Lightning is a brilliant and powerful display of electrostatics in nature. It is inescapable of the attention of humans. Lightning storms were not invented or planned, so all we can do is observe them and their impacts.
How Lightning is Formed
As a general statement, lightning is caused by polarization in clouds. In turn, this causes unequal distributions of charges between the earth and clouds, so lightning strikes to redistribute these charges.
Polarization of Clouds
The precursor of a lightning strike is the polarization of positive and negative charges in a cloud, in which the top of the cloud becomes positive and the bottom becomes negative. There are two mechanisms that enable a cloud to become polarized.
The first mechanism is the collision of water droplets in clouds. In the storm clouds, there are small water droplets and ice particles moving in a turbulent fashion. Water from the ground evaporates and rises up to these storm clouds. As this water approaches the clouds, it becomes water droplets and enters the cloud. These new water droplets collide with those already present in the cloud, and electrons are ripped off of the rising droplets, causing a separation between positively charged water droplets and negative electrons.
The second mechanism involves electrons being separated from ice. In higher altitudes, the temperature lowers and water droplets freeze. These droplets freeze together such that there is a negative, frozen center and a positive, liquid outer. Wind blows and pulls the water away from the frozen core. This water is positively charged and rises in the cloud, while the negatively charged ice falls to the bottom portion of the cloud.
Electric Field and Earth
As static energy builds in the cloud, its electric field expands and eventually extends toward the earth. Since the bottom of the cloud is negatively charged, the negative charges on the earth are repelled, so the earth's surface becomes positively charged.
The strong electric field also ionizes the air. Under regular circumstances, the air would not be conductive enough to allow lightning. The electric field causes molecules in the air to shed their outer electrons, and the air is now a mix of positive ions and free electrons. This conductive plasma now makes it possible for lightning to strike.
Pathway of Lightning
Now that the air is conductive, excess electrons in the cloud zig zag through the air. This group of electrons is called the step leader, and it is not the actual lightning strike. The step leader creates a pathway that the lightning will travel along.
As the step leader, which is negatively charged, approaches earth, the quantity of positive charge on earth increases and it surges upwards to meet the step leader. This positive charge is called the streamer.
When the step leader and streamer meet, they create a complete conductive pathway that electrons can travel along in the form of a lightning strike.
Lightning and Thunder
Many trillions of electrons traverse the conductive pathway in a millisecond, creating lightning. Multiple secondary strikes can occur so quickly that they are perceived by the human eye as part of the initial strike.
Sometimes, lightning can glow with a blue or purple tint. This is because it is a characteristic of ionized air molecules, especially nitrogen and hydrogen.
The column that lightning travels down can reach temperatures as high as 30,000°C, which is significantly hotter than the sun. This intense temperature heats the air around the lightning, causing it to expand and then cool quickly. This causes a shockwave to be sent outwards, which we know as thunder.
Lightning and Atmospheric Chemistry
At the high temperatures of lightning, there is enough energy in the atmosphere for nitrogen and oxygen to combine and form nitrogen oxides (NOx). Some common nitrogen oxides are NO and NO2. These nitrogen oxides cause a change in ground-level ozone (O3). This increase in nitrogen oxides and ozone occurs 3-8 miles above Earth's surface and surpasses the amount of these chemicals that are created by human activity.
Typically in the United States, the burning of fossil fuels is the main cause of nitrogen oxides, which leads to the formation of ozone near the Earth's surface. During summer months, lightning activity increases nitrogen oxides by as much as 90% and ozone by more than 30% in the Earth's troposphere, which is located 3-8 miles above the Earth's surface.
Depending on where ozone is located, it can help or harm life on Earth. Most ozone resides in the stratosphere, which is a region of atmosphere located 8-25 miles above the surface of the Earth. The ozone here helps to shield life on Earth from the sun's harmful ultraviolet radiation. At the surface, ozone is harmful to plants and can cause damage to lung tissue. In the tropopause, a region of the atmosphere that stretches from Earth's surface to 8 miles high, ozone is a radioactive gas that can affect climate.
Nitrogen oxide that is released close to Earth's surface by the burning of fossil fuels undergoes chemical reactions before being transported upward. However, lightning releases nitrogen oxides directly into and throughout the entire troposphere. The lightning source for nitrogen oxides over North America is large, so it has implications on nitrogen oxides in the troposphere over other parts of the world.