It is reported in the media that currently in Australia there are dozens of wildfires caused by lightning strikes. I am familiar with the mechanism of ordinary British lightning involving cumulonimbus and moisture, but I don't understand the Australian mechanism where hot,bone dry conditions cause lightning strikes on a massive scale, apparently from cloudless or near cloudless skies. The role of high temperatures and near cloudless skies obviously makes vegetation highly inflammable, but solar radiation hot enough to heat it to ignition temperature would kill everyone in Australia, so contrary to what some people appear to believe it can't be that. Perhaps someone can offer an explanation.

  • $\begingroup$ This is interesting. I once was tought that a lighting can travel some distance horizontally and hit the ground away from the thunderstorm. My interpretation is that these strikes were rather thunderstorms from frontal systems moving in than "clear air" strikes. Are the any links that describe what happened ? $\endgroup$ – user18411 Dec 27 '19 at 20:50
  • $\begingroup$ Example: live.staticflickr.com/7667/17059820722_d3cc2ecd36_b.jpg $\endgroup$ – user18411 Dec 27 '19 at 20:56
  • $\begingroup$ This looks suspiciously like conventional cumulonimbus which I already understand. As I said in my question, it is the moisture-free Australian lightning which needs explanation. $\endgroup$ – Michael Walsby Dec 27 '19 at 21:14
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    $\begingroup$ I think the problem might be the assumption of completely cloudless skies. The western US is, I'm told, quite similar to much of Australia. While skies here are generally clear in summer, and the relative humidity is low, quite often cumulonimbus clouds build up in the afternoon, leading to lightning that sparks wildfires. There may not be rain associated with these clouds, or the rain may not reach the ground (en.wikipedia.org/wiki/Virga ) The storms can also be scattered events, with plenty of clear sky between. $\endgroup$ – jamesqf Dec 28 '19 at 19:46
  • $\begingroup$ To make th comparison more valid you need to wit until there is a long drought in western USA which bakes the vegetation and makes it very combustible. In UK long droughts are accompanied by day after day of clear skies, as in 1976 and in lesser droughts since.. $\endgroup$ – Michael Walsby Dec 28 '19 at 20:22

Revised answer

TL;DR After having just heard from two experts who actually work in this field(firestorms and dry lightning) in Australia I believe the lightning seen is what is known as "dry lightning" comes from isolated thunderstorms and sometimes storms associated with minor troughs and fronts that come through Australia and also originating from a pyrocumulonimbus cloud. So @David Hammen and @jamesql are correct in their observations that the large scale dry lightning is indeed originating from isolated thunderstorms as well as storms associated with minor troughs and fronts. This dry lightning the experts confirm is associated with little rain.

In addition there have also been several dry lightning strikes from pyrocumulonimbus clouds. As an example for today over 20 dry lightning strikes have been recorded originating from pyrocumulonimbus clouds.

The major driver for the current drought like conditions over Australia is the large anomalous positive Indian Ocean Dipole (positive IOD) that has persisted well into boreal autumn/astral spring.

What the experts mention is that while the dry hot conditions are sporadic (2-4 days) they are persisting over over many weeks.

The following articles written by two researchers Firestorms and flaming tornadoes: how bushfires create their own ferocious weather systems and Firestorms: the bushfire/thunderstorm hybrids we urgently need to understand provide information on the dynamics behind firestorms and the formation of pyrocumulus clouds and the associated dry lightning.

Following is a summary of those articles and the conditions that lead to dry lightning from pyrocumulonimbus clouds.

Not all bushfires lead to firestorms. If a bushfire has sufficient area the upward movement of air causes the fire to interact with the atmosphere above it and form what is known as a pyrocloud. If in addition there is an atmospheric instability then this process leads to the formation of a pyrocumulonimbus cloud . Normally in meteorology we are taught of heat radiating from the ground but in this case the upward movement of air is due to the heat emanating from the fire.

Again from mesoscale meteorology one hears the word "downdrafts". Similarly in this context the appropriate word is "downbursts" and these are vertical drafts of air that hit the ground and move about in all directions. The unsettled conditions cause embers to carry over large distances.

These firestorms produce dry lightning that potentially can spark new fires that may end up creating a larger flaming zone.

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    $\begingroup$ @MichaelWalsby It appears from your multiple comments and answers/questions that you are obsessed with World War II. However I think the linked wikipedia article does suggest that Hiroshima and Nagasaki may have had possible pyrocumulonimbus clouds associated with it. Lightning one cannot confirm $\endgroup$ – gansub Dec 28 '19 at 10:30
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    $\begingroup$ Downvoted because you are taking the OP's claim that the lightning hitting Australia comes from cloudless skies as gospel truth. This is not the case. This is the time of year when Australia gets a lot of run of the mill (i.e., water-driven) thunderstorms. $\endgroup$ – David Hammen Dec 29 '19 at 7:39
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    $\begingroup$ Hi. I think your update with OLR map overstates it as evidence for the OP's "lightning strikes on a massive scale, apparently from cloudless or near cloudless skies". The climatological OLR anomaly shows less frequent convection/cloud, but you haven't made the link with actual lightning events. Those dry lightning forecasts usually occur in a context of other regional weather (e.g., troughs, wet lightning). There's a lot of meteorology going on in each case and there's a danger of us leaving readers with a misleadingly simple picture. $\endgroup$ – Deditos Dec 29 '19 at 13:39
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    $\begingroup$ ITCZ and monsoon are different things, guys. Does the first affect Australia at all ? And if i got it right it is not questioned that initial sparks came from regular, though possibly dry (water does not reach the ground) thunderstorms. These exist in Europe as well, though they are rare. I have no problem with that. $\endgroup$ – user18411 Dec 29 '19 at 14:16
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    $\begingroup$ @ebv the ITCZ here is the monsoon trough. Yes the northern areas are affected by tropical weather every year. Frontal activity only affects mid latitudes of Australia. It is a big country. $\endgroup$ – gansub Dec 29 '19 at 14:23

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