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

　　答案与题解：

　　1. A第一段第二句说明了选择预报方法应考虑的一些因素，其中包括 B、C和 D中提到的所能获得的信息、预报者的实际经验和特定天气状况给预报造成的困难程度。 A是正确的选项，因为文中未提到天气预报员的想象力。

　　2. B 答案可在第二段第二句中直接找到，其后是具体例子，用于说明 persistence method只有在天气状况基本不变的情况下才能有效使用。

　　3. D 第三段昀后两句提供了答案。另外，该题的理解还须结合对第二段的理解。

　　4. C 第四段的第二句告诉我们， the climatology method需要取多年积累起来的气象数据的平均值。第三句提供了具体例子。第五段的第二句和第三句说明， analog method也需要比较和对比历史上某一天的气象状况。

　　5. A昀后一段的第五句和第六句提供了答案。

　　译文：

天气预报的方法

　　天气预报可用几种不同的方式来制作。一个天气预报员采用什么方法来制作天气预报是由其经验决定的，预报员可利用的信息的数量，预报情况的困难程度，预报中需要的准确度和置信度。

　　每一种方法是持续性的方法。这是制作天气预报最简单的方法。持续性方法假设在天气预报时，条件是不会发生变化的。例如，如果今天是晴，87度，持续性方法会预测明天也会是晴，87度。如果今天的降雨量是2英寸，持续性方法会预测明天也会是2英寸。然而，如果天气状况一天天地显著变化，持续性方法常会出错，所以这不是制作天气预报的最佳方法。

　　趋势性方法包括测定锋运动的速度和方向，高、低压的中心和多云、降水地区。利用这些信息，预报员能预测将来的某个时间什么地区会出现同样特征的天气状况。例如，如果一个风暴在你居住地以西100英里且以每天250英里的速度向东移近。运用趋势性方法，你能预测这个风暴在4天以后将到达你所在的地区。当某一大气现象长时期以同一速度向同一方向运动时，趋势性方法很有效。如果它们减速、加速变化或方向转变，这种制作天气预报的方法可能会不准确。

　　气候学方法是另一种制作天气预报的简单方法。这种方法通过计算多年来积累的天气状况值的平均数来制作天气预报。例如，如果你用气候学方法来预测7月4日纽约的天气状况，你会浏览每年7月4日纽约的天气数据记录，然后计算其平均值。气候学方法只有当天气状况与所选时间的预期天气状况相似时才有效，若天气状况和所取时间的天气状况非常不同，运用气候学方法常会失败。

　　相似物方法是有点儿复杂的制作天气预报的方法。它包括测量今天的天气状况和回想过去和今天天气状况相似的一天，预报员会预测现在的天气将发生和过去那天同样的变化。相似物方法很难被应用，因为事实上不可能找到所期望的同样的天气状况。各种不同的天气特征极少同时出现在与前次出现时一样的地点。即使现在的天气状况与其过去的相似物有一点小差别都能造成不同的结果。

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