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Lightning

Lightning. Review: Different types of precipitation Depends on atmospheric temperature and winds . Snow Rain Sleet Freezing rain Graupel/hail. Lightning: Introduction. Lightning is a discharge of electricity, a giant spark

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Lightning

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  1. Lightning

  2. Review: Different types of precipitationDepends on atmospheric temperature and winds Snow Rain Sleet Freezing rain Graupel/hail

  3. Lightning: 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.

  4. Video: Lightning in super slow motion http://www.youtube.com/watch?v=RLWIBrweSU8

  5. Processes of Lightning Formation 1. Charge separation. Charge layers in the cloud are formed by the transfer of positive ions from warmer graupel to colder ice crystal when they collide with each other. 2. Stepped leader. When the negative charge near the bottom of the cloud is large enough to overcome the air's resistance, a stepped leader forms. 3. Return stroke.A region of positive ions move from the ground toward this charge, which then forms a return stroke into the cloud. 4. Dart leader. Not all of the first stroke neutralizes the negatively charged ions and results in another leader in 1/10 of a second

  6. Lightning: 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

  7. Thunder • Charge differences between the thunderstorm and ground can cause lightning strokes of 30,000°C, and this rapid heating of air will creates an explosive shock wave called thunder. • It takes about 3 seconds for thunder to travel 1 kilometer (5 sec per 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.

  8. Refraction of thunder Sound waves move faster in warm air than they do in cool air. Typically, the air temperature decreases with height. When this occurs, thunder will normally have an audible range up to 10 miles (16 km). However, when the air temperature increases with height, called an inversion, sound waves are refracted (bent back toward the earth) as they move due to their faster motion in the warmer air effectively amplifying the thunder.

  9. Global distribution of lightning strikes Significant difference between continent and ocean

  10. Annual mean number of days with thunder

  11. U.S Distribution of lightning time

  12. Video: All Alone in the Night - Time-lapse footage of the Earth as seen from the ISS http://www.youtube.com/watch?v=FG0fTKAqZ5g

  13. Climate impacts of lightning (1) Nitrogen cycle The enormous energy of lightning breaks nitrogen molecules and enables their atoms to combine with oxygen in the air forming nitrogen oxides. These dissolve in rain, forming nitrates, that are carried to the earth. Contributes some 5– 8% of the total nitrogen fixed. Was probably the only source of fixed nitrogen for the earliest forms of life.

  14. Climate impacts of lightning (2) Ozone production Ozone was named after the Greek verb ozein (ὄζειν, "to smell"), from the peculiar odor in lightning storms. Nitrogen oxides produced by lightning can react with others in the presence of sunlight to produce ozone. Since most lightning occurs inside a storm, the added ozone tends to show up several miles high rather than near the earth's surface, so it doesn't add significantly to ozone pollution at ground level.

  15. Climate impacts of lightning(3) Damages Annual mean fatalities 69 for U.S., 600 world-wide. Hundreds of people are permanently injured Annual mean property loss $38 million for U.S. Lightning is the leading cause of wildfires (25,000 each year) Especially caused by lightning in dry thunderstorms (virga)

  16. Lightning Safety Practice the following when lightning is present: • Always take cover in a building. • Do not stand under a tree or other tall object that might serve as a lightning rod. • Avoid standing on mountain summits, ridges, rooftops, or other high areas. • Avoid caves. • Avoid open water (pool, lake, or hot tub).

  17. Summary • Two types of lightning (cloud-to-cloud 80%, cloud-to-ground 20%) • 4 steps of lightning development. • How fast does thunder travel? • Climate impacts of lightning: nitrogen cycle, ozone, wildfire • Lightning safety

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