What causes the direction of lightning flashes? Announcing the arrival of Valued Associate #679: Cesar Manara Planned maintenance scheduled April 23, 2019 at 23:30 UTC (7:30pm US/Eastern) 2019 Moderator Election Q&A - Question CollectionVoltage and current of positive lightningLightning and Charge DisplacementWhat causes charge buildup in a cloud before a lightning storm?Why is there lightning where there are no clouds?Lightning Conductor WorkingLightning, Electrical breakdown, and Currentwhy does lightning always come from the clouds?Noise heard simultaneously with lightningIs wearing an insulated metal armor against a lightning safe?Why does Lightning have branches?

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What causes the direction of lightning flashes?



Announcing the arrival of Valued Associate #679: Cesar Manara
Planned maintenance scheduled April 23, 2019 at 23:30 UTC (7:30pm US/Eastern)
2019 Moderator Election Q&A - Question CollectionVoltage and current of positive lightningLightning and Charge DisplacementWhat causes charge buildup in a cloud before a lightning storm?Why is there lightning where there are no clouds?Lightning Conductor WorkingLightning, Electrical breakdown, and Currentwhy does lightning always come from the clouds?Noise heard simultaneously with lightningIs wearing an insulated metal armor against a lightning safe?Why does Lightning have branches?










24












$begingroup$


During a lightning strike, the flashes appear as cloud to ground or cloud to cloud .Why is this the only manner of propagation? Why do the flashes not go upwards from the clouds?
Pardon me if the question was silly.










share|cite|improve this question









New contributor




Maan is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
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$endgroup$







  • 2




    $begingroup$
    The question wasn't silly!
    $endgroup$
    – Cai
    Apr 17 at 16:23






  • 1




    $begingroup$
    Somewhat relevant question on another stack exchange site: earthscience.stackexchange.com/questions/580/…
    $endgroup$
    – fyrepenguin
    Apr 18 at 5:31















24












$begingroup$


During a lightning strike, the flashes appear as cloud to ground or cloud to cloud .Why is this the only manner of propagation? Why do the flashes not go upwards from the clouds?
Pardon me if the question was silly.










share|cite|improve this question









New contributor




Maan is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.







$endgroup$







  • 2




    $begingroup$
    The question wasn't silly!
    $endgroup$
    – Cai
    Apr 17 at 16:23






  • 1




    $begingroup$
    Somewhat relevant question on another stack exchange site: earthscience.stackexchange.com/questions/580/…
    $endgroup$
    – fyrepenguin
    Apr 18 at 5:31













24












24








24


1



$begingroup$


During a lightning strike, the flashes appear as cloud to ground or cloud to cloud .Why is this the only manner of propagation? Why do the flashes not go upwards from the clouds?
Pardon me if the question was silly.










share|cite|improve this question









New contributor




Maan is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.







$endgroup$




During a lightning strike, the flashes appear as cloud to ground or cloud to cloud .Why is this the only manner of propagation? Why do the flashes not go upwards from the clouds?
Pardon me if the question was silly.







electricity everyday-life atmospheric-science lightning






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New contributor




Maan is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.











share|cite|improve this question









New contributor




Maan is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.









share|cite|improve this question




share|cite|improve this question








edited Apr 17 at 19:17







Maan













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asked Apr 17 at 9:17









MaanMaan

18519




18519




New contributor




Maan is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
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New contributor





Maan is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.






Maan is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.







  • 2




    $begingroup$
    The question wasn't silly!
    $endgroup$
    – Cai
    Apr 17 at 16:23






  • 1




    $begingroup$
    Somewhat relevant question on another stack exchange site: earthscience.stackexchange.com/questions/580/…
    $endgroup$
    – fyrepenguin
    Apr 18 at 5:31












  • 2




    $begingroup$
    The question wasn't silly!
    $endgroup$
    – Cai
    Apr 17 at 16:23






  • 1




    $begingroup$
    Somewhat relevant question on another stack exchange site: earthscience.stackexchange.com/questions/580/…
    $endgroup$
    – fyrepenguin
    Apr 18 at 5:31







2




2




$begingroup$
The question wasn't silly!
$endgroup$
– Cai
Apr 17 at 16:23




$begingroup$
The question wasn't silly!
$endgroup$
– Cai
Apr 17 at 16:23




1




1




$begingroup$
Somewhat relevant question on another stack exchange site: earthscience.stackexchange.com/questions/580/…
$endgroup$
– fyrepenguin
Apr 18 at 5:31




$begingroup$
Somewhat relevant question on another stack exchange site: earthscience.stackexchange.com/questions/580/…
$endgroup$
– fyrepenguin
Apr 18 at 5:31










4 Answers
4






active

oldest

votes


















23












$begingroup$

They exist, they're just rarer, which is why you don't often see them. They're called gigantic jets, and they connect storm clouds to the reservoir of charge in the ionosphere. Here's one that was photographed during Tropical Storm Cristobal in 2008 (source: https://www.livescience.com/10572-gigantic-lightning-jets-shoot-clouds-space.html): enter image description here



Unfortunately, not very much is known about the conditions under which they form, so there's not really a good explanation why they're so rare at the moment.






share|cite|improve this answer









$endgroup$








  • 7




    $begingroup$
    See also: sprites, jets, and elves. Generally classed as upper-atmospheric lightning (which has a few more photos).
    $endgroup$
    – Eric Towers
    Apr 17 at 12:48






