How far in advance should I check the weather forecast?

You can blame your localweatherforecast all you want, write angry letters to meteorologists and even take matters into your own hand with a little homespunmeteorology. In the end, however, the fault lies not with your local weather anchor, but with the rich complexity behind every breeze,cloudor sprinkle that takes place inEarth'satmosphere.

It's easy to get lost staring at the swirling currents in a Jacuzzi -- and not just if you've been in there for 30 minutes. If left unused and empty, you have nothing but a tub full of hotwater. But what happens when you turn on the jets or adjust the thermostat? What happens when, a few margaritas to the wind, you and your friends finally tumble in for a dip?

The Jacuzzi jets churn the waters to life, the rise in temperature creates more steam and the bevy of bodies force new currents in the chlorine-rich water. Various competing and random causes create an entire system of effects. Now imagine a Jacuzzi filled with roughly 5.5-quadrillion-tons (4.99-quadrillion-metric-tons) of gas and stirred to life by such factors as gravity, solar radiation, terrestrial radiation, temperature, pressure, topography and human civilization. You're lounging in Earth's atmosphere, a vast, gaseous body so complicated that even our most powerfulcomputerscan't always anticipate it more than a few moves into the future.

The unruly atmosphere, however, does adhere to definite physical laws and follows established patterns, which we callclimate. Meteorologists predict the weather by examining past patterns and comparing them to observable atmospheric phenomena to create a model of the future. In this respect, it's like owning a pet. You know how Fido normally behaves given various conditions. As such, you can gauge current or upcoming conditions and make a decent guess as to how he'll behave.

The weather, however, is far more complicated and temperamental.

In anticipating tomorrow'sweathertoday, the simplest tool at ameteorologist'sdisposal ispersistence forecasting. It may sound ridiculous in an age of Dopplerradar, but this method essentially breaks down to, "Whatever the weather is doing today, it will continue to do tomorrow." This method isn't perfect, but it serves as a dependable groundwork for forecasting.

Meteorologists corroborate persistence forecasting withsynoptic forecasting, which applies our knowledge of atmospheric laws, andstatistical forecasting, which factors in records of past patterns. Finally,computer forecastingcalculates all this data to produce models of what the future may hold.Numerical weather prediction modelingapplies synoptic forecasting to current conditions, whilemodel output statisticsfigures in statistical forecasting.

有了这项技术,我们可以建立一个模型tomorrow's weather may look like. Yet the sheer number of variables renders even a short-term prediction fallible. One tiny miscalculation and the whole model can change drastically -- a fact you may know as thebutterfly effect. When you look an additional day into the future, the model becomes even more uncertain. This is because each subsequent weather model is generated from the preceding weather model.

Computermodels are based typically on one-hour increments: one hour into the future, two hours into the future, three hours into the future -- each based on the one before it. Each model can introduce new errors and compound errors in the previous model. As such, the further into the future a computer model predicts, the more room for error there is.

Meteorology technology continues to improve, but for the foreseeable future, the weather forecast will be more accurate the closer you are to the day in question. The 24-hour forecast is more dependable than the three-day forecast, which is in turn more dependable than the 10-day forecast. However, don't be afraid to employ a little persistence forecasting yourself. If your area typically experiences heavy rains in April, then guess what month you shouldn't schedule the next church picnic for?

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