Why aren’t positive cloud-to-ground lightning strikes associated with the precipitation core of a storm?

Why aren’t positive cloud-to-ground lightning strikes associated with the precipitation core of a storm?

Thunderstorms are powerful atmospheric phenomena characterized by heavy rain, strong winds, and lightning. Within a thunderstorm, lightning can take several forms, including cloud-to-ground (CG) lightning, which occurs when an electrical discharge travels between the thundercloud and the ground. CG strikes can be further divided into positive and negative strikes, depending on the charge polarity of the cloud and ground. While negative CG strikes are commonly associated with the precipitation core of a storm, positive CG strikes tend to occur in a different location. This article explores the reasons for the absence of positive CG lightning in the precipitation core of a storm.

The role of charge separation

To understand why positive CG lightning strikes are not associated with the precipitation core of a storm, it is crucial to examine the process of charge separation within a thunderstorm. Thunderstorms are fueled by the upward movement of warm, moist air, resulting in the formation of towering cumulonimbus clouds. Within these clouds, the rising air carries water droplets and ice particles, creating a complex electric field through the interaction of different charge carriers.

Typically, cumulonimbus clouds exhibit a charge separation phenomenon known as polarization, where positive and negative charges are separated vertically within the cloud. The lower part of the cloud becomes negatively charged, while the upper part becomes positively charged. This charge separation is driven by several factors, including the collision and separation of ice particles and supercooled water droplets within the cloud. As a result, negative CG lightning strikes are more common because they involve the discharge of the negative charge in the lower part of the cloud to the positively charged ground.

Positive CG Lightning and Upper Positively Charged Regions

Positive CG strikes, on the other hand, occur when there is a discharge from the positively charged upper regions of a thunderstorm cloud to the negatively charged ground. These positive charges are typically located above the main precipitation core of the storm. Several factors contribute to the formation of positively charged regions within a thunderstorm cloud. One of the primary mechanisms is the process of ice crystal graupel coagulation, in which supercooled water droplets freeze onto ice crystals to form larger ice particles known as graupel. These graupel particles become positively charged and are transported by updrafts to the upper regions of the cloud.
Positive CG lightning is therefore more commonly associated with the anvil top or overshooting tops of thunderstorm clouds than with the precipitation core. The anvil top is the flat, spreading upper portion of the cloud, while overshooting tops occur when strong updrafts push a portion of the cloud above the equilibrium level. These regions are characterized by significant positive charge concentrations, making them conducive to the occurrence of positive CG lightning. However, it is important to note that positive CG strikes are relatively rare compared to negative CG strikes.

Implications and Safety Considerations

The distinction between positive and negative CG lightning strikes has important implications for safety considerations during thunderstorms. Negative CG strikes, which are more common and associated with the precipitation core, pose a greater risk to people on the ground. These strikes can occur within several kilometers of the storm and are responsible for the majority of lightning-related injuries and fatalities.
Positive CG lightning, on the other hand, tends to occur farther away from the storm, often extending several miles beyond the edge of the storm. However, they can still pose a threat to people in open areas or engaged in outdoor activities. It is worth noting that positive CG strikes are often more intense and have a higher peak current than negative CG strikes. Therefore, it is critical to stay informed of weather conditions and take shelter indoors when thunderstorms are nearby, regardless of the type of CG lightning expected.

In summary, positive CG lightning is typically not associated with the precipitation core of a storm due to vertical charge separation within thunderstorm clouds. While negative CG lightning occurs between the negatively charged lower portion of the cloud and the ground, positive CG lightning originates from the positively charged upper regions of the cloud. Understanding the factors that contribute to the formation of positively charged regions in thunderstorms is essential for assessing lightning risks and promoting safety during severe weather events.

FAQs

1. Why aren’t positive cloud-to-ground lightning strikes associated with the precipitation core of a storm?

Positive cloud-to-ground lightning strikes are not commonly associated with the precipitation core of a storm because they originate from the anvil region of a thunderstorm rather than the main updraft region where the heaviest precipitation occurs.

2. What causes positive cloud-to-ground lightning strikes to occur away from the precipitation core?

Positive cloud-to-ground lightning strikes occur away from the precipitation core because they are typically initiated by the presence of large ice particles in the upper regions of a thunderstorm. These ice particles are carried upward by powerful updrafts and accumulate in the anvil region, creating a charge separation that leads to positive lightning.

3. How does the charge separation in the anvil region contribute to positive cloud-to-ground lightning strikes?

The charge separation in the anvil region of a thunderstorm occurs due to the collision and separation of ice particles. The smaller ice particles acquire a negative charge, while the larger particles, such as hailstones, become positively charged. This charge separation sets the stage for positive cloud-to-ground lightning strikes to occur.

4. Are positive cloud-to-ground lightning strikes less common than negative ones?

Yes, positive cloud-to-ground lightning strikes are less common than negative ones. Negative lightning strikes account for the majority of cloud-to-ground lightning events, typically outnumbering positive lightning strikes by a ratio of approximately 10 to 1.

5. What are the characteristics of positive cloud-to-ground lightning strikes?

Positive cloud-to-ground lightning strikes tend to be more powerful and longer-lasting than negative lightning strikes. They can also travel greater distances horizontally and have been observed to strike from the anvil region of a thunderstorm to the ground many miles away from the storm’s main updraft.