前程百利小编为考生带来托福阅读背景之雷雨的介绍。雷阵雨(Thundershowers)是一种天气现象，表现为大规模的云层运动，比阵雨要剧烈的多，还伴有放电现象，常见于夏季。希望大家可以从中理解并掌握一些专业词汇，同时学习相关背景知识。
Despite their dramatic behavior, thunderstorms are the smallest of the different kinds of storms. They average around 16 km (10 mi) across, but they often occur in lines that can be a few hundred miles long or clusters a couple hundred miles across. Thunderstorms are often part of larger storms such as hurricanes. While thunderstorms occur most commonly in warm weather, they can happen in the winter. Sometimes snow will fall from a thunderstorm instead of rain; this is called a “thunder snow.”
Every thunderstorm includes lightning because lightning causes thunder. Lightning creates a danger in any thunderstorm, no matter how large or small. Many thunderstorms drench the countryside with heavy rain, so you also have to worry about flash floods—that is, floods that rise very quickly, sometimes running over the banks of normally small streams. If a thunderstorm is strong enough, it can also create hail, or balls of ice that fall from the storm. Hailstones are usually less than 1.3 cm (0.5 in) across. Occasionally hailstones as big as softballs fall during storms, posing considerable danger because such large stones fall faster than 160 km/h (100 mph).
Some especially dangerous thunderstorms create winds called downbursts, which descend from the clouds and hit the ground going faster than 160 km/h (100 mph). Downbursts are sometimes concentrated in small areas less than 4 km (2.5 mi) across called microbursts. Thunderstorms also sometimes create tornadoes in the warm air that’s rising from the ground. Unlike microbursts, tornado winds spin in a circle as the air rises. Tornado winds can blow faster than 400 km/h (250 mph), but twisters this strong are rare.
以上就是前程百利小编为大家带来的托福阅读背景，平时的阅读练习是托福考试阅读备考的重要一环。希望大家利用好每份材料，尽可能多地从中获取知识。预祝大家托福考试取得好成绩!
