外语学习-day2

英语

Fire, then ice

Wild weather spans the world

• ON SEPTEMBER 7TH in Denver, Colorado, the temperature reached 34°C，6°C
  above what is normal for the time of year. The city was sitting under the dome of hot air encouraging record fires across the American West. The next day snow started to fall. By midnight the temperature was below freezing. What happened?

ecouraging：推动，造成

• The immediate cause was the polar jet stream(极地喷射气流), a world-girdling(环球) high-altitude(高空) wind driven by temperature differences between Arctic air to the north and warmer air to the south. Its meandering(蜿蜒) path is set by patterns of high and low pressure known as Rossby waves(罗斯贝波). And because fluid dynamic are never simple, the jet stream exerts its own influence in trun(气流反过来影响) upon these guiding waves(这些导向型的罗斯贝波).

• As the jet stream passed to the north of the high pressure over America's west coast, the big temperature difference between that hot air and what lay farther to the north added to its energy. This extra impetus meant that as the jet came down the east side of the high-pressure zone it curved back on itself towards the west. In meteorogical terms, the Rossby wave broke. In this case the breaking waves's white water took the form of large quantities of cold air from Canadian north that were suddenly pulled south, and which crashed down on Colorado.