Which of the following is NOT one of the types of lightning?

The correct answer is that "Ground-to-sky" is not considered one of the primary types of lightning. The common classifications of lightning include in-cloud lightning, which occurs within a single cloud; cloud-to-ground lightning, which is the most familiar form, where the electricity travels from a cloud to the earth's surface; and cloud-to-cloud lightning, where the discharge occurs between two separate clouds.

The reason "Ground-to-sky" is not a standard type of lightning is that lightning generally originates in clouds due to the separation of electrical charges and moves downwards towards the ground or across clouds. While there are phenomena such as upward lightning or sprites (which occur above thunderstorms), these are not classified as "Ground-to-sky" lightning in the conventional sense. Understanding these distinctions is crucial in meteorology and aviation safety, as different types of lightning can pose varying risks to aircraft and ground-based operations.