whisky-dave expressed precisely :
When they explained it on the TV durign a weather documentary they said it
wass the chance of rain andn showed how they calculated it.
In waether there';s a few things that can happen due to changign Highs and
lows and various 'gulf' type streams the forcaster can guess which will
effect which but it's a probabilty rather than a known outcome, so they use
the % term.

I suppose they could use odds like bookies.

I was wondering about that yesterday, reading the local forecast. It
was showing various percentages for each hour, 20%, 30%, 40%, 70%. Even
threats of thunderstorms. Does the first 20% mean that there is a 20%
risk of some rain within that hour, or something else?

In fact what happened was someone 2 miles away saw some brief and light
rain, hardly worth a mention. We had cloudy, no rain fell at all, no
thunder, the sun came out when it was supposed to be 70% and it was hot
and very humid.