Clouds are classified according to their（）.

单项选择题Clouds appearing in the following order: cirrus，cirrostratus，altostratus，stratus，and nimbostratus usually indicate the approach of a(n)（）.

A.warm front
B.occluded front
C.medium front
D.cold front

单项选择题Cloud formations are minimal when the（）.

A.surface temperature and temperature aloft are equal
B.surface temperature and temperature aloft differ greatly
C.barometric pressure is very low
D.relative humidity is very high

单项选择题Cirrus clouds followed by cirrostratus then altostratus，stratus，and occasionally nimbostratus indicate the approach of a(n)（）.

A.cold front
B.warm front
C.tropical front
D.occluded front

单项选择题Brief，violent showers frequently accompanied by thunder and lightning are usually associated with（）.

A.passage of a warm front
B.passage of a cold front
C.winds shifting counterclockwise in the Northern Hemisphere
D.stationary high pressure systems

单项选择题As a cold front passes an observer，pressure（）.

A.drops and winds become variable
B.rises and winds become gusty
C.drops and winds become gusty
D.rises and winds become variable