If a physical system is sufficiently complex then it also will exhibit randomness. Small differences in the initial conditions of the Earth’s atmosphere can lead to large, unpredictable changes in the evolution of weather systems. This is known as ‘chaos theory’. The random number generator at www.random.org uses atmospheric noise as a source for random numbers, because the atmosphere is a chaotic system.
Some argue that although chaotic, due to the atmosphere being controlled by classical physics, it is a deterministic system (unlike a quantum system), therefore, the numbers produced are not truly random. However, for the atmosphere to be a truly predictable system, one would need to know the position and velocity of every molecule in the planet’s weather systems. This is clearly not possible and the inaccuracy of current weather forecasts show the reality of how unpredictable weather systems are.