仪表进近图上通常有一个20km的距离圈，此距离圈是以（）为中心的。

Clear ice is more dangerous than rime ice because it （ ） ．

Clear icing would likely be found in （ ） clouds.

Pitot tube icing affects （ ） ．

（ ） are necessary for a substantial buildup of ice on an aircraft.

（ ） is the most dangerous type of structural icing.

（ ） stage of a thunderstorm might be considered the most violent.

（ ） are associated with the mature stage of the thunderstorm.

The cumulus stage of the thunderstorm is marked by （ ） ．

（ ） cloud is associated with fast-moving cold fronts.

（ ） generally produces the most violent flying weather.