第四十三篇
Forecasting Methods
There are several different methods that can be used to create a forecast.The method a forecaster chooses depends upon the experience of the forecaster, the amount of information available to the forecaster, the level of difficulty that the forecast situation presents, and the degree of accuracy or confidence（可信度） needed in the forecast.
1.What factor is NOT mentioned in choosing a forecasting method?
A.Imagination of the forecaster
B.Necessary amount of information
C.Practical knowledge of the forecaster
D.Degree of difficulty involved in forecasting
The first of these methods is the persistence¹ method; the simplest way of producing a forecast.The persistence method assumes that the conditions at the time of the forecast will not change.For example, if it is sunny and 87 degree today, the persistence method predicts that it will be sunny and 87 degree tomorrow.If two inches rain fell today, the persistence method would predict two inches of rain for tomorrow.However, if weather conditions change significantly for day to day, the persistence method usually breaks down and is not the best forecasting method to use.
2.Persistence method will work well
A.if weather conditions change greatly from day to day.
B.if weather conditions do not change much.
C.on sunny days.
D.on rainy days.
The trends method involves determining the speed and direction of movement for fronts（锋）, high and low pressure center, and areas of clouds and precipitation（降水）.Using this information, the forecaster can predict where he or she expects those features to be at some future time.For example, if a storm system（天气特征） is 1,000 miles west of your location and moving to the east at 250 miles per day, using the trends method you would predict it arrive in your area in 4 days.The trends method works well when systems continue to move at the same speed in the same direction for a long period of time.If they slow down, speed up, change intensity², or change direction, the trends forecast will probably not works as well.
3.The limitation of the trends method is the same as the persistence method in that
A.it makes predictions about weather.
B.it makes predictions about precipitation.
C.the weather features need to be well defined.
D.the weather features need to be constant for a long period of time.
The climatology method is another simple way of producing a forecast.This method involves averaging weather statistics accumulated over many years to make the forecast.For example, if you were using the climatology method to predict the weather for New York City on July 4th, you would go through all the weather data that has been recorded for every July 4th and take an average.（The climatology method works well when the weather pattern is similar to that expected for the chosen time of year.If the pattern is quite unusual for the given time of year, the climatology method will often fail.） （这一部分试题中没有提问，因此可以不读，接着做下面的题。）
4.Which method may involve historical data?
A.The trends method
B.The analog³ method
C.Both climatology method and analog method
D.The trends method and the persistence method.
The analog（相似物） method is slightly more complicated method of producing a forecast.It involves examining today's forecast scenario⁴ and remembering a day in the past when the weather scenario looked very similar （an analog）.The forecaster would predict that the weather in this forecast will behave the same as it did in the past.The analog method is difficult to use because it is virtually （=actually） impossible to find a predict analog.Various weather features rarely align⁵ （结盟） themselves in the same locations as they were in the previous time.Even small differences between the current time and the analog can lead to very different results.
5.It will be impossible to make weather forecast using the analog method
A.when the current weather scenario differs from the analog method.
B.when the current weather scenario is the same as the analog method.
C.when the analog is over ten years old.
D.when the analog is a simple repetition of the current weather scenario.

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