导读：Emily 前几天经历了一场龙卷风，所以今天我们就来聊聊极端恶劣天气-龙卷风的形成，以及应对龙卷风的措施。红色字体标注的为相关术语。

Transcript:

A popular American documentary Television series called ‘storm chasers’ was premiered on October 17, 2007 on the Discovery Channel. It showed how the storm chasers pursue tornadoes in Tornado Alley in the United States.

If you barely know his program. Last year, there was another Chinese celebrity reality show named ‘Life’s Encounter’ which received a score of 8.9 out of 10 on Douban. In Series 1, Episode 2, the host Aya and her guest took an adventure with storm chasers. This unique experience triggered their thinking about life and marriage.

When I saw this Episode, I didn’t realize that the tornado will come through my living area someday.

However, it just did.

The unpredicted tornado ripped roofs off buildings in the community and destroyed power lines. The amount of devastation I saw pushed me to conduct a research about it.

What is a tornado?

A tornado is a tube of violently spinning air that touches the ground. Wind inside the tornado spins fast, but the actual ‘circle’ of wind around them is huge. This makes tornadoes very dangerous.^[1]

Tornadoes can occur in countries such as India, Brazil, and Italy. But nearly three-quarters of tornadoes around the world occur within the U.S. due to its unique topography(地形).

Why are tornadoes common in the U.S.?

Tornado Alley

Tornado Alley is a colloquial term for the area of the United States where tornadoes are most frequent.^[2]

The tornado alley（龙卷风走廊） is a media-driven term, its boundaries can be different based on different criteria. But we know the tornadoes most likely happen in the areas between the Rocky Mountains（落基山脉） and the Appalachian Mountains（阿巴拉契亚山脉）. Please remember no state is entirely free of tornadoes.^[3]

How do tornadoes form?

To make a long story short, most tornadoes form from a thunderstorm.

As we can see from the picture above, cool dry air from the Rockies and warm moist air of the Gulf of Mexico are the basic ingredients for a thunderstorm（雷暴天气）.

when the two masses meet, the atmosphere is becoming unstable.

Warm, less heavy air masses develop into Cumulus clouds and continue to grow vertically. Then the clouds will go on to form a dense cumulonimbus cloud（积雨云）.

In this circumstance where wind direction and speed varies at high altitudes, the air will swirl（旋转） horizontally like a cylinder.

Warm updraughts（上升气流） tilt the cylinder vertically.

The thunderstorm with deep rotating updrafts is known as ‘supercell（超级单体）’.

Falling downdraughts of cold air within supercell help the cylinder stretch toward the ground. When it reaches the ground, it becomes a tornado.

How is tornado strength rated?

If you happen to see a tornado report, you will notice each tornado has a rating scale from EF0 to EF5.

The EF Scale is the standard way to measure tornadoes based on wind damage. ^[4]

Tornado safety

Tornadoes can cause massive damage, especially for mobile home and car. The high-speed wind can toss them through the air without effort. Therefore if you are in a car or mobile home during a tornado, what you should do is to get out of the car or mobile home and take shelter in near building.

If you are inside a building, go to the low level of the building, find an inner room or hallway without windows and get under a piece of sturdy furniture if it is possible.

习语时间

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cut across

英文释义：Go beyond, transcend
中文释义：超越，抄近路穿过
例子：

cut a deal

英文释义：Offer or arrange an agreement or compromise
中文释义：达成协议
例子：

知识点：cut a deal with…与…达成协议。如果你平时看一些外刊，会发现 enter a deal with 也是一个意思。

cut adrift

英文释义：Separated or detached; freed.
中文释义：使分开，独立谋生
例子：

cut and dried

英文释义：Ready-made, predetermined and not changeable.
中文释义：老生常谈
例子：

cut and paste

英文释义：Describing a patched-up job or trivial work. 
中文释义：剪和贴
例子：

cut and run

英文释义：Clear out, escape, desert, as in He wished he could just cut and run
中文释义：落荒而逃，慌忙逃走
例子：

cut a wide swath

英文释义：Draw a lot of attention, make a considerable display
中文释义：炫耀，夸耀，自以为了不起
例子：he was new to the company, he cut a wide swath
知识点：cut a wide swath through our town.招摇过市。对比一下，中文还是言简意赅。

cut back

英文释义： Reduce, decrease
中文释义：减少，缩减。
例子：

知识点：cut back on social media。这个短语还可以当修剪枝叶讲，比如：It is time to cut back on these trees.

cut both ways

英文释义：Have a mixed effect, have advantages and disadvantages. 
中文释义：各有利弊，双刃剑
例子：

cut capers

英文释义： Frolic or romp
中文释义：嬉戏
例子：The children cut capers in the pile of raked leaves.
Emily 解说：raked leaves 落叶。capers 是一种西餐里的材料，叫酸豆。下边这张图可以帮助我们记忆。