A.perpendicular to the isobars in their warm sectorsB.p……

单项选择题Cumulonimbus clouds can produce（）.

A.dense fog and high humidity
B.gusty winds，thunder，rain or hail，and lightning
C.clear skies with the approach of a cold front
D.a rapid drop in barometric pressure followed by darkness

单项选择题Cumulonimbus clouds are most likely to accompany a(n)（）.

A.high pressure system
B.cold front
C.warm front
D.occluded front

单项选择题Clouds that form as small white flakes or scaly globular masses covering either small or large portions of the sky are（）.

A.cirrus
B.cirrostratus
C.altostratus
D.cirrocumulus

单项选择题Clouds are classified according to their（）.

A.size
B.moisture content
C.altitude and how they were formed
D.location in a front

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