Photo: Trees at sunset

Photograph by John Eastcott and Yva Momatiuk

Weather is the state of the atmosphere at a specific time and place, with respect to temperature, precipitation, and other factors such as cloudiness. Weather is generated by many forces, some obvious, some not. Warm, humid air masses blowing in from oceans, for example, fuel rains. Sunlight heats the land, generating thermals that help produce summer thunderstorms.

Mountains and cities also affect the weather. In mountains this occurs because the wind must rise as it crosses over the ridge. This lifts the air, causing it to cool. That produces clouds, rain, or snow.

Cities, on the other hand, produce urban "heat islands" where roads, parking lots, and rooftops warm in the sun. This not only raises the city's temperature, but it can affect the weather, producing thunderstorms in some cities or altering storm tracks in others.

Predicting the Weather

Weather forecasting is the art of predicting what will happen in the future. In its simplest form, it's merely a matter of looking out the window to see what types of clouds are around and which way they are moving. Knowledge of local weather patterns can then allow fairly good predictions for the next 12 to 24 hours.

Professional forecasters have a wide variety of other tools. Weather stations scattered around the globe allow them to make detailed weather maps, as do satellites, which allow forecasters to see what is happening far out to sea, where there are no weather stations. Weather balloons and radar also contribute.

Nevertheless, long-run weather forecasting is notoriously difficult. That's because weather prediction involves a mathematical concept called chaos theory, in which extremely small errors in measuring today's weather conditions can snowball into large, seemingly random, errors in long-range forecasts.

It has been said, for example, that a butterfly flapping its wings today in China could produce (or prevent) tornadoes two weeks from now in Kansas. While this so-called butterfly effect is undoubtedly overstated, the basic concept is simple: Even the most minor factors can alter long-term weather forecasts.

Most weather forecasters believe that accurate forecasting more than two weeks into the future will always be impossible. Today, anything beyond five to seven days involves substantial guesswork and is often wrong.

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