Does anyone know what “27% chance of precipitation” means in weathermanese?

Does it mean that the probabilty of some measurable preciptation during the day is 27%, or does it mean that, at any given moment durng the day, there is a 27% probability that it is then raining?

I’m not sure inquiring minds want to know this, but I need to pack for a trip.

**Update** First reply is in:

*My understanding is that it means that during the time for which the forecast is supposed to be good, 27% of the area covered by the forecast*

*
**will experience precipitation. In other words, if a forecast is for a one-hundred square mile area, 27 of those square miles will get rained upon.*

**Second update** A reader points me to the Environment Canada website with the following explanation:

*Probability forecasts cannot be used to predict when, where or how much precipitation will occur. For example, a 60% probability of snow today does not mean that it will snow during 60% of the day. However, the probability figure does mean that there is a 60% chance of a measurable amount of snow falling at that location.*

If that’s right, and if there’s a 27% chance of some rain this afternoon, then in most places the probability that it will be raining at any given moment must be much smaller than 27%. I’m leaving my raincoat home.

## Author: Mark Kleiman

Professor of Public Policy at the NYU Marron Institute for Urban Management and editor of the *Journal of Drug Policy Analysis*. Teaches about the methods of policy analysis about drug abuse control and crime control policy, working out the implications of two principles: that swift and certain sanctions don't have to be severe to be effective, and that well-designed threats usually don't have to be carried out.
Books:
*Drugs and Drug Policy: What Everyone Needs to Know* (with Jonathan Caulkins and Angela Hawken)
*When Brute Force Fails: How to Have Less Crime and Less Punishment* (Princeton, 2009; named one of the "books of the year" by *The Economist*
*Against Excess: Drug Policy for Results* (Basic, 1993)
*Marijuana: Costs of Abuse, Costs of Control* (Greenwood, 1989)
UCLA Homepage
Curriculum Vitae
Contact: Markarkleiman-at-gmail.com
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