A "tornado outbreak" is often defined as six or more tornadoes produced by the same weather system within a day.

VOA: special.2010.05.11

It's just minus the sum of all these things, because of course these are just defined as the work done by the environment to the system.

等于负的这些东西的和，因为它们的定义是,环境对系统所做的功。

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