CAMP SPRINGS, Md. — Every few minutes, rain or shine, day and night, a giant computer here runs millions of numbers through equations that meteorologists use in forecasting weather conditions around the world.
But high technology isn't always enough.
"It's always a good idea to look out the window," acknowledges Richard W. Schwerdt, glancing into a bright, sunny afternoon outside the National Weather Service's World Weather Building.
By Looking at Sky
That caution by Schwerdt, who produces forecasts for Washington, reflects the way people have forecast the weather for thousands of years--by looking at the sky.
But a good weather eye is getting a lot of help from sophisticated computers these days. The Weather Service forecasts are becoming increasingly accurate and detailed, looking further into the future.
"Forecasts of three days are now as accurate as forecasts of 1 1/2 days were a dozen years ago," says Ronald L. Lavoie of the weather service's Office of Meteorology in Silver Spring, several miles north of Schwerdt's forecast center and the computer operation. Correspondingly, the five-day forecasts are as accurate now as the two-day predictions once were.
"Our data shows a steady improvement over a 20-year period in local forecasts for 24-, 36- and 48-hour periods," adds Paul D. Polger, who is in charge of tracking the accuracy of the weather service forecasts. "The improvement is statistically significant."
'They Have Improved'
Joe D'Aleo, director of meteorology for The Weather Channel, an Atlanta-based national cable TV service, agrees. "I think they have improved, and the prospect is good for further improvement.
"Their ability to detect and communicate severe weather has improved with satellites and radar. Also, their communications system improvements allow them to more quickly process messages and then transmit forecasts and warnings."
A big gain has been in the three-to-five-day forecasts, which simply didn't exist a decade or two ago. That was just too far in the future to predict the weather with accuracy.
Now, Weather Service meteorologists routinely forecast the weather five days ahead with confidence--"with some skill," in Lavoie's words.
And, although they won't call it a forecast yet, a group headed by Donald Gilman is even using computers to generate a weather "outlook" for 30 days and 90 days in the future, although it's a pretty general picture.
Series of Plateaus
A chart showing how National Weather Service forecasts over the last few decades compare to what actually happened outdoors looks like a series of plateaus--a step up each time a new computer allows more detail to be used in calculating the current and future weather.
"If you're looking at the progression over a period of time, 30 years ago we used fairly subjective means, drawing lines on charts and moving the lines forward," Lavoie says.
The big computers came into their own in the mid-1950s and since then the improvement has been in steps. "Each more powerful computer allows a more realistic model to be run, including better physics and so forth," Lavoie says.
The starting point for any forecast is, of course: What is the weather doing now?
To find out, meteorologists in more than a hundred countries around the world--and at hundreds of places in the United States--take detailed measurements twice a day.
Greenwich Mean Time
This is done everywhere at the same time, at noon and midnight Greenwich mean time--7 a.m. and 7 p.m. EST--so that the result is a detailed picture of the weather of the whole world.
Those calculations are shared by all countries and fed into computers, which calculate what is happening.
Then, using about six basic equations for winds, temperature, humidity, precipitation and other factors, the computer calculates what will be happening in the world's weather in a few minutes and draws that worldwide picture.
From that, it calculates what the situation will be in another few minutes, and so on into the future, 10 or 15 minutes--or less--at a time.
The length of those forward steps varies, depending on the detail of the information and which weather data is being used, explains John Brown, chief of the development division at the computer operation.
Some steps move forward in 75-second increments while others don't have to be calculated as often.
'Faster Than Weather'
"It really takes a supercomputer to do this," Brown chuckles. "You have to do it faster than the weather evolves, or you aren't making a forecast."
The computer calculates these steps for six hours, prints out a weather map of the expected conditions, then goes at it again to extend the conditions to 12 hours.
"The trouble is, of course, that there are simplifications in the physics and there are features that you haven't been able to fully identify and observe, which grow and begin to make the projection incorrect," Lavoie explains.
So, he says, "tat essentially produces a limit on the usefulness of the prediction, which is currently at about six or seven days."