Charge Generation in Thunderstorms

Hello Everyone!

Today's blog is about a weather phenomenon we observe quite a lot during the summer months, and yet it is something that we don't fully understand. This weather phenomenon is lighting. But before lighting can occur, a mechanism  is needed to produce the charge buildup to electrify a thundercloud. 

All clouds are electrified to some degree.The distribution of charges in thunderstorms has been investigated with special radiosondes (called altielectrographs), by measuring the changes in the electric field at the ground that accompany lightning flashes, and with instrumented aircraft's. These studies have shown that, on average, thunderstorms contain positive charges in their upper regions, negative charges lower down but above the 0 degree C isotherm, and a much smaller pocket of positive charges just below the melting point.

     Schematic diagram to show the distribution of charges in a thunderstorm.

The magnitudes of the lower negative charge and the upper positive charge are approx. 10 –100 coulombs (hereafter symbol “C). The location of the negative charge (called the main charging zone) is rather well defined between the -10 degree C and about -20 degree C temperature levels. The positive charge is distributed in a more diffuse region above the negative charge. Although there have been a few reports of lightning from warm clouds, the vast majority of thunderstorms occur in cold clouds.

In order to understand the theories of thunderstorm electrification, I will first give a brief explanation of what the thermoelectric effect in ice is.  Let us consider a rod of ice warmed at one end and cooled at the other, so that a steady temperature difference ΔT is maintained between its two ends. Some of the water molecules in ice are always dissociated into positive and negative ions and the number of these ions is greater at higher temperatures. Therefore, the warmer end of the ice rod will contain more positive and negative ions than the colder end. Since ions migrate from regions of high concentration to regions of low concentration, both the positive and negative ions will tend to  migrate from the warmer toward the colder end of the ice rod. However, the mobility of the negative ions in ice is essentially zero, whereas that of the positive ions is quite high. Therefore, the positive ions migrate toward the cold end where they build up a positive space charge which eventually prevents any further migration of positive ions into the region. Hence, under steady-state conditions, a potential difference ΔV is established along the rod, with the cold end positively charged and the warm end negatively charged.

An important observational result, which provides the basis for most theories of thunderstorm electrification, is that the onset of strong electrification follows the occurrence (detected by radar) of heavy precipitation within the cloud in the form of graupel or hailstones. Most theories assume that as a graupel particle or hailstone (hereafter called the rimer) falls through a cloud it is charged negatively due to collisions with small cloud particles (droplets or ice), giving rise to the negative charge in the main charging zone. The corresponding positive charge is imparted to cloud particles as they rebound from the rimer, and these small particles are then carried by updrafts to the upper regions of the cloud. The exact conditions and mechanism by which a rimer might be charged negatively, and smaller cloud particles charged positively, have been a matter of debate for some hundred years. The main theories of charge generation in thunderstorms revolve around the thermoelectric effect and induction charging (not explained in this blog post).

I will now describe briefly a proposed mechanism for charge transfer between a rimer and a colliding ice crystal that appears promising. Laboratory experiments show that electric charge is separated when ice particles collide and rebound. The magnitude of the charge is typically about 10 fC per collision, which is close to the rate of charge generation in thunderstorms. The sign of the charge received by the rimer depends on temperature, the liquid water content of the cloud, and the relative rates of growth from the vapour phase of the rimer and the ice crystals. If the rimer grows more slowly by vapour deposition than the ice crystals, the rimer receives negative charge and the ice crystals receive the corresponding positive charge. Because the latent heat released by the freezing of supercooled droplets on a rimer as it falls through a cloud will raise the surface temperature of the rimer above ambient temperatures, the rate of growth of the rimer by vapour deposition will be less than that of ice crystals in the cloud. Consequently, when an ice crystal rebounds from a rimer, the rimer should receive a negative charge and the ice crystal a positive charge, as required to explain the main distribution of charges in a thunderstorm.

Diagram: Ice particles collide with a hailstone whose surface is warmed by riming. Ice particles rebound with positive charge and hailstone receives negative charge.

The charge transfer appears to be due to the fact that positive ions move through ice much faster than negative ions. As new ice surface is created by vapour deposition, the positive ions migrate rapidly into the interior of the ice, leaving the surface negatively charged. During a collision material from each of the particles is mixed, but negative charge is transferred to the particle with the slower growth rate. In some thunderstorms, a relatively weak positive charge is observed just below the main charging zone. This may be associated with the charging of solid precipitation during melting or to mixed phase processes.

    A more detailed charge distribution between two thunderstorm clouds.