The holiday season requires a close eye on the weather. Modern people do this, not by looking at the sky, but by consulting a smartphone. Yahoo, the most popular weather app, tells you the probability of rain, hour by hour.

But you don’t want to know the probability that it will rain. It either will rain or it won’t and you want to know which of these outcomes will occur. And you can never know whether such a prediction was accurate. Both rain and no rain are consistent with the claim that the probability that it would rain was 10 per cent.

The British meteorological office gives some help in interpretation. It suggests that if the probability of rain was 10 per cent you might be willing to hang your sheets out to dry, but not a shirt needed for an important dinner. While this may be useful advice, however, it is not an explanation of the meaning of the statement that the probability of rain is 10 per cent.