4 ms·

> Even on sunny days with cloudless skies, the air carries a voltage of around 100 volts for every meter above the ground. Wait, what !?

Pick-A-Hill2019 6y ago
Got to admit I too thought that claim merited a [citation needed] so I tried some internet googling to try and find something that backed up those numbers and drew a blank. I have never seen a ~100v per vertical metre potential difference in any cables or equipment that I've run. That includes a long SWA cable from a buildings' floor level to the roof-top end point (roughly 40 metres - so that would equal 40x 100v = 4kV). [Edit to add - Longest vertical cable run I have dealt with was 180m / 41 floors for a four-core fire rated comms cable] [Double Edit, Sorry.] The only source I was able to find was this "Or, if we suspend a metal object outdoors, we’ll find that it spontaneously charges up to a certain voltage: about +100VDC for every meter of altitude above the ground." from https://www.quora.com/Can-electric-current-charge-flow-through-air https://www.quora.com/Can-electric-current-charge-flow-throu...?
- mncharity 6y ago
  > googling [...] drew a blank Global atmospheric electric circuit; atmospheric electric field (or potential gradient); Carnegie curve; electric field mill sensor. > those numbers It varies a lot. Time of day, location, conditions. Perhaps usually more than -100 V/m, and less than 200 V/m or so? Nothing compared to the 10 kV of zapping doorknobs in winter. > never seen [in...] buildings' The e-field contours follow the outside of a building, rather than cutting through it. And of a person. And more inconveniently, sometimes of a sensor pole. It's long seemed to me a pity this isn't used more in intro physics E&M. Been known for one or two+ centuries. Maybe with drone-sized less-expensive e-field sensors it will get more visibility?
  - Pick-A-Hill2019 6y ago
    Have an upvote for the rabbit hole although I still don't see any solid evidence backing up the 100v per metre claim unless scaled to ground vs. a theoretical probe placed 5+ kilometres up in the clouds.
    - mncharity 6y ago
      You can watch it live[1] at UK Reading's Atmospheric Observatory. Currently (00-ish UTC), the last 12-ish hours there, have been between 100 and 150 V/m. Preceded by 6-ish hours of high-variability, say -50 to 100 with excursions down by -200 V/m. And then 6-ish hours of 50 to 100 V/m. Aside from the 6 hrs of variability, which look like a front coming in, the shape seems stereotypical Carnegie curve. [1] http://www.met.reading.ac.uk/weatherdata/Reading_daily_AWS_graphs.html#8 http://www.met.reading.ac.uk/weatherdata/Reading_daily_AWS_g...
      - BlueTemplar 6y ago
        Right, the voltage is huge compared to household electronics, however the amperage is tiny...
georgecmu 6y ago
See https://www.feynmanlectures.caltech.edu/II_09.html#:~:text=9%E2%80%931The%20electric%20potential,volts%2Fm%20in%20the%20air https://www.feynmanlectures.caltech.edu/II_09.html#:~:text=9.... Reference: Chalmers, J. Alan, Atmospheric Electricity, Pergamon Press, London (1957). 9–1 The electric potential gradient of the atmosphere On an ordinary day over flat desert country, or over the sea, as one goes upward from the surface of the ground the electric potential increases by about 100 volts per meter. Thus there is a vertical electric field E of 100 volts/m in the air.

Pick-A-Hill2019 6y ago

Got to admit I too thought that claim merited a [citation needed] so I tried some internet googling to try and find something that backed up those numbers and drew a blank. I have never seen a ~100v per vertical metre potential difference in any cables or equipment that I've run. That includes a long SWA cable from a buildings' floor level to the roof-top end point (roughly 40 metres - so that would equal 40x 100v = 4kV). [Edit to add - Longest vertical cable run I have dealt with was 180m / 41 floors for a four-core fire rated comms cable] [Double Edit, Sorry.] The only source I was able to find was this "Or, if we suspend a metal object outdoors, we’ll find that it spontaneously charges up to a certain voltage: about +100VDC for every meter of altitude above the ground." from https://www.quora.com/Can-electric-current-charge-flow-through-air https://www.quora.com/Can-electric-current-charge-flow-throu...?

mncharity 6y ago
> googling [...] drew a blank Global atmospheric electric circuit; atmospheric electric field (or potential gradient); Carnegie curve; electric field mill sensor. > those numbers It varies a lot. Time of day, location, conditions. Perhaps usually more than -100 V/m, and less than 200 V/m or so? Nothing compared to the 10 kV of zapping doorknobs in winter. > never seen [in...] buildings' The e-field contours follow the outside of a building, rather than cutting through it. And of a person. And more inconveniently, sometimes of a sensor pole. It's long seemed to me a pity this isn't used more in intro physics E&M. Been known for one or two+ centuries. Maybe with drone-sized less-expensive e-field sensors it will get more visibility?
- Pick-A-Hill2019 6y ago
  Have an upvote for the rabbit hole although I still don't see any solid evidence backing up the 100v per metre claim unless scaled to ground vs. a theoretical probe placed 5+ kilometres up in the clouds.
  - mncharity 6y ago
    You can watch it live[1] at UK Reading's Atmospheric Observatory. Currently (00-ish UTC), the last 12-ish hours there, have been between 100 and 150 V/m. Preceded by 6-ish hours of high-variability, say -50 to 100 with excursions down by -200 V/m. And then 6-ish hours of 50 to 100 V/m. Aside from the 6 hrs of variability, which look like a front coming in, the shape seems stereotypical Carnegie curve. [1] http://www.met.reading.ac.uk/weatherdata/Reading_daily_AWS_graphs.html#8 http://www.met.reading.ac.uk/weatherdata/Reading_daily_AWS_g...
    - BlueTemplar 6y ago
      Right, the voltage is huge compared to household electronics, however the amperage is tiny...

georgecmu 6y ago

See https://www.feynmanlectures.caltech.edu/II_09.html#:~:text=9%E2%80%931The%20electric%20potential,volts%2Fm%20in%20the%20air https://www.feynmanlectures.caltech.edu/II_09.html#:~:text=9.... Reference: Chalmers, J. Alan, Atmospheric Electricity, Pergamon Press, London (1957). 9–1 The electric potential gradient of the atmosphere On an ordinary day over flat desert country, or over the sea, as one goes upward from the surface of the ground the electric potential increases by about 100 volts per meter. Thus there is a vertical electric field E of 100 volts/m in the air.