  • 1




    $begingroup$
    @EricTowers Indeed, there's a lot of strange stuff that goes on with lightning-like phenomena above the clouds. Sprites and elves didn't really fit the description of "lightning, but traveling upwards from a cloud," which is why they weren't originally mentioned, but they're still super interesting.
    $endgroup$
    – probably_someone
    Apr 17 at 12:55






  • 20




    $begingroup$
    I would hazard that another part of the reason you don't often see lightning going up from the top of the clouds is that, when it does happen, there is generally a cloud in the way.
    $endgroup$
    – Chronocidal
    Apr 17 at 13:55






  • 3




    $begingroup$
    "They exist, they're just rarer, which is why you don't often see them." Another, perhaps bigger reason for not seeing lightning above clouds might be that clouds are blocking your view?
    $endgroup$
    – JiK
    Apr 17 at 14:23










  • $begingroup$
    @PeterA.Schneider : Just what do you think a blue jet is? (Link is to a photo by Thijs Bors in Australia’s Northern Territory of a blue jet above a thunderstorm, observable as an upward bolt at the cloud.)
    $endgroup$
    – Eric Towers
    Apr 18 at 2:02


















6












$begingroup$

According to Wikipedia, charge separation in a thunderstorm occurs as follows:




The differences in the movement of the precipitation cause collisions to occur. When the rising ice crystals collide with graupel, the ice crystals become positively charged and the graupel becomes negatively charged. ... The updraft carries the positively charged ice crystals upward toward the top of the storm cloud. The larger and denser graupel is either suspended in the middle of the thunderstorm cloud or falls toward the lower part of the storm




A lightning flash consists of multiple rapid discharge events in which current flows both from cloud to ground and vice versa.






share|cite|improve this answer









$endgroup$








  • 1




    $begingroup$
    OK -- and why exactly does the lightning strike from positive to negative? Is that always so with arcs? If so, again: Why? ;-)
    $endgroup$
    – Peter A. Schneider
    Apr 17 at 17:29



















6












$begingroup$

If you look at slow-motion videos of lightning strikes you'll notice that a "pilot lightning", called a step leader, originates indeed in the cloud and propagates, branching out roughly radially from its point of origin.



The general direction is certainly dictated by the gradient of the electric field while the chaotic element is introduced by the explosive expansion of the air turning into plasma.



Why does it start in the cloud and not on the ground? The general answer is that the lightning will start where the field strength is sufficient to cause an arc; presumably the concentrated charges in the cloud create an inhomogeneous electric field with a strong gradient close to the charge in the cloud, where the field strength then should be higher than in the surroundings.



This hypothesis is consistent with the observation that the step leaders actually grow in many directions, but generally radially away from the point of origin, minus some chaotic element. This direction of growth indicates the direction of the field driving it: Primarily radially away from the charge in the cloud, and only secondarily towards the ground.



Close to flat ground the electric field will be near-homogeneous because the ground is a relatively good conductor: Charges dissipate until balanced, equaling out field inhomogeneities. That's why a lightning bolt will emerge from the clouds before the field strength for an arc will be reached on the ground.



Pointy spires etc. — as opposed to flat ground — do have inhomogeneous fields at the tip so that some "positive arcing" starts there, known as St. Elmo's Fire. I'm not sure whether a true lightning bolt could emerge from such a positive tip. Coincidentally I stumbled over a recent video which seems to show an incident where a lightning boolt indeed emerges from the tip of Toronto's CN Tower, in this Newsflare article.. There are multiple bolts in the video; the one at 10s clearly appears to originate at the top of the tower and works its way up into the sky. That's also supported by the fact that it branches out away from the tower, much like "normal" lightning branches out from a point in the cloud. This indicates the radial orientation of the electric field around the tip of the tower.



The eventual main lightning bolt which emerges once a conductive path between the separated charges has been established is not "directed"; it simply is a runaway discharge through the plasma arc created by the step leader.






share|cite|improve this answer











$endgroup$








  • 1




    $begingroup$
    It might also be worth mentioning upward streamers as well if you are talking about the step leaders.
    $endgroup$
    – fyrepenguin
    Apr 18 at 5:30


















1












$begingroup$

The lightning has a visible direction of propagation, from top to down, that is normally the same as the electrons it is carrying: free electrons get accelerated (downward) by the electric field and ionize other molecules, thus progressively creating the lightning plasma pathway.



Another reason why lightnings start from top, is that air at higher altitudes is less dense, and need less voltage to initiate a spark. With a uniform electric field, a spark will always start from high altitude, and propagate downward by the accelerated electrons.
Paschen Law






share|cite|improve this answer











$endgroup$












  • $begingroup$
    Generally speaking, positive charges accumulate at the top of a thundercloud, and negative charges accumulate at the bottom.
    $endgroup$
    – gandalf61
    Apr 17 at 10:14






  • 4




    $begingroup$
    It is now well known that the beginning of a lightning strike occurs with multiple leader ionisation paths starting from the ground upwards these paths are extremely faint. The first of these to reach a sufficiently high altitude and closest to the charge accumulation in the cloud becomes the conduction path for the main down-strike which is visible.
    $endgroup$
    – Alopex
    Apr 17 at 11:32






  • 1




    $begingroup$
    Oh noo I was all wrong
    $endgroup$
    – patta
    Apr 17 at 11:33










  • $begingroup$
    Is it possible that the field is stronger in the cloud? On the ground charges are likely dissipated; most places will be close to, well, ground neutral. The cloud, however, is a confined volume of charges which will "attract field lines", if you want. The necessary breakdown field strength will likely be reached close to the cloud.
    $endgroup$
    – Peter A. Schneider
    Apr 17 at 17:39











