A.each time the chart is changedB.once per monthC.once ……

单项选择题If an observer in the Northern Hemisphere faces the surface wind，the center of low pressure is to his（）.

A.left，slightly behind him
B.right，slightly behind him
C.left，slightly in front of him
D.right，slightly in front of him

单项选择题Cyclones tend to move（）.

A.perpendicular to the isobars in their warm sectors
B.parallel to the isobars in their warm sectors
C.parallel to the line of the cold front
D.perpendicular to the line of the cold front

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