Turbulence is the irregular motion of air caused by obstructions to airflow around the plane.

And this can be caused by many factors, but is most often caused by unstable weather patterns that trigger storms.

And if you are thinking, "Wouldn't the pilot know there's a storm ahead?" turbulence can usually be detected on the plane's weather radar, if they contain water particles.

And pilots also receive preflight weather briefings that include information about potential turbulence.

But as the name clear-air suggests, there are no warnings when the plane is about to encounter such sudden and severe swirls of air.

Looking at the number of injuries, I believe that the aircraft, the flight, encountered clear-air turbulence.

A plane of that size, a Boeing 777, can be tossed about violently.

If it's caused by normal turbulence, the cloud-related turbulence, the pilot could have avoided and will have avoided that.

But clear-air turbulence, there's no way of knowing when it's going to happen, where and the severity or intensity of the turbulence.

According to the National Weather Service in the US, about 75% of CAT incidents happen in clear air, although they're not restricted to cloud-free air.

Here's what we know about CAT.

It usually happens in high altitudes of 15,000ft above ground level.

SQ321 was cruising at 37,000ft.

CATs can happen when two masses of air meet at different velocities, pressure or temperature, such as when a jet stream interacts with calmer air.

Other sources of CAT include airflow over mountainous terrain, as well as when planes pass through the vicinity of thunderstorms, such as wind passing horizontally 5,000ft above a cumulonimbus cloud.

There is currently no technology to detect the phenomenon.

Let's retrace the flight of SQ321.

At 10: 38pm London time on Monday (May 20), SQ321 departed London's Heathrow Airport.

There were 221 passengers and 18 crew members on board the Boeing 777-300ER jet.

The London Heathrow to Singapore Changi route is routinely a 13-hour journey.

About 10 hours into the flight, when the crew was serving breakfast, the plane entered an area over the Irrawaddy basin in Myanmar, and that's when things went wrong.

SQ321 encountered unexpected extreme turbulence and began tilting upwards.

Altitude data from FlightRadar24 shows the aircraft was thrust up and down over the course of a minute.

At one point, it reached about 400ft or 120 meters above its original altitude, before dramatically dropping back to cruising altitude of 37,000ft.

If you look at the plane's vertical speed, which is the rate at which it ascends or descends, it surged to 1,664ft per minute and then dived to -1536ft per minute, all in a matter of three seconds.

Weather forecasting service AccuWeather would later report explosive thunderstorms had been developing nearby just after 4pm Singapore time.

The flight was diverted to Bangkok and the pilot declared a medical emergency.

He landed the plane at Bangkok's Suvarnabhumi Airport at 4:45pm Singapore time.