  • $begingroup$
    @Alopex - Interesting. Please convert your comment to answer, and add resources. Should be easy if it is well known :-)
    $endgroup$
    – Peter M.
    Apr 17 at 19:28











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4 Answers
4






active

oldest

votes








4 Answers
4






active

oldest

votes









active

oldest

votes






active

oldest

votes









23












$begingroup$

They exist, they're just rarer, which is why you don't often see them. They're called gigantic jets, and they connect storm clouds to the reservoir of charge in the ionosphere. Here's one that was photographed during Tropical Storm Cristobal in 2008 (source: https://www.livescience.com/10572-gigantic-lightning-jets-shoot-clouds-space.html): enter image description here



Unfortunately, not very much is known about the conditions under which they form, so there's not really a good explanation why they're so rare at the moment.






share|cite|improve this answer









$endgroup$








  • 7




    $begingroup$
    See also: sprites, jets, and elves. Generally classed as upper-atmospheric lightning (which has a few more photos).
    $endgroup$
    – Eric Towers
    Apr 17 at 12:48






  • 1




    $begingroup$
    @EricTowers Indeed, there's a lot of strange stuff that goes on with lightning-like phenomena above the clouds. Sprites and elves didn't really fit the description of "lightning, but traveling upwards from a cloud," which is why they weren't originally mentioned, but they're still super interesting.
    $endgroup$
    – probably_someone
    Apr 17 at 12:55






  • 20




    $begingroup$
    I would hazard that another part of the reason you don't often see lightning going up from the top of the clouds is that, when it does happen, there is generally a cloud in the way.
    $endgroup$
    – Chronocidal
    Apr 17 at 13:55






  • 3




    $begingroup$
    "They exist, they're just rarer, which is why you don't often see them." Another, perhaps bigger reason for not seeing lightning above clouds might be that clouds are blocking your view?
    $endgroup$
    – JiK
    Apr 17 at 14:23










  • $begingroup$
    @PeterA.Schneider : Just what do you think a blue jet is? (Link is to a photo by Thijs Bors in Australia’s Northern Territory of a blue jet above a thunderstorm, observable as an upward bolt at the cloud.)
    $endgroup$
    – Eric Towers
    Apr 18 at 2:02















23












$begingroup$

They exist, they're just rarer, which is why you don't often see them. They're called gigantic jets, and they connect storm clouds to the reservoir of charge in the ionosphere. Here's one that was photographed during Tropical Storm Cristobal in 2008 (source: https://www.livescience.com/10572-gigantic-lightning-jets-shoot-clouds-space.html): enter image description here



Unfortunately, not very much is known about the conditions under which they form, so there's not really a good explanation why they're so rare at the moment.






share|cite|improve this answer









$endgroup$








  • 7




    $begingroup$
    See also: sprites, jets, and elves. Generally classed as upper-atmospheric lightning (which has a few more photos).
    $endgroup$
    – Eric Towers
    Apr 17 at 12:48






  • 1




    $begingroup$
    @EricTowers Indeed, there's a lot of strange stuff that goes on with lightning-like phenomena above the clouds. Sprites and elves didn't really fit the description of "lightning, but traveling upwards from a cloud," which is why they weren't originally mentioned, but they're still super interesting.
    $endgroup$
    – probably_someone
    Apr 17 at 12:55






  • 20




    $begingroup$
    I would hazard that another part of the reason you don't often see lightning going up from the top of the clouds is that, when it does happen, there is generally a cloud in the way.
    $endgroup$
    – Chronocidal
    Apr 17 at 13:55






  • 3




    $begingroup$
    "They exist, they're just rarer, which is why you don't often see them." Another, perhaps bigger reason for not seeing lightning above clouds might be that clouds are blocking your view?
    $endgroup$
    – JiK
    Apr 17 at 14:23










  • $begingroup$
    @PeterA.Schneider : Just what do you think a blue jet is? (Link is to a photo by Thijs Bors in Australia’s Northern Territory of a blue jet above a thunderstorm, observable as an upward bolt at the cloud.)
    $endgroup$
    – Eric Towers
    Apr 18 at 2:02













23












23








23





$begingroup$

They exist, they're just rarer, which is why you don't often see them. They're called gigantic jets, and they connect storm clouds to the reservoir of charge in the ionosphere. Here's one that was photographed during Tropical Storm Cristobal in 2008 (source: https://www.livescience.com/10572-gigantic-lightning-jets-shoot-clouds-space.html): enter image description here



Unfortunately, not very much is known about the conditions under which they form, so there's not really a good explanation why they're so rare at the moment.






share|cite|improve this answer









$endgroup$



They exist, they're just rarer, which is why you don't often see them. They're called gigantic jets, and they connect storm clouds to the reservoir of charge in the ionosphere. Here's one that was photographed during Tropical Storm Cristobal in 2008 (source: https://www.livescience.com/10572-gigantic-lightning-jets-shoot-clouds-space.html): enter image description here



Unfortunately, not very much is known about the conditions under which they form, so there's not really a good explanation why they're so rare at the moment.







share|cite|improve this answer












share|cite|improve this answer



share|cite|improve this answer










answered Apr 17 at 9:38









probably_someoneprobably_someone

19.2k12962




19.2k12962







  • 7




    $begingroup$
    See also: sprites, jets, and elves. Generally classed as upper-atmospheric lightning (which has a few more photos).
    $endgroup$
    – Eric Towers
    Apr 17 at 12:48






