Yeah, we know it's hard to keep that sunny smile on your face when you're juggling all the variables that go into an accurate weather forecast.

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Why can't scientists accurately predict the weather?

While most scientists are revered for making sense of our complex universe (Einstein is practically a hero), meteorologists often face ridicule. How can we put a person on the moon or foretell planetary alignments years in advance, yet still fail to put together accurate weather forecasts?

First, to give credit where credit is due: Weather forecasters have improved their game significantly over the last 20 years. The three-day forecasts they deliver today are better than the one-day forecasts they delivered 20 years ago. They're also much better equipped to provide advanced warnings of severe weather, doubling the lead times for tornado warnings and giving people an extra 40 minutes to escape flash floods.

Modern meteorologists wouldn't be nearly so accurate without numerical forecasting, which uses mathematical equations to predict the weather. Such forecasting requires powerful computers and lots of observational data collected from land, sea and air. A single weather station would never be able to collect so much information. Instead, thousands of stations across the globe are linked and their data pooled. Some of these stations -- ground-based wind gauges (what meteorologists call anemometers), rain collectors and temperature sensors -- resemble those used by amateur weather watchers. Others lie far out at sea, strapped to buoys. And still others travel on commercial airliners or shipping vessels, collecting weather data as passengers and goods are moved from point A to point B. Finally, weather satellites and balloons provide information from the upper regions of the atmosphere. Satellites photograph Earth's weather from their orbit in space, while balloons monitor upper-air data over a particular location.

Collectively, all of these sensors and gauges produce more than 1 million weather-related observations every day. A normal computer -- the kind you buy at your local electronics store -- would choke on all of this data. Luckily, meteorologists can rely on supercomputers, crazy-fast machines that perform millions of calculations per second. In the United States, these computers are housed at the National Centers for Environmental Prediction (NCEP), located in Camp Springs, Md. There, weather observations stream into a supercomputer's brain, which uses complex mathematical models to predict how, based on the incoming data, weather conditions might change over time. The computer's output form the basis of almost every forecast broadcast on radio and television channels across America.

Despite our best efforts at predicting the weather, scenes like this -- near downtown Nashville, Tenn., on May 7, 2010, after the Cumberland Rive rose to its highest level in over 70 years -- remain a part of our future.

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Partly Cloudy with a Chance of Chaos

You might think that the National Centers for Environmental Prediction's supercomputers could never make mistakes, but even their abilities aren't up to the enormous challenge of weather forecasting. That's because they must take into account several large-scale phenomena, each of which is governed by multiple variables and factors. For example, they must consider how the sun will heat the Earth's surface, how air pressure differences will form winds and how water-changing phases (from ice to water or water to vapor) will affect the flow of energy. They even have to try to calculate the effects of the planet's rotation in space, which moves the Earth's surface beneath the atmosphere. Small changes in any one variable in any one of these complex calculations can profoundly affect future weather.

In the 1960s, an MIT meteorologist by the name of Edward Lorenz came up with an apt description of this problem. He called it the butterfly effect, referring to how a butterfly flapping its wings in Asia could drastically alter the weather in New York City. Today, Lorenz is known as the father of chaos theory, a set of scientific principles describing highly complex systems, such as weather systems, where small changes in initial conditions radically change the final results. Because of chaos, there is a limit to how accurate weather forecasts can be. Lorenz set this limit at two weeks.

Modern meteorologists use state-of-the-art technology and techniques to tame chaos, such as the ensemble forecast, which consists of several forecasts, each one based on slightly different starting points. If each prediction in the ensemble looks the same, then the weather is likely to "behave." If any prediction looks radically different, then the weather is more likely to "misbehave."

Meteorologists also rely on Doppler radar to monitor weather conditions more effectively and improve forecasts. Doppler radar requires a transmitter to emit radio waves into the sky. The waves strike atmospheric objects and bounce back. Clouds moving away from the transmitter return different kinds of waves than clouds moving toward the transmitter. A computer in the radar converts data about the reflected radio waves into pictures showing cloud coverage and bands of precipitation, as well as wind speeds and direction.

Because of this technology, meteorologists can now predict the weather better than ever, especially when they limit how far they look into the future. For example, up to 12 hours out, meteorologists offer fairly reliable forecasts of general conditions and trends. Unfortunately, thanks to chaos, they will never be able to predict the weather with absolute certainty, which is how surprise storms -- tornadoes and torrential, flooding rains -- continue to devastate communities with little warning. For this reason, it might be best to carry an umbrella, even on days forecasted to be bright and sunny.

Lots More Information

Related HowStuffWorks ArticlesMore Great LinksSources
  • Dye, Lee. "Can We Control the Weather? Maybe." ABC News. Aug. 3, 2005 (June 29, 2010)http://abcnews.go.com/Technology/DyeHard/story?id=1001079&page=1
  • Monastersky, Richard. "Forecasting is No Picnic." Scientific American Presents Weather: What We Can and Can't Do About It. Spring 2000.
  • Rosenfeld, Jeffrey. "The Butterfly That Roared." Scientific American Presents Weather: What We Can and Can't Do About It. Spring 2000.
  • Williams, Jack. The Weather Book. Vintage Books, 1997.