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)（）.
单项选择题 Brief，violent showers frequently accompanied by thunder and lightning are usually associated with（）.
单项选择题 As a cold front passes an observer，pressure（）.
