A bit of Meteorology

My question about weather forecasters. When they say, for example, that the chance of rain is 30%, what exactly do they mean? Are they speaking of the chance that it will rain specifically where I happen to be or anywhere in the large metropolitan area where their broadcast is heard? Surely, those are different probabilities. Also, what period of time is implied? The chance that it will rain sometime this week is different than the chance of it raining in the next hour. It seems to me that weather forecasters should specify the geographic area and the period of time whenever they give us probabilities. What do you think?
- Richard Treptow
Chicago State University, Chicago, IL, USA

The significance of the percentage in a forcast is based upon history. If there is a 30% chance of rain for a the current weather conditions, then 30% of past times that the weather was like it is currently, it rained. As far as period of time, that is generally given. Something like "a 30% chance of rain till noon, then partly cloudy in the afternoon." Geographical location has to be general in order for them to make any prediction about the weather. Weather conditions are too changing to specify them to a very small area (like one building).

(published on 10/22/2007)