31、根据材料,回答31-36问题。
Forecasting Methods There are several different methods that can be used to create a forecast. The methodforecaster chooses depends upon the experience of the forecaster, the amount of informationavailable to the forecaster, the level of difficulty that the forecast situation presents, and thedegree of accuracy or confidence needed in the forecast.
The first of these methods is the persistence method; the simplest way of producing aforecast. The persistence method assumes that the conditions at the time of the forecast will notchange. For example, if it is sunny and 87 degree today, the persistence method predicts that itwill be sunny and 87 degree tomorrow. If two inches of rain fell today, the persistence methodwould predict two inches of rain for tomorrow.However, if weather conditions changesignificantly from day to day, the persistence method usually breaks down and is not the bestforecasting method to use.
The trends method involves determining the speed and direction of movement for fronts,high and low pressure centers, and areas of clouds and precipitation. Using this information, theforecaster can predict where he or she expects those features to be at some future time. Forexample, if a storm system is 1,000 miles west of your location and moving to the east at 250miles per day, suing the trends method you would predict it to arrive in your area in 4 days. Thetrends method works well when systems continue to move at the same speed in the samedirection for a long period of time. If they slow down, speed up, change intensity, or changedirection, the trends forecast will probably not work as well.
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