The physics of lightning

Triboelectric nanogenerators are an energy harvesting technology that relies on the coupling effects of contact electrification and electrostatic induction between two solids or a liquid and a solid. Here, we present a triboelectric nanogenerator that can work based on the interaction between two pure liquids. A liquid–liquid triboelectric nanogenerator is achieved by passing a liquid droplet through a freely suspended liquid membrane. We investigate two kinds of liquid membranes: a grounded membrane and a pre-charged membrane. The falling of a droplet (about 40 μL) can generate a peak power of 137.4 nW by passing through a pre-charged membrane. Moreover, this membrane electrode can also remove and collect electrostatic charges from solid objects, indicating a permeable sensor or charge filter for electronic applications. The liquid–liquid triboelectric nanogenerator can harvest mechanical energy without changing the object motion and it can work for many targets, including raindrops, irrigation currents, microfluidics, and tiny particles. Triboelectric nanogenerators harvest energy by contacting two solids or a liquid and a solid. Here the authors use a conductive liquid membrane as a permeable electrode to demonstrate triboelectrification via liquid–liquid contact by passing liquid droplets through a liquid membrane to generate power.

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Power generation from the interaction of a liquid droplet and a liquid membrane.

This review is an overview of progress in understanding the theory and observation of the global atmospheric electric circuit, with the focus on its dc aspects, and its short and long term variability. The effects of the downward ionosphere-earth current density, Jz, on cloud microphysics, with its variability as an explanation for small observed changes in weather and climate, will also be reviewed. The global circuit shows responses to external as well as internal forcing. External forcing arises from changes in the distribution of conductivity due to changes in the cosmic ray flux and other energetic space particle fluxes, and at high magnetic latitudes from solar wind electric fields. Internal forcing arises from changes in the generators and changes in volcanic and anthropogenic aerosols in the troposphere and stratosphere. All these result in spatial and temporal variation in Jz.Variations in Jz affect the production of space charge in layer clouds, with the charges being transferred to droplets and aerosol particles. New observations and new analyses are consistent with non-negligible effects of the charges on the microphysics of such clouds. Observed effects are small, but of high statistical significance for cloud cover and precipitation changes, with resulting atmospheric temperature, pressure and dynamics changes. These effects are detectable on the day-to-day timescale for repeated Jz changes of order 10%, and are thus second order electrical effects. The implicit first order effects have not, as yet, been incorporated into basic cloud and aerosol physics. Long term (multidecadal through millennial) global circuit changes, due to solar activity modulating the galactic cosmic ray flux, are an order of magnitude greater at high latitudes and in the stratosphere, as can be inferred from geological cosmogenic isotope records. Proxies for climate change in the same stratified depositories show strong correlations of climate with the inferred global circuit variations.The theory for electrical effects on scavenging of aerosols in clouds is reviewed, with several microphysical processes having consequences for contact ice nucleation; effects on droplet size distributions; precipitation and cloud lifetimes. There are several pathways for resulting macroscopic cloud changes that affect atmospheric circulation; including enhanced ice production and precipitation from clouds in cyclonic storms, with latent heat release affecting cyclone vorticity; and cloud cover changes in layer clouds that affect the atmospheric radiation balance. These macroscopic consequences of global circuit variability affecting aerosols–cloud interactions provide explanations for the many observations of short term and long term changes in clouds and climate that correlate with measured or inferred Jz and cosmic ray flux changes due to external or internal forcing, and lead to predictions of additional effects.

https://iopscience.iop.org/article/10.1088/0034-4885/71/6/066801/pdf

The global atmospheric electric circuit and its effects on cloud microphysics

On the evening of 28 July 1997 the city of Fort Collins, Colorado, experienced a devastating flash flood that caused five fatalities and over 200 million dollars in damage. Maximum accumulations of rainfall in the western part of the city exceeded 10 in. in a 6-h period. This study presents a multiscale meteorological overview of the event utilizing a wide variety of instrument platforms and data including rain gauge, CSU—CHILL multiparameter radar, Next Generation Radar, National Lightning Detection Network, surface and Aircraft Communication Addressing and Reporting System observations, satellite observations, and synoptic analyses. Many of the meteorological features associated with the Fort Collins flash flood typify those of similar events in the western United States. Prominent features in the Fort Collins case included the presence of a 500-hPa ridge axis over northeastern Colorado; a weak shortwave trough on the western side of the ridge; postfrontal easterly upslope flow at low levels; w...

/pdf/mesoscale-and-radar-observations-of-the-fort-collins-flash-19z3mmyc1k.pdf

Mesoscale and Radar Observations of the Fort Collins Flash Flood of 28 July 1997

Both the amplitude and the phase of the ionospheric potential and Carnegie curve of atmospheric electricity are considered to distinguish causes for the negatively charged earth in fair weather. Satellite-observed longitudinal distributions of electrical activity are convolved with local diurnal variations of cloud-to-ground lightning and point discharge current to produce universal diurnal variations which are compared with the Carnegie curve. The amplitude ratio (maximum-minimum)/mean) for the predicted universal diurnal variation of point discharge shows good agreement with the Carnegie curve, whereas the predicted amplitude ratio for lightning is 2–3 times greater. These comparisons suggest that conduction current other than lightning is the dominant charging agent for the Earth's surface.

