Right about tonight, wrong about next week
Why do forecasts blur after three days?
A forecast measures this morning's sky and calculates tomorrow. But what if this morning's number was very slightly off? What you have here is not the real atmosphere but the very simple model Lorenz used. The wall appears all the same.
Board 1 · Two people's forecasts
One person wrote down this morning's temperature as 20.000°, the other as 20.001°. No thermometer could tell them apart. If both calculate ten days ahead by the same method, what happens to the weather in the two towns? Use the slider to add more days.
Difference between the two readings 0.001°
20.000° and 20.001°
How many days ahead? 14 days
Board 2 · Run it fifty times at once
So a real forecast changes the reading a little each time and calculates fifty times. Count any run that comes out below 20° as rain, move the day, and see how many of the fifty fall on the rain side.
Which day shall we look at? day 1
The story behind this board
It reads far more easily after you have tried it.
A better computer will not bring this wall down, because this morning's sky can never be measured with infinite accuracy. Between one weather station and the next there is nothing, and over the sea and high in the air the gaps are wider still. So today's forecasts are not calculated once but many times, and are given as a probability. Not for lack of confidence, but because that is the honest answer.