# Query

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.

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