So, since tiny changes in the starting conditions of a weather system can make significant differences to the outcome, when making a forecast we have to try to take into account what might be happening now, as well as what might happen in the future to affect the atmosphere. The best we can do is to produce a range of forecasts, with some indication of what is most likely to happen.
To help illustrate this, consider throwing two dice instead of one:
With two dice, the probability of throwing a combined score of a number between 2 and 12 is not the same. There is only one combination of number that would give you a 2 or a 12 (two 1s or two 6s respectively) but, for example, for a combined score of 4 you could throw a 3 and a 1, two 2s or a 1 and a 3 - so you are 3 times as likely to throw 4 as 2 or 12. There are most possible ways of throwing a combined score of 7, and no way at all of throwing a 1 or 13 or more.
Move the slider to pick a number and throw the dice a large number of times. Notice the shape of the graph that is produced - the middle numbers are rolled more often than the smallest or largest numbers.
This sort of shape of graph is very common. For example, temperature measurements will often show a similar distribution, although temperature can of course take any value, not just the numbers one to twelve.
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