Watch: Pilot films cloud twice Mount Everest's height. Know the science behind

A pilot over Sweden filmed a thunderstorm anvil cloud from 40,000 feet. Here is the science behind why this almost never happens.

In Short

• Pilot Cesar Sirbu filmed the anvil cloud over Sweden at 40,000 feet.
• The cloud top forms where the troposphere meets the stratosphere.
• Lightning inside heats air to 30,000 degrees Celsius, hotter than the Sun.

It sounds like something out of a science fiction film. A pilot is cruising at 40,000 feet over Sweden when, just outside the cockpit window, a cloud the size of a mountain range sits perfectly still against the blue sky, lightning quietly flickering in its belly.

Pilot Cesar Sirbu captured this, and the video went viral on social media. What Sirbu filmed is called a cumulonimbus incus, or an anvil cloud.

Here is what that actually means. A cumulonimbus is a thunderstorm cloud. The word incus is Latin for anvil, which describes the flat, hammer-like top the cloud develops at its peak.

Most of us see these from below, as dark, bruised masses before a storm. Sirbu saw one from the side, at the same height as its top. That almost never happens.

WHY DOES THE CLOUD GO FLAT AT THE TOP?

The atmosphere has a ceiling. It is called the tropopause, the invisible boundary where the troposphere, the weather layer of the atmosphere, ends and the stratosphere begins.

Below it, the temperature drops as you go higher. Above it, that pattern reverses. When a thunderstorm's powerful updrafts, or columns of rising air moving at up to 100 kilometres per hour, push moist air to this ceiling, the air cannot rise further.

It spreads sideways instead, creating that flat, fibrous top made of ice crystals.

The anvil in Sirbu's video stretches hundreds of kilometres, carried by upper-level winds.

WHAT IS CAUSING THE LIGHTNING?

Deep inside the storm, between minus 10 and minus 20 degrees Celsius, tiny hailstone-like pellets called graupel collide with smaller ice crystals.

These collisions transfer electric charge: the graupel falls with a negative charge, the lighter ice crystals rise with a positive one.

Voltage builds to millions of volts until the air breaks down and lightning flashes.

Each lightning bolt heats the surrounding air to roughly 30,000 degrees Celsius, briefly hotter than the surface of the Sun.

WHY IS THIS VIEW SO RARE?

Airlines require pilots to stay at least 37 kilometres from severe thunderstorms. A clear, side-on view from a commercial cockpit, at the exact altitude of the anvil, requires an unusual combination of safe distance, ideal flight path, and fortunate timing. Sirbu had all three.

The storm itself was likely 10 to 15 kilometres tall, roughly the height of two Mount Everests stacked together.

This is not a special effect. This is just weather on Earth, doing what it always has, while most of us sleep.

- Ends