DNA: Singapore Airlines tragedy- What's in-flight turbulence?

Singapore Airlines' Boeing 777-300ER flight had taken off from London and was supposed to land at Changi Airport in Singapore, but 10 hours after takeoff, the flight got stuck in air turbulence due to bad weather at 37 thousand feet in Myanmar airspace. After which the plane came down from a height of 37 thousand feet to 31 thousand feet within 3 minutes. After which an emergency landing of this plane was made in Bangkok