  • 1




    $begingroup$
    @EricTowers Indeed, there's a lot of strange stuff that goes on with lightning-like phenomena above the clouds. Sprites and elves didn't really fit the description of "lightning, but traveling upwards from a cloud," which is why they weren't originally mentioned, but they're still super interesting.
    $endgroup$
    – probably_someone
    Apr 17 at 12:55






  • 20




    $begingroup$
    I would hazard that another part of the reason you don't often see lightning going up from the top of the clouds is that, when it does happen, there is generally a cloud in the way.
    $endgroup$
    – Chronocidal
    Apr 17 at 13:55






  • 3




    $begingroup$
    "They exist, they're just rarer, which is why you don't often see them." Another, perhaps bigger reason for not seeing lightning above clouds might be that clouds are blocking your view?
    $endgroup$
    – JiK
    Apr 17 at 14:23










  • $begingroup$
    @PeterA.Schneider : Just what do you think a blue jet is? (Link is to a photo by Thijs Bors in Australia’s Northern Territory of a blue jet above a thunderstorm, observable as an upward bolt at the cloud.)
    $endgroup$
    – Eric Towers
    Apr 18 at 2:02












  • 7




    $begingroup$
    See also: sprites, jets, and elves. Generally classed as upper-atmospheric lightning (which has a few more photos).
    $endgroup$
    – Eric Towers
    Apr 17 at 12:48






  • 1




    $begingroup$
    @EricTowers Indeed, there's a lot of strange stuff that goes on with lightning-like phenomena above the clouds. Sprites and elves didn't really fit the description of "lightning, but traveling upwards from a cloud," which is why they weren't originally mentioned, but they're still super interesting.
    $endgroup$
    – probably_someone
    Apr 17 at 12:55






  • 20




    $begingroup$
    I would hazard that another part of the reason you don't often see lightning going up from the top of the clouds is that, when it does happen, there is generally a cloud in the way.
    $endgroup$
    – Chronocidal
    Apr 17 at 13:55






  • 3




    $begingroup$
    "They exist, they're just rarer, which is why you don't often see them." Another, perhaps bigger reason for not seeing lightning above clouds might be that clouds are blocking your view?
    $endgroup$
    – JiK
    Apr 17 at 14:23










  • $begingroup$
    @PeterA.Schneider : Just what do you think a blue jet is? (Link is to a photo by Thijs Bors in Australia’s Northern Territory of a blue jet above a thunderstorm, observable as an upward bolt at the cloud.)
    $endgroup$
    – Eric Towers
    Apr 18 at 2:02







7




7




$begingroup$
See also: sprites, jets, and elves. Generally classed as upper-atmospheric lightning (which has a few more photos).
$endgroup$
– Eric Towers
Apr 17 at 12:48




$begingroup$
See also: sprites, jets, and elves. Generally classed as upper-atmospheric lightning (which has a few more photos).
$endgroup$
– Eric Towers
Apr 17 at 12:48




1




1




$begingroup$
@EricTowers Indeed, there's a lot of strange stuff that goes on with lightning-like phenomena above the clouds. Sprites and elves didn't really fit the description of "lightning, but traveling upwards from a cloud," which is why they weren't originally mentioned, but they're still super interesting.
$endgroup$
– probably_someone
Apr 17 at 12:55




$begingroup$
@EricTowers Indeed, there's a lot of strange stuff that goes on with lightning-like phenomena above the clouds. Sprites and elves didn't really fit the description of "lightning, but traveling upwards from a cloud," which is why they weren't originally mentioned, but they're still super interesting.
$endgroup$
– probably_someone
Apr 17 at 12:55




20




20




$begingroup$
I would hazard that another part of the reason you don't often see lightning going up from the top of the clouds is that, when it does happen, there is generally a cloud in the way.
$endgroup$
– Chronocidal
Apr 17 at 13:55




$begingroup$
I would hazard that another part of the reason you don't often see lightning going up from the top of the clouds is that, when it does happen, there is generally a cloud in the way.
$endgroup$
– Chronocidal
Apr 17 at 13:55




3




3




$begingroup$
"They exist, they're just rarer, which is why you don't often see them." Another, perhaps bigger reason for not seeing lightning above clouds might be that clouds are blocking your view?
$endgroup$
– JiK
Apr 17 at 14:23




$begingroup$
"They exist, they're just rarer, which is why you don't often see them." Another, perhaps bigger reason for not seeing lightning above clouds might be that clouds are blocking your view?
$endgroup$
– JiK
Apr 17 at 14:23












$begingroup$
@PeterA.Schneider : Just what do you think a blue jet is? (Link is to a photo by Thijs Bors in Australia’s Northern Territory of a blue jet above a thunderstorm, observable as an upward bolt at the cloud.)
$endgroup$
– Eric Towers
Apr 18 at 2:02




$begingroup$
@PeterA.Schneider : Just what do you think a blue jet is? (Link is to a photo by Thijs Bors in Australia’s Northern Territory of a blue jet above a thunderstorm, observable as an upward bolt at the cloud.)
$endgroup$
– Eric Towers
Apr 18 at 2:02











6












$begingroup$

According to Wikipedia, charge separation in a thunderstorm occurs as follows:




The differences in the movement of the precipitation cause collisions to occur. When the rising ice crystals collide with graupel, the ice crystals become positively charged and the graupel becomes negatively charged. ... The updraft carries the positively charged ice crystals upward toward the top of the storm cloud. The larger and denser graupel is either suspended in the middle of the thunderstorm cloud or falls toward the lower part of the storm




