# What is the cause for turbulence?

The transition from laminar flow to turbulent flow is usually told interms of Reynolds numbers. The Reynolds number is $Re=UL/\nu$ where $U$ is the speed, $L$ is dimensional length and $\nu$ is the viscosity. Even if we say the Reynold's number tells us whether the flow is turbulent or not, I have never seen a unique $Re$ that differentiates laminar from turbulence. My question is then, is the cause of turblence perfectly known? is it velocity, length or viscosity that highly determines the flow condition? Is there a unique Reynold's number that separates laminar flow from turbulent?