The local diurnal variation of cloud electrification and the global diurnal variation of negative charge on the Earth

Some aspects of the correlation between lightning and rain activities in thunderstorms

In August 1989 the electric field was measured below a thunderstorm at Kennedy Space Center for more than 1 hour at several levels up to 800 m. No substantial precipitation reached the ground at the experimental site until after this data collection period, which included the thunderstorm approach and the triggering of four lightning flashes. The sensors at altitude were suspended from a tethered balloon and that at the surface was a standard field mill. The maximum value detected was 65 kV/m at 603 m while at the same time the steady surface field did not exceed 5 kV/m. Applying Gauss's law to the electric field data, we have calculated average charge densities within the layers formed by the levels where the sensors were located. Corona ions produced at the surface were detected up to the upper layer between 436 m and 603 m, and the densities calculated reached values close to 1 nC m−3 within each layer. The current densities were also evaluated at the sensor levels; the one calculated at 436 m ranges about 1.4 nA m−2 which means that a large proportion of corona ions were carried up to several hundreds of meters above ground.

Multilevel measurement of the electric field underneath a thundercloud: 2. Dynamical evolution of a ground space charge layer

On August 7, 1996, a flood of the Rio Gallego, in Aragon (Spanish central Pyrenees) wiped out the Biescas' campground, killing more than 80 persons. From the observations of two meteorological radars, this thunderstorm exhibited some of the radar characteristics of a supercell. It spread almost exactly over the Rio Gallego catchment area, up-stream of Biescas, and it remained above this area for about two hours, without any advection. The rainfall reached 100 mm h−1 and the total cumulative amount locally reached 200 mm. The cloud-to-ground (CG) flash density averaged over 5 × 5 km² areas, was exceptionally high, around 2 km−2, and the peak flash rate averaged over 5-minute periods reached 11.6 min−1 within the cell area. The evolution of the CG flash rate and the radar reflectivity were closely correlated. The flash rate reached high values before the arrival of the precipitation at the ground. We observe also a very good agreement between the location of the intense rain at the ground and the high CG flash density. The rainfall water volume per flash was 3×104 m³, a value in agreement with previous work. We propose that quantitative measurements of the lightning activity may provide valuable information for flash-flood nowcasting and short-term forecasting in mountainous areas.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/98GL00517

The CG lightning activity of a storm causing a flash‐flood

Electric field measurements above ground have shown that the space charge layer created by corona at ground level shields the background electric field produced by the thundercloud. Therefore it is expected that this space charge layer can also influence the conditions required to initiate upward lightning from tall objects. For this reason, a numerical model that describes the evolution of the main electrical parameters below a thunderstorm is used to compute the space charge layer development. The time variation of the electric field measured at 600 m above ground during the 1989 rocket triggered lightning experiment at the Kennedy Space Center (Florida) is used to drive the model. The obtained space charge density profiles are used to compute the conditions required to initiate stable upward lightning positive leaders from tall towers. Corona at the tip of the tower is neglected. It is found that the space charge layer significantly affects the critical thundercloud electric fields required to initiate upward lightning leaders from tall objects. The neutral aerosol particle concentration is observed to have a significant influence on the space charge density profiles and the critical thundercloud electric fields, whereas the corona current density does not considerably affect the results for the cases considered in the analysis. It is found that a lower thundercloud electric field is required to trigger a lightning flash from a tall tower or other tall slender grounded structure in the case of sites with a high neutral aerosol particle concentration, like polluted areas or coastal regions.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2006JD008308

Effect of the space charge layer created by corona at ground level on the inception of upward lightning leaders from tall towers

In order to study the formation and evolution of the charge layer created by a corona at the ground underneath thunderclouds, we have developed a new system of electric field detection. It is composed of five self-contained sensors designed to measure the ambient electric field at various altitudes between the surface and a tethered balloon that suspends the entire system of equipment. Each sensor can detect ambient field intensities up to 150 kV/m with an accuracy of 1.5% and without any detectable influence of a local corona effect on the measurement. A standard shutter field mill detects the surface electric field. The data collected aloft are telemetered via a 400-MHz frequency band. An adapted data processing system provides a real-time display of the multilevel evolution of the electric field within the first hundreds of meters above the ground. The digitized data can be used to display the instantaneous vertical profile of field intensity on various time scales.

Serge Chauzy

Papers

Some aspects of the correlation between lightning and rain activities in thunderstorms

Multilevel measurement of the electric field underneath a thundercloud: 2. Dynamical evolution of a ground space charge layer

The CG lightning activity of a storm causing a flash‐flood

Effect of the space charge layer created by corona at ground level on the inception of upward lightning leaders from tall towers

Multilevel measurement of the electric field underneath a thundercloud: 1. A new system and the associated data processing