A lightning flash consists of multiple rapid discharge events in which current flows both from cloud to ground and vice versa.






share|cite|improve this answer









$endgroup$








  • 1




    $begingroup$
    OK -- and why exactly does the lightning strike from positive to negative? Is that always so with arcs? If so, again: Why? ;-)
    $endgroup$
    – Peter A. Schneider
    Apr 17 at 17:29
















6












$begingroup$

According to Wikipedia, charge separation in a thunderstorm occurs as follows:




The differences in the movement of the precipitation cause collisions to occur. When the rising ice crystals collide with graupel, the ice crystals become positively charged and the graupel becomes negatively charged. ... The updraft carries the positively charged ice crystals upward toward the top of the storm cloud. The larger and denser graupel is either suspended in the middle of the thunderstorm cloud or falls toward the lower part of the storm




A lightning flash consists of multiple rapid discharge events in which current flows both from cloud to ground and vice versa.






share|cite|improve this answer









$endgroup$








  • 1




    $begingroup$
    OK -- and why exactly does the lightning strike from positive to negative? Is that always so with arcs? If so, again: Why? ;-)
    $endgroup$
    – Peter A. Schneider
    Apr 17 at 17:29














6












6








6





$begingroup$

According to Wikipedia, charge separation in a thunderstorm occurs as follows:




The differences in the movement of the precipitation cause collisions to occur. When the rising ice crystals collide with graupel, the ice crystals become positively charged and the graupel becomes negatively charged. ... The updraft carries the positively charged ice crystals upward toward the top of the storm cloud. The larger and denser graupel is either suspended in the middle of the thunderstorm cloud or falls toward the lower part of the storm




A lightning flash consists of multiple rapid discharge events in which current flows both from cloud to ground and vice versa.






share|cite|improve this answer









$endgroup$



According to Wikipedia, charge separation in a thunderstorm occurs as follows:




The differences in the movement of the precipitation cause collisions to occur. When the rising ice crystals collide with graupel, the ice crystals become positively charged and the graupel becomes negatively charged. ... The updraft carries the positively charged ice crystals upward toward the top of the storm cloud. The larger and denser graupel is either suspended in the middle of the thunderstorm cloud or falls toward the lower part of the storm




A lightning flash consists of multiple rapid discharge events in which current flows both from cloud to ground and vice versa.







share|cite|improve this answer












share|cite|improve this answer



share|cite|improve this answer










answered Apr 17 at 10:11









gandalf61gandalf61

802210




802210







  • 1




    $begingroup$
    OK -- and why exactly does the lightning strike from positive to negative? Is that always so with arcs? If so, again: Why? ;-)
    $endgroup$
    – Peter A. Schneider
    Apr 17 at 17:29













  • 1




    $begingroup$
    OK -- and why exactly does the lightning strike from positive to negative? Is that always so with arcs? If so, again: Why? ;-)
    $endgroup$
    – Peter A. Schneider
    Apr 17 at 17:29








1




1




$begingroup$
OK -- and why exactly does the lightning strike from positive to negative? Is that always so with arcs? If so, again: Why? ;-)
$endgroup$
– Peter A. Schneider
Apr 17 at 17:29





$begingroup$
OK -- and why exactly does the lightning strike from positive to negative? Is that always so with arcs? If so, again: Why? ;-)
$endgroup$
– Peter A. Schneider
Apr 17 at 17:29












6












$begingroup$

If you look at slow-motion videos of lightning strikes you'll notice that a "pilot lightning", called a step leader, originates indeed in the cloud and propagates, branching out roughly radially from its point of origin.



The general direction is certainly dictated by the gradient of the electric field while the chaotic element is introduced by the explosive expansion of the air turning into plasma.



Why does it start in the cloud and not on the ground? The general answer is that the lightning will start where the field strength is sufficient to cause an arc; presumably the concentrated charges in the cloud create an inhomogeneous electric field with a strong gradient close to the charge in the cloud, where the field strength then should be higher than in the surroundings.



This hypothesis is consistent with the observation that the step leaders actually grow in many directions, but generally radially away from the point of origin, minus some chaotic element. This direction of growth indicates the direction of the field driving it: Primarily radially away from the charge in the cloud, and only secondarily towards the ground.



Close to flat ground the electric field will be near-homogeneous because the ground is a relatively good conductor: Charges dissipate until balanced, equaling out field inhomogeneities. That's why a lightning bolt will emerge from the clouds before the field strength for an arc will be reached on the ground.



Pointy spires etc. — as opposed to flat ground — do have inhomogeneous fields at the tip so that some "positive arcing" starts there, known as St. Elmo's Fire. I'm not sure whether a true lightning bolt could emerge from such a positive tip. Coincidentally I stumbled over a recent video which seems to show an incident where a lightning boolt indeed emerges from the tip of Toronto's CN Tower, in this Newsflare article.. There are multiple bolts in the video; the one at 10s clearly appears to originate at the top of the tower and works its way up into the sky. That's also supported by the fact that it branches out away from the tower, much like "normal" lightning branches out from a point in the cloud. This indicates the radial orientation of the electric field around the tip of the tower.



The eventual main lightning bolt which emerges once a conductive path between the separated charges has been established is not "directed"; it simply is a runaway discharge through the plasma arc created by the step leader.






share|cite|improve this answer











$endgroup$








  • 1




    $begingroup$
    It might also be worth mentioning upward streamers as well if you are talking about the step leaders.
    $endgroup$
    – fyrepenguin
    Apr 18 at 5:30















6












$begingroup$

If you look at slow-motion videos of lightning strikes you'll notice that a "pilot lightning", called a step leader, originates indeed in the cloud and propagates, branching out roughly radially from its point of origin.



