民航各类飞行的依据是（）。

The three stages of a thunderstorm are （ ） ．

（ ） is not included in the three conditions necessary for thunderstorm formation.

An area of severe weather often forms well （ ） the fast-moving cold front.

（ ） would decrease the stability of an air mass.

When the temperature rises, first of all,

Airmasses are classified according to the regions where they originate. They are generally divided into polar or tropical to identify （ ） characteristics.

Which of the following is not a general weather characteristic associated with cold fronts?

When flight speed is constant, the greater the air density,

A warm front is one where （ ） ．

A cold front is one where （ ） ．