ST L3: Lightning/ From Heat to Motion Flashcards
What is incandesce?
The glow in the air caused by heat (warmer than the surface of the sun!) from electrical discharge (lightning).
What is lightning?
Electrical discharge through air that heats the air to temperatures warmer than the surface of the sun. Th path of air instantly heated by lightning explosively expands to create a shock wave, which changes into the sound waves of thunder further from the lightning bolt.
Describe how lightning in a thunderstorm occurs.
Positive and negative electrical charge separates within the cloud.
Where does negative charge build up? Positive charge?
lower level in the cloud. positive charge builds up near the top of the cloud.
What is voltage? How does this relate to lightning?
Electrical force caused by buildup of negative charge. Electrical force becomes so strong an electrical breakdown occurs, resulting in lightning flashes.
How does the cloud return to equilibrium?
Lightning transfers the charge.
Describe the process of how lightning strikes the Earth.
Lightning begins with an electrical breakdown in the cloud. Breakdown causes electrical discharge called a downward propagating leader. Just before reaching ground, many small upward leaders rise to meet it. At least one of the upward leaders connect with the downward propagating leader. When they meet, the charge from the cloud transfers to the ground.
How fast is lightning?
Moves at hundreds of miles per second.
What is a return stroke? How hot is it? How fast?
Sudden, super fast transfer of charge from the cloud to the ground. It produces temperatures exceeding fifty thousand degrees F in a fraction of a second. Hotter than any other natural phenomenon on Earth.
How is a shock wave produced?
Air expanding explosively. We hear it as thunder.
Describe what happens once lightning finds a path to the ground.
Once lightning finds a path to the ground, it forms a channel through the air. Makes it easier for addition return strokes to transfer remaining charge from the cloud.
How many return strokes usually involved in lightning we see?
3 or 4, sometimes more.
Why do we see lightning strokes flicker?
More charge is being transferred to the ground following the same channel.
How much % of cloud-to-ground lightning is positively charged?
10%
Which flashes are generally more powerful? What does that entail?
positive flashes. they have higher chance of starting fires and damaging electrical power lines.
How can cloud-to-ground strikes be further classified?
by whether negative or positive charge moves to the ground
Where do negative strikes form?
Cloud base. They are more numerous.
Where do positive strikes form?
Thunderstorm anvil. Positive strikes are less frequent, often much stronger, and most likely to start wild fires. (In Canada, 10-25% of cloud-to-ground lightning is positive)
How do electrical charges build up in the thunderstorms?
From freezing and collision of cloud droplets and ice crystals. When the electrical potential (charge difference) between cloud and ground become great enough, the air ionizes. Air becomes conductor instead of insulator along the ionized path. Stepped leader nearing ground causes ground streamers of electrons rise from tall trees, poles, buildings to meet stepped leader.
What are graupeln?
ice crystals
How great of a breakdown potential is needed to ionize air?
3 billion volts.
What is a stepped leader?
Path of ionized air rapidly stepping down from the cloud.
What occurs once the stepped leader connects to the ground streamers?
Electrons flow in a massive current called a return stroke (we see the return stroke).
How strong is the current of return stroke?
(2 000 to 200 000 amperes)
What tears the ionized path apart?
wind
List the types of lightning.
cloud-to-ground lightning upward-moving lightning (or ground-to-cloud) intracloud lightning cloud-to-air lightning anvil crawler lightning bead lightning/chain lightning
Describe upward-moving/ground-to-cloud lightning
Often has strange smooth and fluid channel. Extremely rare type of lightning before Industrial Revolution. Tall man-made structures trigger the discharge.
Before the Industrial Revolution, how did upward-moving lightning occur?
From mountain peaks
What helps trigger upward-moving lightning these days?
Tall man-made structures, increasing skyscrapers and radio antennae make upward-moving lightning more common.
Where do most lightning discharges occur? What is this type of lightning called?
Only in clouds.
It is called intracloud lightning. Sometimes, it leaps out into air. Called cloud-to-air lightning.
What is bead lightning or chain lightning?
When main lightning channel begins to cool, sometimes decays in glowing fragments. This string of dots is the bead or chain lightning.
What is ribbon lightning?
Parallel successive flashes or strokes offset from each other. Occurs when strong winds blow the flashing channel (lightning sailing).
Where is lightning mainly found in North America
Southeast US. (where most thunderstorms occur) Florida has highest lightning strikes. Known as lightning alley.
How do thunderstorms get triggered in Alberta? In Ontario near the Great Lakes?
Alberta(most frequent lightning): updrafts of air along the Rockies
Ontario (highest lightning density): lake-breeze fronts
What is lightning flash density?
Flash density is the number of flashes per square kilometre per year
How to be safe when lightning is present?
a. Monitor weather conditions
b. Follow 30/30 rule
c. Stay indoors
What is the 30/30 rule for lightning?
If 30 seconds or less between the flash is seen and a bang is heard, then move indoors and stay there until 30 minutes after last lightning or thunder.