The general direction is certainly dictated by the gradient of the electric field while the chaotic element is introduced by the explosive expansion of the air turning into plasma.



Why does it start in the cloud and not on the ground? The general answer is that the lightning will start where the field strength is sufficient to cause an arc; presumably the concentrated charges in the cloud create an inhomogeneous electric field with a strong gradient close to the charge in the cloud, where the field strength then should be higher than in the surroundings.



This hypothesis is consistent with the observation that the step leaders actually grow in many directions, but generally radially away from the point of origin, minus some chaotic element. This direction of growth indicates the direction of the field driving it: Primarily radially away from the charge in the cloud, and only secondarily towards the ground.



Close to flat ground the electric field will be near-homogeneous because the ground is a relatively good conductor: Charges dissipate until balanced, equaling out field inhomogeneities. That's why a lightning bolt will emerge from the clouds before the field strength for an arc will be reached on the ground.



Pointy spires etc. — as opposed to flat ground — do have inhomogeneous fields at the tip so that some "positive arcing" starts there, known as St. Elmo's Fire. I'm not sure whether a true lightning bolt could emerge from such a positive tip. Coincidentally I stumbled over a recent video which seems to show an incident where a lightning boolt indeed emerges from the tip of Toronto's CN Tower, in this Newsflare article.. There are multiple bolts in the video; the one at 10s clearly appears to originate at the top of the tower and works its way up into the sky. That's also supported by the fact that it branches out away from the tower, much like "normal" lightning branches out from a point in the cloud. This indicates the radial orientation of the electric field around the tip of the tower.



The eventual main lightning bolt which emerges once a conductive path between the separated charges has been established is not "directed"; it simply is a runaway discharge through the plasma arc created by the step leader.






share|cite|improve this answer











$endgroup$








  • 1




    $begingroup$
    It might also be worth mentioning upward streamers as well if you are talking about the step leaders.
    $endgroup$
    – fyrepenguin
    Apr 18 at 5:30













6












6








6





$begingroup$

If you look at slow-motion videos of lightning strikes you'll notice that a "pilot lightning", called a step leader, originates indeed in the cloud and propagates, branching out roughly radially from its point of origin.



The general direction is certainly dictated by the gradient of the electric field while the chaotic element is introduced by the explosive expansion of the air turning into plasma.



Why does it start in the cloud and not on the ground? The general answer is that the lightning will start where the field strength is sufficient to cause an arc; presumably the concentrated charges in the cloud create an inhomogeneous electric field with a strong gradient close to the charge in the cloud, where the field strength then should be higher than in the surroundings.



This hypothesis is consistent with the observation that the step leaders actually grow in many directions, but generally radially away from the point of origin, minus some chaotic element. This direction of growth indicates the direction of the field driving it: Primarily radially away from the charge in the cloud, and only secondarily towards the ground.



Close to flat ground the electric field will be near-homogeneous because the ground is a relatively good conductor: Charges dissipate until balanced, equaling out field inhomogeneities. That's why a lightning bolt will emerge from the clouds before the field strength for an arc will be reached on the ground.



Pointy spires etc. — as opposed to flat ground — do have inhomogeneous fields at the tip so that some "positive arcing" starts there, known as St. Elmo's Fire. I'm not sure whether a true lightning bolt could emerge from such a positive tip. Coincidentally I stumbled over a recent video which seems to show an incident where a lightning boolt indeed emerges from the tip of Toronto's CN Tower, in this Newsflare article.. There are multiple bolts in the video; the one at 10s clearly appears to originate at the top of the tower and works its way up into the sky. That's also supported by the fact that it branches out away from the tower, much like "normal" lightning branches out from a point in the cloud. This indicates the radial orientation of the electric field around the tip of the tower.



The eventual main lightning bolt which emerges once a conductive path between the separated charges has been established is not "directed"; it simply is a runaway discharge through the plasma arc created by the step leader.






share|cite|improve this answer











$endgroup$



If you look at slow-motion videos of lightning strikes you'll notice that a "pilot lightning", called a step leader, originates indeed in the cloud and propagates, branching out roughly radially from its point of origin.



The general direction is certainly dictated by the gradient of the electric field while the chaotic element is introduced by the explosive expansion of the air turning into plasma.



Why does it start in the cloud and not on the ground? The general answer is that the lightning will start where the field strength is sufficient to cause an arc; presumably the concentrated charges in the cloud create an inhomogeneous electric field with a strong gradient close to the charge in the cloud, where the field strength then should be higher than in the surroundings.



This hypothesis is consistent with the observation that the step leaders actually grow in many directions, but generally radially away from the point of origin, minus some chaotic element. This direction of growth indicates the direction of the field driving it: Primarily radially away from the charge in the cloud, and only secondarily towards the ground.



Close to flat ground the electric field will be near-homogeneous because the ground is a relatively good conductor: Charges dissipate until balanced, equaling out field inhomogeneities. That's why a lightning bolt will emerge from the clouds before the field strength for an arc will be reached on the ground.



