那些试飞飞机并未遇到整片天空火山灰浓度高于每立方米400毫克，即本底水准20倍的区域。

Those flights have encountered no patches of sky with an ash density of more than 400 micrograms per cubic metre, 20 times the background level.

youdao

那些试飞飞机并未遇到整片天空火山灰浓度高于每立方米400毫克，即本底水准20倍的区域。

Those flights have encountered no patches of sky with an ash density of more than 400 micrograms per cubic metre, 20 times the background level.

youdao