Weather is one thing we all have in common, a phenomenon we all share. With its unpredictability is an example of a mysterious order in chaos in the Earth’s atmosphere.

Imagine a set of sensors in the atmosphere, a distance between them equal to 1 meter, which can measure temperature, velocity and direction of the wind, atmospheric pressure, and humidity. Imagine also that we know all the rules and equations describing the movement of air and other processes happening in the atmosphere. Does it mean that we will be able to make long-term predictions of the weather? Unfortunately no.

Scientists use differential equations, which are very similar to those describing movements of air or liquids, trying to predict the weather. So all the rules should be, and probably they are, correct. Well, therefore where is a mistake? The answer is simple, but may sound strange – in calculations. Even when using the best and the fastest computer, during every iteration an error became bigger and bigger - eventually it is as big as the result. So predicting means just guessing.

One of the first scientists who realised that long-term predictions are unreliable was the meteorologist Edward Lorenz. He put three differential equations into his computer, put also some starting data and began his experiment. After some time (quite long, because in the middle of the 50’s computers worked really slowly) he had a set of numbers describing the weather for a given period of time in the nearest future. A few days later he decided to continue his experiment and put into his computer some results from the previous session. He expected findings to be the same (or very similar) in both cases, but he was wrong. After several iterations Lorenz noticed, that the results differs, and this difference is growing very rapidly in every step. In 20^th iteration the error was bigger than an outcome – the predicting ended and started guessing. It means, there is the same possibility that after 20 days it will be raining or there will be beautiful, sunny weather.