Pointy spires etc. — as opposed to flat ground — do have inhomogeneous fields at the tip so that some "positive arcing" starts there, known as St. Elmo's Fire. I'm not sure whether a true lightning bolt could emerge from such a positive tip. Coincidentally I stumbled over a recent video which seems to show an incident where a lightning boolt indeed emerges from the tip of Toronto's CN Tower, in this Newsflare article.. There are multiple bolts in the video; the one at 10s clearly appears to originate at the top of the tower and works its way up into the sky. That's also supported by the fact that it branches out away from the tower, much like "normal" lightning branches out from a point in the cloud. This indicates the radial orientation of the electric field around the tip of the tower.



The eventual main lightning bolt which emerges once a conductive path between the separated charges has been established is not "directed"; it simply is a runaway discharge through the plasma arc created by the step leader.







share|cite|improve this answer














share|cite|improve this answer



share|cite|improve this answer








edited Apr 18 at 10:44

























answered Apr 17 at 19:05









Peter A. SchneiderPeter A. Schneider

1,233613




1,233613







  • 1




    $begingroup$
    It might also be worth mentioning upward streamers as well if you are talking about the step leaders.
    $endgroup$
    – fyrepenguin
    Apr 18 at 5:30












  • 1




    $begingroup$
    It might also be worth mentioning upward streamers as well if you are talking about the step leaders.
    $endgroup$
    – fyrepenguin
    Apr 18 at 5:30







1




1




$begingroup$
It might also be worth mentioning upward streamers as well if you are talking about the step leaders.
$endgroup$
– fyrepenguin
Apr 18 at 5:30




$begingroup$
It might also be worth mentioning upward streamers as well if you are talking about the step leaders.
$endgroup$
– fyrepenguin
Apr 18 at 5:30











1












$begingroup$

The lightning has a visible direction of propagation, from top to down, that is normally the same as the electrons it is carrying: free electrons get accelerated (downward) by the electric field and ionize other molecules, thus progressively creating the lightning plasma pathway.



Another reason why lightnings start from top, is that air at higher altitudes is less dense, and need less voltage to initiate a spark. With a uniform electric field, a spark will always start from high altitude, and propagate downward by the accelerated electrons.
Paschen Law






share|cite|improve this answer











$endgroup$












  • $begingroup$
    Generally speaking, positive charges accumulate at the top of a thundercloud, and negative charges accumulate at the bottom.
    $endgroup$
    – gandalf61
    Apr 17 at 10:14






  • 4




    $begingroup$
    It is now well known that the beginning of a lightning strike occurs with multiple leader ionisation paths starting from the ground upwards these paths are extremely faint. The first of these to reach a sufficiently high altitude and closest to the charge accumulation in the cloud becomes the conduction path for the main down-strike which is visible.
    $endgroup$
    – Alopex
    Apr 17 at 11:32






  • 1




    $begingroup$
    Oh noo I was all wrong
    $endgroup$
    – patta
    Apr 17 at 11:33










  • $begingroup$
    Is it possible that the field is stronger in the cloud? On the ground charges are likely dissipated; most places will be close to, well, ground neutral. The cloud, however, is a confined volume of charges which will "attract field lines", if you want. The necessary breakdown field strength will likely be reached close to the cloud.
    $endgroup$
    – Peter A. Schneider
    Apr 17 at 17:39











  • $begingroup$
    @Alopex - Interesting. Please convert your comment to answer, and add resources. Should be easy if it is well known :-)
    $endgroup$
    – Peter M.
    Apr 17 at 19:28















1












$begingroup$

The lightning has a visible direction of propagation, from top to down, that is normally the same as the electrons it is carrying: free electrons get accelerated (downward) by the electric field and ionize other molecules, thus progressively creating the lightning plasma pathway.



Another reason why lightnings start from top, is that air at higher altitudes is less dense, and need less voltage to initiate a spark. With a uniform electric field, a spark will always start from high altitude, and propagate downward by the accelerated electrons.
Paschen Law






share|cite|improve this answer











$endgroup$












  • $begingroup$
    Generally speaking, positive charges accumulate at the top of a thundercloud, and negative charges accumulate at the bottom.
    $endgroup$
    – gandalf61
    Apr 17 at 10:14






  • 4




    $begingroup$
    It is now well known that the beginning of a lightning strike occurs with multiple leader ionisation paths starting from the ground upwards these paths are extremely faint. The first of these to reach a sufficiently high altitude and closest to the charge accumulation in the cloud becomes the conduction path for the main down-strike which is visible.
    $endgroup$
    – Alopex
    Apr 17 at 11:32






  • 1




    $begingroup$
    Oh noo I was all wrong
    $endgroup$
    – patta
    Apr 17 at 11:33










  • $begingroup$
    Is it possible that the field is stronger in the cloud? On the ground charges are likely dissipated; most places will be close to, well, ground neutral. The cloud, however, is a confined volume of charges which will "attract field lines", if you want. The necessary breakdown field strength will likely be reached close to the cloud.
    $endgroup$
    – Peter A. Schneider
    Apr 17 at 17:39











  • $begingroup$
    @Alopex - Interesting. Please convert your comment to answer, and add resources. Should be easy if it is well known :-)
    $endgroup$
    – Peter M.
    Apr 17 at 19:28













1












1








1





$begingroup$

The lightning has a visible direction of propagation, from top to down, that is normally the same as the electrons it is carrying: free electrons get accelerated (downward) by the electric field and ionize other molecules, thus progressively creating the lightning plasma pathway.



Another reason why lightnings start from top, is that air at higher altitudes is less dense, and need less voltage to initiate a spark. With a uniform electric field, a spark will always start from high altitude, and propagate downward by the accelerated electrons.
Paschen Law






share|cite|improve this answer











$endgroup$



The lightning has a visible direction of propagation, from top to down, that is normally the same as the electrons it is carrying: free electrons get accelerated (downward) by the electric field and ionize other molecules, thus progressively creating the lightning plasma pathway.



