Reported June 2008

WHAT'S THE FORECAST: Weather forecasting is the application of science and technology to predict the state of the atmosphere for a future time and a given location. Humankind has attempted to predict the weather since ancient times. For millennia people have tried to forecast the weather. In 650 B.C., the Babylonians predicted the weather from cloud patterns. In about 340 B.C., Aristotle described weather patterns in Meteorologica. Chinese weather prediction lore extends at least as far back as 300 B.C. Ancient weather forecasting methods usually relied observed patterns of events. For example, it might be observed that if the sunset was particularly red, the following day often brought fair weather. This experience accumulated over the generations to produce weather lore. Today, weather forecasts are made by collecting data about the current state of the atmosphere and using computer models of the atmospheric processes to project how the atmosphere will evolve.

HOW DO METEOROLOGISTS PREDICT THE WEATHER? Weather forecasting is an inexact science, and many different methods are used in combination to make a prediction. The "climatology method" averages weather statistics gathered over many years for a specific region to make a prediction, but it only works when the weather pattern is similar to what is expected for the chosen time of year. Taking it to the next level, meteorologists examine a forecast and then look to a day in the past where the conditions were very similar -- and then predict that the current weather is expected to behave like that. However, tiny differences in conditions can make big differences in the actual weather. Radar systems send a signal -- usually microwaves -- towards an object or region, and then analyze how long it takes for the signal to be reflected back. Those results can reveal valuable information about the weather.

WHAT IS DOPPLER RADAR: Doppler radar uses a well-known effect of light called the Doppler shift. Just as a train whistle will sound higher as it approaches a platform and then become lower in pitch as it moves away, light emitted by a moving object is perceived to increase in frequency (a blue shift) if it is moving toward the observer; if the object is moving away from us, it will be shifted toward the red end of the spectrum. Doppler radar sends out radio waves that bounce off objects in the air, such as raindrops or snow crystals, and then measures how much the frequency changes in returning radio waves to better determine wind direction and speed.

The American Meteorological Society contributed to the information contained in the TV portion of this report.

If you would like more information, please contact:

Jeff Lazo
Project Scientist, National Center for Atmospheric Research
(303) 497-2857
lazo@ucar.edu

American Meteorological Society
Boston, MA 02108-3693
(617) 227-2425
http://www.ametsoc.org