Forecasting Methods

　　There are several different methods that can be used to create a forecast. The method 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 needed in the forecast.

　　The first of these methods is the persistence 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 of rain fell today, the persistence method would predict two inches of rain for tomorrow. However, if weather conditions change significantly from day to day, the persistence method usually breaks down and is not the best forecasting 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, the forecaster can predict where he or she expects those features to be at some future time. For example, if a storm system is 1,000 miles west of your location and moving to the east at 250 miles per day, suing the trends method you would predict it to 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 intensity, or change direction, the trends forecast will probably not work as well.

　　The climatology method is another simple way of producing a forecast. This method involves averaging weather statistics accumulated over man 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 only 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.

　　The analog method is a slightly more complicated method of producing a forecast. It involves examining today's forecast scenario 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 impossible to find a predict analog. Various weather features rarely align themselves in the same locations they were in the previous time. Even small differences between the current time and the analog can lead to very different results.

　　31 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.

　　32 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

　　33 The limitation of the trends method is the same as the persistence method in that __________.

　　A it makes predications about weather

　　B it makes predications 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

　　34 Which method may involve historical weather data? __________

　　A The trends method.

　　B The analog method.

　　C Both climatology method and analog method.

　　D The trends method and the persistence method.

　　35 It will be impossible to make weather forecast using the analog method __________.

　　A when the current weather scenario differs from the analog

　　B when the current weather scenario is the same as the analog

　　C when the analog is over ten years old

　　D when the analog is a simple repetition of the current weather scenario

　　答案與題解：

　　1. A第一段第二句說明了選擇預報方法應考慮的一些因素，其中包括 B、C和 D中提到的所能獲得的信息、預報者的實際經驗和特定天氣狀況給預報造成的困難程度。 A是正確的選項，因為文中未提到天氣預報員的想象力。

　　2. B 答案可在第二段第二句中直接找到，其后是具體例子，用于說明 persistence method只有在天氣狀況基本不變的情況下才能有效使用。

　　3. D 第三段昀后兩句提供了答案。另外，該題的理解還須結合對第二段的理解。

　　4. C 第四段的第二句告訴我們， the climatology method需要取多年積累起來的氣象數據的平均值。第三句提供了具體例子。第五段的第二句和第三句說明， analog method也需要比較和對比歷史上某一天的氣象狀況。

　　5. A昀后一段的第五句和第六句提供了答案。

　　譯文：

天氣預報的方法

　　天氣預報可用幾種不同的方式來制作。一個天氣預報員采用什么方法來制作天氣預報是由其經驗決定的，預報員可利用的信息的數量，預報情況的困難程度，預報中需要的準確度和置信度。

　　每一種方法是持續性的方法。這是制作天氣預報最簡單的方法。持續性方法假設在天氣預報時，條件是不會發生變化的。例如，如果今天是晴，87度，持續性方法會預測明天也會是晴，87度。如果今天的降雨量是2英寸，持續性方法會預測明天也會是2英寸。然而，如果天氣狀況一天天地顯著變化，持續性方法常會出錯，所以這不是制作天氣預報的最佳方法。

　　趨勢性方法包括測定鋒運動的速度和方向，高、低壓的中心和多云、降水地區。利用這些信息，預報員能預測將來的某個時間什么地區會出現同樣特征的天氣狀況。例如，如果一個風暴在你居住地以西100英里且以每天250英里的速度向東移近。運用趨勢性方法，你能預測這個風暴在4天以后將到達你所在的地區。當某一大氣現象長時期以同一速度向同一方向運動時，趨勢性方法很有效。如果它們減速、加速變化或方向轉變，這種制作天氣預報的方法可能會不準確。

　　氣候學方法是另一種制作天氣預報的簡單方法。這種方法通過計算多年來積累的天氣狀況值的平均數來制作天氣預報。例如，如果你用氣候學方法來預測7月4日紐約的天氣狀況，你會瀏覽每年7月4日紐約的天氣數據記錄，然后計算其平均值。氣候學方法只有當天氣狀況與所選時間的預期天氣狀況相似時才有效，若天氣狀況和所取時間的天氣狀況非常不同，運用氣候學方法常會失敗。

　　相似物方法是有點兒復雜的制作天氣預報的方法。它包括測量今天的天氣狀況和回想過去和今天天氣狀況相似的一天，預報員會預測現在的天氣將發生和過去那天同樣的變化。相似物方法很難被應用，因為事實上不可能找到所期望的同樣的天氣狀況。各種不同的天氣特征極少同時出現在與前次出現時一樣的地點。即使現在的天氣狀況與其過去的相似物有一點小差別都能造成不同的結果。

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