Calibrating Rainfall Goes High-Tech

You don’t need a weather report to know that if you spent much time outside over the last few days, you got wet.

But just how wet?

Only a few years ago, when weather forecasters relied solely on rain gauges to determine rainfall totals, you couldn’t get an accurate reading for your neighborhood unless one of the gauges happened to be nearby.

Your precipitation total might not be the same as in the neighborhood down the road, and it could be considerably less or more than in other parts of Ventura County.

Rain gauges are still an indispensable tool for meteorologists. But weather officials now combine those data with information from a national network of high-tech weather radars and computers to deliver a detailed portrait of how much rain has fallen, within a mile, on any given day in almost any part of the United States.

That marriage of old and new technology was used in making this map, which shows--with far more accuracy than previously possible over a large area--how much precipitation fell on Ventura County from 1:30 p.m. Sunday to 4 p.m. Tuesday.

The rain gauge part of the picture came from Ventura County monitoring stations. The radar data came from the National Weather Service’s Doppler radar unit on Sulphur Mountain, near Ojai.

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The radar--the location of which is marked by the center of the beam’s searchlight effect--can estimate the amount of precipitation in an area about a mile square.

“We use the radar to take exact slices of a storm in much the same way that a CAT scan takes slices of the body,” said Mike Smith, who is president of WeatherData Inc., a Kansas City company that provides weather data to The Times.

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Rain in Your Neighborhood

This map shows the cumulative amount of rain that fell on Ventura County from a storm system that hovered over the region from Sunday afternoon through Tuesday evening. The bulk of the precipitation came down on Tuesday. This information was gathered by Doppler radar unit on Sulphur Mountain that measures the amount of precipitation with square-miles area.

Source: WeatherData Inc.