As a key to tunnel smoke control, the critical velocity refers to a minimum vertical ventilation velocity which can effectively control the smoke under the direction of fire source without reversing.

临界风速是隧道烟气控制的关键所在，其指隧道火灾过程中能有效控制烟气于火源下风方向而不发生逆流的最小纵向通风风速。

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Fire groups were fighting the fire on Sunday when a sudden change of wind direction caused "a huge fireball".

周日，消防队正在灭火，突然风向改变，造成“一个巨大的火球”。

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In case of fire, please follow the direction of arrow and take the nearest staircase.

如发生火警，可按箭头指示方向，选择最近的楼梯逃生。

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San Manuel Fire at the direction of the Tribe has routinely answered calls beyond the reservation during times of community and nationwide emergency.

WHITEHOUSE: Michael smith | The White House

Meanwhile, in Tunis, the government charged that Libya's rocket-firing at Tunisia is a violation of its sovereignty and puts citizens and infrastructure at risk, even though Tripoli pledged several times not to fire in the direction of Tunisia, the foreign ministry source told the Tunisian News Agency.

CNN: STORY HIGHLIGHTS

This was actually quite enjoyable, and after a few more matches I actually had the ability to spray some fire in the general direction of opposing aircraft.

FORBES: World of Warplanes: First Impressions