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Q&A: Understanding the chance of rain

Pedestrians cross Kennedy Boulevard in downtown Tampa during a recent light rain.


Pedestrians cross Kennedy Boulevard in downtown Tampa during a recent light rain.

Figuring the chance of rain

Exactly what is meant when a weatherman says that there is, for example, a 30 percent chance of rain? Please explain what they mean when they give a percentage for the chance of rain, and how the figure is determined.

Forecasts are done locally from 122 National Weather Service forecast offices and are provided to local news outlets along with the computer guidance that led to the forecast. Many broadcasters use the weather service forecast directly, others study the computer information and adjust it based on their local knowledge and still others incorporate information from private forecasting services.

When the forecast says there is, say, a 30 percent chance of rain, it means that in any given place within the forecast area there is a 30 percent chance of rain falling and a 70 percent chance of no rain.

It does not mean it has rained three times in the last 10 years on this date, though historical data is included in computer programs used to forecast weather.

It may or may not mean that 30 percent of the area will receive rain.

What we call the "chance of rain" in any given location is known as the probability of precipitation, or PoP, in weather talk. According to the National Weather Service, you calculate PoP by multiplying the confidence that precipitation will fall somewhere in the forecast area by the percentage of the area that will get a measurable amount, if it falls anywhere.

An example: The forecaster knows precipitation is going to fall — so confidence is 100 percent — and has concluded that 40 percent of the area will get measurable rain. That works out to a 40 percent chance of rain in any given spot in the forecast area.

Or, let's say the forecaster is 50 percent sure precipitation will occur, and expects measurable rain over 80 percent of the area if it does fall. In this case, you end up with — once again — a 40 percent chance of rain.

Chemical salts help check fires

What are the fire retardant chemicals they are dumping on the forest fires out in California? How effective are they?

The planes are dropping a commercial product called Phos-Chek. It's a dry powder that's mixed with water. About 80 percent of what comes out of the plane is water. The main ingredient in Phos-Chek is "chemical salts," which is fertilizer such as ammonium phosphate and diammonium sulfate.

A red dye is part of the mix, too. The dye helps the pilot see where the retardant already has been dropped and where to begin the next drop.

Water alone won't stop the fire for long, as it evaporates too quickly. Once the ground cover is coated with the mix, it will resist fires until the next big rain.

Q&A: Understanding the chance of rain 08/21/08 [Last modified: Tuesday, November 1, 2011 5:33pm]
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