Beyond two or three days, the world's best weather forecasts are doubtful, and beyond six or seven they are worthless.  The Butterfly Effect is the reason. For small pieces of weather--to a global forecaster, small can mean thunderstorms and blizzards  --any prediction becomes worse rapidly. Errors and uncertainties increase, from dust devils and storms up to continent-size eddies   that only satellites can see. The modern weather models work with net-like points sixty miles apart, and even so, some starting data have to be guessed, since ground stations and satellites cannot see everywhere. But suppose the earth could be covered with sensors placed one foot apart, rising at one-foot intervals all the way to the top of the atmosphere. Suppose every sensor gives perfectly accurate readings of temperature, pressure, humidity  , and any other data a weatherman would want. Exactly at noon a powerful computer takes all the data and calculates what will happen at each point at 12.01, then 12.02, then 12.03... the computer will still be unable to predict whether Princeton will have sun or rain one month away. At noon the spaces between the sensors will hide fluctuations   that the computer will not know about. By 12.01, those fluctuations will already have created small errors one foot away. Soon the errors will have added to the ten-foot scale, and so on up to the size of the globe. A weather forecast   _   in the world.
is reliable within one or two days is doubtful beyond 24 hours becomes useless beyond two or three days is still worthwhile in seven days