Where are safe indoor places during lightning storms?
fully enclosed metal vehicle with the windows rolled up;
Metal cars/vehicles (with windows up) and aircraft are safe because the lightning flows on the outside of the skin of the vehicle, leaving the people inside unharmed.
Modern composite (fiber glass) aircraft contain an embedded mesh of wires for the purpose of safely conducting the lightning electricity away from people and sensitive instruments
substantial permanent building
When indoors, what should you do during lightning event?
Don’t touch electrical appliances like wired phones. (cordless and cell phones are OK)
Don’t touch any plumbing connected by metal pipes.
Stay away from windows
When outdoors, what to do during lightning event?
Avoid unsafe areas like small structures, huts, rain shelters, nearby metallic objects (pole, trees, water, hill top, etc.)
If you feel the hair standing up on your head or arms, there is a good chance that you are about to be struck by lightning in the next second or so. Move immediately so that you are not the highest point.
if caught in the open and cannot find a safe place, in desperation do the “Lightning-Safety Crouch” with feet together, hands over ears
What is a “bolt from the blue?”
When lightning shoots out of the side of a storm and hits the ground ten miles away from the storm.
What to do if someone is struck by lightning?
Try to revive them with CPR.
How many deaths does lightning cause a year on average? How many injuries per year? Where do most of the casualties occur?
93 deaths, 300 injuries. Most take place outdoors during the summer, mainly in the afternoon and early evening.
How many cloud-to-ground strikes per year?
8 million. Causes 6-12 deaths every year.
When are lightning storms most frequent in the US?
Mid to late summer, when there are more thunderstorms.
How far away is the lightning?
Count the number of seconds between when you see the lightning and hear the thunder. Divide that number by 3 to estimate the range in kilometers to the lightning.
For Example: a 9 second difference means the lightning is 3 km away.
What does buoyancy do? What is it the dominant process of?
creates vertical motions (up and down drafts). dominant process in thunderstorms.
What does pressure do? What is it the dominant process of?
creates horizontal motions/winds.
dominant process of cyclones.
What are important for hurricanes?
buoyancy and pressure.
What makes a storm organized?
If they tend to perpetuate themselves with positive feedback (drawing in fuel by converting temperature into motion)
____ creates winds
forces. It causes air parcels to accelerate in the direction of the force.
What is an air parcel?
An object that is a blob of air, about the size of a toy balloon. When air parcel moves, there is nothing more than wind. (horizontal or vertical)
______ alters buoyancy to drive vertical winds.
Temperature
Warm air wants to ___, thereby creating ___. Cold air wants to ___, thereby creating ____.
rise; updrafts; sink; down drafts
Which type of air is denser?
Colder air. Results in downward buoyancy force, while warmer air is less dense and results in an upward buoyancy force.
______ alters pressure to drive horizontal winds.
temperature
A pressure difference across a horizontal distance creates a horizontal ______ force that can accelerate air parcels to create winds.
pressure-gradient
What is a pressure-gradient force?
A change of pressure between one air parcel and its neighbour.
As the hurricane develops, its center (____) becomes a ring of _____. ______ in these storms makes the core _____ relative to the outside
core; thunderstorms; Condensation; warmer
The horizontal pressure gradient at the top of the hurricane creates outward ____ winds. These winds remove air molecules from the ____.
Fewer molecules in the core causes ____ pressure at the surface (as shown in the figure below), because pressure is the ____ of all the overlying air. This low pressure at the bottom of the core creates a ____ ____ that sucks in air. This gives the ____ ____ into the bottom of a hurricane. This inflow ____ in more fuel (warm humid air), making the hurricane stronger.
spiraling; core
lower; weight; pressure gradient; spiral inflow; advects
Pressure decreases with height in the atmosphere in what manner?
Exponential. Rate of decrease with height depending on how densely packed the air molecules are.
Pressure decreases more gradually with height in which kind of air? Why?
Warmer air, because the warm-air column takes up more vertical space.
What concept ties the vertical and horizontal motions (winds) together? Describe the concept
the concept of continuity. the air molecules tend to spread themselves smoothly and evenly, and don’t leave any gaps (i.e., they don’t leave a vacuum), and don’t get bunched together. the air is continuous
the vertical motion generated initially by buoyancy within the parcel will generate a _____ motion in the air just outside and under the parcel.
horizontal
the ____moving air creates gaps or bunched-up air molecules slightly off to the sides of the rising air parcel, which drive other vertical motions
horizontally
the air parcel is rising because of its ____, and there is a ____ of the surrounding environmental air out of the way above the parcel, and filling back in under the parcel (Figure ST.61e). In real life, this circulation develops smoothly and continuously as the warm parcel begins to rise, and creates only ____ pressure changes, not a complete ____.
buoyancy; circulation; small; vacuum
air parcels that are forced horizontally will cause ____ circulations to move the surrounding air out of the way and to deposit it back behind the air parcel. Thus, vertical and horizontal motions are linked, by the effect of ____.
vertical; continuity
____ create winds. Temperature alters ____, to create vertical winds. Temperature alters ____, to drive horizontal winds. Continuity links vertical and horizontal winds into ____.
Forces; buoyancy; pressure; circulations