Another reason why lightnings start from top, is that air at higher altitudes is less dense, and need less voltage to initiate a spark. With a uniform electric field, a spark will always start from high altitude, and propagate downward by the accelerated electrons.
Paschen Law







share|cite|improve this answer














share|cite|improve this answer



share|cite|improve this answer








edited Apr 17 at 10:57

























answered Apr 17 at 9:57









pattapatta

77648




77648











  • $begingroup$
    Generally speaking, positive charges accumulate at the top of a thundercloud, and negative charges accumulate at the bottom.
    $endgroup$
    – gandalf61
    Apr 17 at 10:14






  • 4




    $begingroup$
    It is now well known that the beginning of a lightning strike occurs with multiple leader ionisation paths starting from the ground upwards these paths are extremely faint. The first of these to reach a sufficiently high altitude and closest to the charge accumulation in the cloud becomes the conduction path for the main down-strike which is visible.
    $endgroup$
    – Alopex
    Apr 17 at 11:32






  • 1




    $begingroup$
    Oh noo I was all wrong
    $endgroup$
    – patta
    Apr 17 at 11:33










  • $begingroup$
    Is it possible that the field is stronger in the cloud? On the ground charges are likely dissipated; most places will be close to, well, ground neutral. The cloud, however, is a confined volume of charges which will "attract field lines", if you want. The necessary breakdown field strength will likely be reached close to the cloud.
    $endgroup$
    – Peter A. Schneider
    Apr 17 at 17:39











  • $begingroup$
    @Alopex - Interesting. Please convert your comment to answer, and add resources. Should be easy if it is well known :-)
    $endgroup$
    – Peter M.
    Apr 17 at 19:28
















  • $begingroup$
    Generally speaking, positive charges accumulate at the top of a thundercloud, and negative charges accumulate at the bottom.
    $endgroup$
    – gandalf61
    Apr 17 at 10:14






  • 4




    $begingroup$
    It is now well known that the beginning of a lightning strike occurs with multiple leader ionisation paths starting from the ground upwards these paths are extremely faint. The first of these to reach a sufficiently high altitude and closest to the charge accumulation in the cloud becomes the conduction path for the main down-strike which is visible.
    $endgroup$
    – Alopex
    Apr 17 at 11:32






  • 1




    $begingroup$
    Oh noo I was all wrong
    $endgroup$
    – patta
    Apr 17 at 11:33










  • $begingroup$
    Is it possible that the field is stronger in the cloud? On the ground charges are likely dissipated; most places will be close to, well, ground neutral. The cloud, however, is a confined volume of charges which will "attract field lines", if you want. The necessary breakdown field strength will likely be reached close to the cloud.
    $endgroup$
    – Peter A. Schneider
    Apr 17 at 17:39











  • $begingroup$
    @Alopex - Interesting. Please convert your comment to answer, and add resources. Should be easy if it is well known :-)
    $endgroup$
    – Peter M.
    Apr 17 at 19:28















$begingroup$
Generally speaking, positive charges accumulate at the top of a thundercloud, and negative charges accumulate at the bottom.
$endgroup$
– gandalf61
Apr 17 at 10:14




$begingroup$
Generally speaking, positive charges accumulate at the top of a thundercloud, and negative charges accumulate at the bottom.
$endgroup$
– gandalf61
Apr 17 at 10:14




4




4




$begingroup$
It is now well known that the beginning of a lightning strike occurs with multiple leader ionisation paths starting from the ground upwards these paths are extremely faint. The first of these to reach a sufficiently high altitude and closest to the charge accumulation in the cloud becomes the conduction path for the main down-strike which is visible.
$endgroup$
– Alopex
Apr 17 at 11:32




$begingroup$
It is now well known that the beginning of a lightning strike occurs with multiple leader ionisation paths starting from the ground upwards these paths are extremely faint. The first of these to reach a sufficiently high altitude and closest to the charge accumulation in the cloud becomes the conduction path for the main down-strike which is visible.
$endgroup$
– Alopex
Apr 17 at 11:32




1




1




$begingroup$
Oh noo I was all wrong
$endgroup$
– patta
Apr 17 at 11:33




$begingroup$
Oh noo I was all wrong
$endgroup$
– patta
Apr 17 at 11:33












$begingroup$
Is it possible that the field is stronger in the cloud? On the ground charges are likely dissipated; most places will be close to, well, ground neutral. The cloud, however, is a confined volume of charges which will "attract field lines", if you want. The necessary breakdown field strength will likely be reached close to the cloud.
$endgroup$
– Peter A. Schneider
Apr 17 at 17:39





$begingroup$
Is it possible that the field is stronger in the cloud? On the ground charges are likely dissipated; most places will be close to, well, ground neutral. The cloud, however, is a confined volume of charges which will "attract field lines", if you want. The necessary breakdown field strength will likely be reached close to the cloud.
$endgroup$
– Peter A. Schneider
Apr 17 at 17:39













$begingroup$
@Alopex - Interesting. Please convert your comment to answer, and add resources. Should be easy if it is well known :-)
$endgroup$
– Peter M.
Apr 17 at 19:28




$begingroup$
@Alopex - Interesting. Please convert your comment to answer, and add resources. Should be easy if it is well known :-)
$endgroup$
– Peter M.
Apr 17 at 19:28










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