Water droplets form in our atmosphere when the air cools to the dewpoint and achieves a relative humidity of 100%. At this temperature, water vapor condenses (changes from a gas to a liquid) and this process occurs on individual tiny dust or aerosol particles called cloud condensation nuclei. These condensation nuclei, which act as a seed for condensation, are only about 0.1 µm in diameter and are rather abundant in the atmosphere, so that when saturation occurs, nearly all the transparent water vapor will condense into opaque water droplets about the size of 1-2 mm and create a cloud (or fog if near the ground).
You would think that if the water droplets that make up a cloud experience an ambient temperature below freezing (0ºC) they would turn to ice by freezing. However, in order for a water droplet to turn from a liquid to a solid, ice nuclei are needed to act as a foundation from which an ice crystal lattice can grow. Unlike the abundant cloud condensation nuclei in our atmosphere, ice nuclei numbers are much less, and therefore, not all the water droplets can freeze to become ice in sub-freezing temperatures. As a result, we find that clouds that are colder than freezing contain liquid water droplets that are referred to as supercooled (i.e., below freezing). In fact, supercooled liquid water can be present in a cloud to temperatures as low as -40ºC.
When supercooled water droplets encounter snowflakes and ice crystals, they freeze to the surface expanding the ice crystal or snowflakes size. Continued exposure to these droplets will cause a snowflake or ice crystal to grow into a small loosely packed ice spheroid known as graupel. This graupel is the first stage in the formation of hail, and along with the much smaller ice crystals, are two of the three necessary components in the cloud electrification process.
A thunderstorm contains updrafts in which unstable air rises. Hydrometeors, which is a scientific name for water or ice particles in the atmosphere, collide in these updrafts due to their different sizes and fall rates. Under specific conditions, these collisions result in a charge transfer between particles, which lead to charge building up on some of the hydrometers. Specifically, when ice crystals collide with graupel in the presence of supercooled water droplets, charge is transferred between these ice particles so that they are left with either a surplus or deficit of electrons following the collision. Most commonly, the smaller and lighter ice crystals are left with a deficit of electrons and therefore are positively charged whereas the graupel typically takes on a negative charge due to a surplus of electrons. The animation below illustrates this process.
Scientists have found that the polarity of the ice particles following collisions depends on the ambient temperature as well as the supercooled water droplet concentration. While in most cases, smaller ice crystals become positively charged, in some cases they can become negative charged by gaining electrons during collisions. As will be shown later, the polarity of the electrification process will then determine the dominant polarity of the subsequent flashes.
As a thunderstorm matures, graupel tends to accumulate in the lower part of the storm since it has a higher fall rate than ice crystals. However, the airmass in which they are both falling has a relative upward motion in an updraft. Over time, this process collisional charging, transport and accumulation results in the formation of charge regions within a cumulus cloud. The charge itself resides on the ice particles and individually each particle contains only a small amount of charge. However, huge numbers of electrified hydrometeors together can collectively create regions with very high voltages.
The simplified model of charge regions in a thunderstorm includes an upper positive charge region dominated by positively charged ice crystals (shown as red in the image below) and a lower negative charge region with primarily graupel and hail containing a surplus of electrons (shown as blue). For the remainder of this discussion, red will be used to designation positive charge (deficit of electrons) and blue negative charge (surplus of electrons). This will apply to charge regions and lightning leaders that form and propagate.
While the simple dipole (positive over negative) model serves well to explain storm electrification, the number and distribution of charge regions can be more complex. Frequently, a lower and relatively smaller positive charge region develops below the main negative charge region later in a storms lifecycle for reasons that are still not well understood. This lower region plays and important role in allowing ground flashes to occur as will be explained later.
Over time, storms can develop very complex charge structures with many charge regions spanning multiple levels. When storms grow upscale and develop into large complexes, charge regions can become horizontally extensive with layers of charge covering 10s to 100s of kilometers while varying altitude with distance.
In discussing fundamental lightning behavior, I will limit the charge regions to the basic tripole of upper positive (red) over middle negative (blue) over lower, smaller positive (red) structure with an inverted tripole consisting of upper negative (blue) over middle positive (red) over lower, smaller negative (blue). Inverted tripole storms can form when thermodynamic and microphysical properties present during collisional charging favor a surplus of electrons on the ice crystals rather than the graupel. Additionally, storms can initially develop a normal charge tripole but later exhibit an inverted structure due to ongoing lightning activity or changing storm morphology.
In addition to the non-inductive electrification process that occurs via ice particle collisions, there are important inductive electrification processes that take place once main charge regions form. Most notably is an inductive attraction of opposite polarity ions in the cloudless regions around storms, which then causes these ions to attach to cloud droplets on the outside boundary of storm clouds. Over time, accumulation of opposite polarity charge on the cloud periphery results in what is referred to as a screening layer of charge. The term “screening layer” describes the reduction in electric field that would be sensed outside the storm resulting from the presence of opposite polarity charge between the main storm charge region and a sensor external to the storm.
Furthermore, opposite polarity charge is induced on the earth’s surface beneath and near a thunderstorm. The Earth is a relatively good conductor and electrons can move in response to induction forces created by storm charge regions. As a result, the surface of the Earth and objects on the surface become charged as thunderstorm charge regions move over an area.
Now that we understand the electrification process, we will move onto how lightning forms as a result of this electrification process.