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Observations
Thu May 13, 2004
Listen in RealAudio 
When you see or hear your local forecaster, did you ever wonder how s/he knows what
the weather is really going to be like tomorrow? Well, it all starts with observations. Hi,
I'm Bryan Yeaton for The Weather Notebook.
Every hour, at hundreds of sites across the nation, the weather is noted. It used to be
done by actual people eyeballing the thermometers, but nowadays, many locations are
monitored by an Automated Surface Observing System. In addition to temperature, they
look at the other flavors of weather: humidity, pressure, wind, cloud cover, precipitation.
When you add all these readings together, you get a pretty good snapshot of what the
weather is doing right now. But in order to figure out what it is going to do, we need to
see the movie, so we put together several hours of pictures, and we can tell how all the
complex components of the atmosphere are interacting.
Then, some of the most powerful computers on the planet use that information to
decide what will probably happen tomorrow, and the next few days.
So, how accurate are the forecasts? It depends on who is interpreting the data, but in
general, the one to two-day forecasts are about 90 percent accurate. Yes, you heard
me correctly. After five days, the rate drops closer to 50 percent, although some would
argue on that point. After five days... well, lets' just say, that it is a toss up between the
computers and throwing darts.
The most vital part of a forecast, though, is still the most basic: the observation.
The Weather Notebook is produced with funding from the National Science Foundation
and Subaru of America.
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