# Why we shouldn't trust long-term weather forecasts

Today we talk about the weather, not the climate, terms that are sometimes confused. About time it is sometimes said: We cannot trust the men (and women) of the time. The lines that follow dismantle this statement. We begin the argument by recalling that in the preparation for the Normandy landings in World War II, a fundamental piece of information was weather forecasting. The best meteorologists in the world, using the measurements and calculations of the time, gave headquarters a forecast that turned out to be reasonably good. Based on that result, the meteorologists of the decade 1950-60 thought that, with better measurements and the use of computing, whose capacity grew from one day to the next, they could predict the weather in the long term and, by manipulating it, even change the climate. of a place For example, dragging clouds so that it rains whenever you want.

A decade later, a meteorologist with a very good mathematical background, Edward Lorenz (1917-2008), who had participated as a young meteorologist in World War II, asked to see the brand-new large computer at MIT (Massachusetts Institute of Technology, Boston). ), where he was a teacher. Although the problem he wanted to solve seemed like a trivial mathematical requirement compared to those of high-energy physicists, they gave him the time. He wanted to find the reason why it was not possible to reliably predict the weather with more than three days. To reduce the complexity of the problem, he greatly simplified the dynamic equations that he had to enter into the computer. It was a dynamic, deterministic, three-dimensional, nonlinear system of equations that today bears his name. This system has no analytical solution, it is necessary to use what are called numerical methods and, if possible, the computer. He did several simulations and in one of them it occurred to him to start in the middle, entering by hand the results obtained the day before.

As the computation time passed, the variables became out of sync and the prediction results increasingly diverged from the previous ones over time. Analyzing this surprising result he realized that it might be associated with rounding off the decimal places in the hand-entered data. He rounded to the thousandth. This rounding equals a very small fluctuation, which he found to be enough to modify the weather forecast. E. Lorenz, like other scientists of the time, thought that small fluctuations had no appreciable effect on large-scale systems, such as the weather. However, his results showed otherwise. There are large-scale systems that can exhibit unpredictable behavior. Something that does not mean that it is not subject to laws. If instead of the simple Lorenz equations, the much more complex current ones are introduced, time is no more predictable. Therefore, do not believe the one who tells you what the weather will be like in a month. Accept only winter is colder than summer.

Years later Lorenz introduced the concept of the "butterfly effect" which became very popular. He did it with the phrase: "The flapping of a butterfly's wings in Brazil can cause a tornado in Texas." With this he introduced the concept of chaos in unstable systems when the initial conditions are slightly changed. Thus, Lorenz has really been the father of the third revolution in science in the 20th century: the physics of chaos. That is why it is unfair that he does not appear on the list of Nobel Prize winners. Let us not forget that chaotic phenomena abound in nature and in human activity: economics, population biology, thermodynamics, chemistry, biomedical sciences (some arrhythmias), in apparently stable systems, such as the planetarium, etc.