are 1km downbursts
• Invisible micro bursts are hazardous for aircrafts
• Gust fronts occur because downburst air hits the ground and spread outward in straight lines
• Haboobs are visible if the ground is dry, and arc clouds are visible if the air is moist-gustnado
• Strong winds can blow down large trees and destroy weak things
• Avoid weak buildings and trees that could fall in a storm

Thunderstorms

• Airports have sensors for thunderstorms, flights avoid thunderstorms, and all pilots are trained
• Virga, a precipitation that evaporates before the ground, lead to downbursts
• Haboobs = sandstorms, hazard of downburst
• Storms can draw in humid air, condense the moisture, and release its heat into the storm, increasing the strength with violent winds
• Concepts include humidity, saturation, latent heat, advection, and adiabatic cooling

Humidity

• Humidity is the amount of water vapor in the air, made of water vapor, nitrogen, oxygen, trace gases, and liquid water droplets.
• Mixing ratio= amount of water vapour divided by the amount of all other gases
• Saturation is an equilibrium between evaporation and condensation
• Water vapor can easily condense into a liquid and an exchange of water molecules occurs between vapor and liquid
• Warmer air can hold more water vapor at equilibrium than colder air
• Air containing the max amount of water vapor is saturated
• Unsaturated air is not cloudy

Advection and Cooling

• Advection is movement of air by the wind
• Cooling is water vapor that can be advected into a thunderstorm by wind.
• Air cools when it is with no heat transfer to the surrounding environment
• Condensation releases latent heat, to provide energy to the storm
• If the humidity value becomes smaller than the actual humidity, condensation occurs
• Condensation releases the stored latent heat back into sensible heat to make storms warmer and even stronger
• Reduces the humidity to the equilibrium and produces or increases liquid cloud drops which can grow to become rain drops
• The release of latent heat increases the strength of the storm

Lecture 3: Tornadoes

• The shapes of tornadoes, what makes them visible, descriptions of wall clouds and where tornadoes form relative to thunderstorms
• Most tornadoes are visible by the droplets (funnel cloud) and/or dust from the ground
• Some tornadoes are invisible
• There a re tornado shapes. shape independent of intensity classification: cone, v-shaped, wedge, cylinder, hour-glass, rope
• Only a small percentages of supercell storms actually produce tornadoes
• Strong tornadoes are associated with supercell storms
• Attached dust or a tornado at the bottom of the thunderstorm suggests the presence of a supercell
• A rotating wall cloud around the mesocyclone leads to supercell formation
• Isolated lowering of the cloud base suggests a wall cloud
• tornadoes come from rotating wall clouds in supercells

Enhanced Fujita Scale

• Use enhanced Fujita scale to identify the tornado intensity
• Movement in North America is usually from SW toward NE
• Movement in any tornado has been observed in another direction
• If in car when a tornado happens, easily able to drive away
• Around 75% of the world's tornadoes occur in North America

Tornado Safety

• Rotational speeds around tornado are much faster than translation (how fast it travels)
• Tornadoes are classified by the enhanced Fujita scale in North America
• Europe uses TORRO scale

Tornado Evolution

• Characteristics of tornado evolution, tornado outbreaks, and mesocyclones
• Tornadoes are short lived, usually lasting a minute
• A path of damage is short, often 1-10s of km long
• Very occasionally, a tornado is able to last for hours
• A tornado outbreak is 6 tornadoes in one day, or many within a week
• Often associated with squall lines along cold fronts
• As cold fronts move toward the southeast, the storms move toward the northeast

Tornado Hazards

• If indoors, get below ground into a basement or storm cellar, and out of mobile homes
• If outdoors, get into a ditch or hole to place body below the “line of fire” of fast moving debris
• If in car, drive out of tornado towards right to left direction
• Tornadoes are most prone in tornado alley in the US
• In Canada, they are most frequent the prairies stretching to Nova Scotia
• Tornado watch is a 6-13 hr forecast for a broad region, and normal activities can continue
• A tornado warning means a tornado was actually detected

Lecture 4: Forces and Winds

• The lecture covers how forces, acceleration, buoyancy, and pressure-gradients relate to winds
• Prognostic equation applies to all objects
• How winds will increase, decrease, or change is dependent on the forces that act on air parcel Forces: -Buoyancy force (vertical) that causes up and downdrafts
• Pressure-gradient force (horizontal) that causes horizontal winds

Atmospheric Buoyancy

• Warm air rises which drives the release of latent heat and the production of violent updrafts
• Forces are used to create winds.
• Temperature determines buoyancy that causes vertical movement
• Pressure determines horizontal forces
• When air hits mountain ranges and is forced to rise, it cools at ~10C/km
• Sumas was drained by European colonization in Abbotsford
• the lake residents built their new home higher up

Atmospheric Continuity

• Continuity concept: air is spread relatively evenly
• Circulation occurs in real life, as air parcels start to move
• Circulation can be driven by buoyancy in the vertical or by horizontal
• Hail and a tornado have similar risk
• in the USA and canada, risk of hail is high in Alberta, Manitoba, and BC
• Risk of hail and rain: avoid the storm at the top, use care as heavy rainfall may cause water

Lecture 5: Hurricanes

• Hurricanes (typhons) have a height of 15 km.
• Wind turns counter-clockwise and the diameter is 150-300 km
• The center (eye) of a hurricane is clear, calm and has a low pressure
• Hurricanes and typhoons are tropical climates with surface winds turning
• relatively clear, calm, and low pressure
• Eye walls are rings of thunderstorms around the center, forming a cylindrical shape
• spiral bands= the thunderstorms extending from the center
• A warm core of warm air causes wind to be faster

Factors Affecting Hurricanes

• Hurricanes depend when SST is > 26 deg Celsius/79F + moisture
• Trade winds move storms
• Winds can weaken storm formation
• Waves are large and generate surges
• Hurricane self-fueling is possible when organized to create their own fuel in humid weather
• During hurricane formation, low pressures create air and condensation increases to create and circulate warm currents of air to make the strom last for several weeks/hour
• Core = eye + eye wall with high water and thunder
• Strong upward drafts, outflow and inflow occur with cycles that create complete rotation of the hurricane
• The risk is greatest during hurricane season

Hurricane Formation

• The tropics are favourable to hurricane formation due the rotation and temperature.
• Hurricanes only persist if central pressure in the eye remains low and the hurricane remains over warm

Storm Surge

• Large scale weather systems can interfere with those hurricanes
• hurricanes head where steer them, and get larger there
• Storms can also cause problems and affect lives (fatal)
• Theyre driven by strong wind that oceans push and can sea water to push towards shore, with high levels and floodings

Risk & Safety

• Its important to plan ahead during hurricanes and get ready for evacuation
• Predictors dont have probabilities
• Canada (especially in the Atlantic provinvces) experiences few hurricanes each decade

Explore the various types of lightning, including intracloud and cloud-to-ground strikes. Learn about positive and negative strikes, and understand the sequence of events in a lightning strike, from charge separation to the return stroke.