The short answer

Clouds form from rising hot air that cools and forms water vapor and eventually ice crystals. The lighter ice crystals rise to higher parts of the cloud while the heavier particles sink down. As ice crystals and heavier particles collide, they create electrical charges that result in negatively charged particles accumulating at the base of the cloud, leading to lightning.

The long answer

Clouds form as hot air rises up from the ground into the atmosphere; this is known as an updraft. As the hot air rises, it reaches colder temperatures and begins to cool and form water vapor. The more water vapor, the larger the cloud.

If the temperature in the cloud falls below water's freezing point, the water vapor begins to turn into ice crystals. Some of these ice crystals collide with water droplets and form larger particles that are called graupel.

Since the graupel particles are heavier, they start to sink down to a lower part of the cloud. Ice crystals are lighter and therefore start to rise to a higher part of the cloud via the updraft.

Along the way, the graupel particles and ice crystals collide into each other which results in electrical charges. The ice crystals become positively charged and the graupel becomes negatively charged.

As more and more negatively-charged graupel particles gather at the base of the cloud, it causes the ground to form more positive charges. This is because negative and positive charges are attracted to each other.

When these opposite charges become strong enough, there is an electrical discharge in the form of lightning. When it hits the ground, it is called cloud-to-ground lightning. There can also be cloud-to-cloud lightning and cloud-to-air lightning.

🧠 Bonus brain points

What is thunder?

Thunder is the sound produced by a lightning strike. The lightning causes the air to heat and expand rapidly. Immediately after, the air cools and contracts quickly. The instantaneous expansion and contraction of air creates the sound wave of thunder.

Since light moves faster than sound, it takes roughly 5 seconds for the sound of thunder to travel a mile. You can use this as a rough estimation of how far away the cloud is by counting seconds after you see a lightning flash (e.g. 10 seconds to hear thunder after a flash means the cloud is 2 miles away).

Sources

NOAA SciJinks. (2022). What Causes Lightning? Retrieved February 25, 2023, from http://www.youtube.com/watch?v=VqXnN_FQfrc

NOAA. (2018, November 27). Understanding Lightning: Thunderstorm Electrification. National Weather Service. Retrieved February 25, 2023, from https://www.weather.gov/safety/lightning-science-electrification

NOAA. (2019, January 4). Understanding lightning: Thunder. National Weather Service. Retrieved February 25, 2023, from https://www.weather.gov/safety/lightning-science-thunder