LightningLightning

Review: Different types of precipitationReview: Different types of precipitationDepends on atmospheric temperature and winds Depends on atmospheric temperature and winds

SnowSnow Rain Rain Sleet Sleet Freezing rain Graupel/hail Freezing rain Graupel/hail

Lightning: IntroductionLightning: Introduction• Lightning is a discharge of

electricity, a giant spark

• 80% of lightning occurs when clouds discharge electricity. This is referred to as cloud-to-cloud lightning; occurring when voltage gradient overcomes the electrical resistance of the air.

• 20% of lightning occurs when electrical discharge travels between the base of the cloud and the surface. This is referred to as cloud-to-ground lightning.

Video: Lightning in super slow Video: Lightning in super slow motionmotion

http://www.youtube.com/watch?v=RLWIBrweSU8

1. Charge separation1. Charge separation. Charge layers in the . Charge layers in the cloud are formed by the transfer of positive cloud are formed by the transfer of positive ions from warmer graupel to colder ice crystal ions from warmer graupel to colder ice crystal when they collide with each other.when they collide with each other.

2. 2. Stepped leader. Stepped leader. When the negative charge When the negative charge near the bottom of the cloud is large enough to near the bottom of the cloud is large enough to overcome the air's resistance, a stepped overcome the air's resistance, a stepped leader forms.leader forms.

3. 3. Return stroke.Return stroke. A region of positive ions A region of positive ions move from the ground toward this charge, move from the ground toward this charge, which then forms a which then forms a return stroke return stroke into the cloud.into the cloud.

4. 4. Dart leaderDart leader. Not all of the first stroke . Not all of the first stroke neutralizes the negatively charged ions and neutralizes the negatively charged ions and results in another leader in 1/10 of a secondresults in another leader in 1/10 of a second

Processes of Lightning Formation

Lightning: Runaway dischargesLightning: Runaway discharges• High-speed moving electrons radiate light as lightning.• Electrons approach the speed of light, gaining energy. Fast

moving electrons create others by colliding with atoms. This can result in an avalanche called runaway electrons.

• When a large number of runaway electrons accumulate in a small volume, the energy is released in a so-called runaway breakdown.

Shao et al. 2011

ThunderThunder• Charge differences between the thunderstorm and ground can Charge differences between the thunderstorm and ground can

cause lightning strokes of 30,000°C, and this rapid heating of air cause lightning strokes of 30,000°C, and this rapid heating of air will creates an explosive shock wave called will creates an explosive shock wave called thunderthunder..

• It takes about 3 seconds for thunder to travel 1 kilometer (It takes about 3 seconds for thunder to travel 1 kilometer (5 sec 5 sec per mileper mile).). A lag in lightning strike and thunder occurs due to sound traveling slower than light.

• When thunder is farther away, the echoing of sound waves off of objects (like buildings and hills) causes thunder to sound rumbling.

Refraction of thunderRefraction of thunder Sound waves move faster in Sound waves move faster in

warm air than they do in cool air. warm air than they do in cool air. Typically, the air temperature Typically, the air temperature

decreases with height. When decreases with height. When this occurs, thunder will normally this occurs, thunder will normally have an audible range up to 10 have an audible range up to 10 miles (16 km).miles (16 km).

However, when the air However, when the air temperature increases with temperature increases with height, called an inversion, height, called an inversion, sound waves are refracted (bent sound waves are refracted (bent back toward the earth) as they back toward the earth) as they move due to their faster motion move due to their faster motion in the warmer air effectively in the warmer air effectively amplifying the thunder.amplifying the thunder.

Global distribution of lightning strikesGlobal distribution of lightning strikes

Significant difference between continent and ocean

Annual mean number of days with Annual mean number of days with thunderthunder

U.S Distribution of lightning timeU.S Distribution of lightning time

Video: Video: All Alone in the Night - All Alone in the Night - Time-lapse footage of the Earth Time-lapse footage of the Earth

as seen from the ISSas seen from the ISS

http://www.youtube.com/watch?v=FG0fTKAqZ5g

Climate impacts of lightning Climate impacts of lightning (1) Nitrogen cycle(1) Nitrogen cycle

The enormous energy of lightning breaks nitrogen molecules and The enormous energy of lightning breaks nitrogen molecules and enables their atoms to combine with oxygen in the air forming enables their atoms to combine with oxygen in the air forming nitrogen oxides. nitrogen oxides.

These dissolve in rain, forming nitrates, that are carried to the earth.These dissolve in rain, forming nitrates, that are carried to the earth. Contributes some 5– 8% of the total nitrogen fixed.Contributes some 5– 8% of the total nitrogen fixed. Was probably the only source of fixed nitrogen for the earliest forms Was probably the only source of fixed nitrogen for the earliest forms

of life. of life.

Climate impacts of lightning Climate impacts of lightning (2) Ozone production(2) Ozone production

Ozone was named after the Greek Ozone was named after the Greek verb ozein (ὄζειν, "to smell"), from verb ozein (ὄζειν, "to smell"), from the peculiar odor in lightning storms.the peculiar odor in lightning storms.

Nitrogen oxides produced by Nitrogen oxides produced by lightning can react with others in the lightning can react with others in the presence of sunlight to produce presence of sunlight to produce ozone. ozone.

Since most lightning occurs inside a Since most lightning occurs inside a storm, the added ozone tends to storm, the added ozone tends to show up several miles high rather show up several miles high rather than near the earth's surface, so it than near the earth's surface, so it doesn't add significantly to ozone doesn't add significantly to ozone pollution at ground level.pollution at ground level.

Climate impacts of lightningClimate impacts of lightning(3) Damages(3) Damages

Annual mean fatalities 69 for U.S., Annual mean fatalities 69 for U.S., 600 world-wide600 world-wide. .

Hundreds of people are Hundreds of people are permanently injuredpermanently injured

Annual mean property loss $38 Annual mean property loss $38 million for U.S.million for U.S.

Lightning is the leading cause of Lightning is the leading cause of wildfires (25,000 each year)wildfires (25,000 each year)

Especially caused by lightning in Especially caused by lightning in dry thunderstorms (virga)dry thunderstorms (virga)

Lightning SafetyLightning Safety

Practice the following when lightning is present:Practice the following when lightning is present:• Always take cover in a building. Always take cover in a building. • Do not stand under a tree or other tall object that might Do not stand under a tree or other tall object that might

serve as a lightning rod.serve as a lightning rod.• Avoid standing on mountain summits, ridges, rooftops, Avoid standing on mountain summits, ridges, rooftops,

or other high areas. or other high areas. • Avoid caves.Avoid caves.• Avoid open water (pool, lake, or hot tub).Avoid open water (pool, lake, or hot tub).

SummarySummary

1.1. Two types of lightning (cloud-to-cloud 80%, cloud-to-Two types of lightning (cloud-to-cloud 80%, cloud-to-ground 20%)ground 20%)

2.2. 4 steps of lightning development. 4 steps of lightning development.

3.3. How fast does thunder travel? How fast does thunder travel?

4.4. Climate impacts of lightning: nitrogen cycle, ozone, Climate impacts of lightning: nitrogen cycle, ozone, wildfirewildfire

5.5. Lightning safetyLightning safety