A line of clouds，sharp changes in wind direction，and squalls are most frequently associated with a(n)（）.

单项选择题A hygrometer is a device used for determining（）.

A.the absolute temperature
B.atmospheric pressure
C.wind velocity
D.relative humidity

单项选择题A generally circular low pressure area is called a(n)（）.

A.cyclone
B.anticyclone
C.cold front
D.occluded front

单项选择题A frontal thunderstorm is caused by（）.

A.pronounced local heating
B.wind being pushed up a mountain
C.a warm air mass rising over a cold air mass
D.an increased lapse rate caused by advection of warm surface air

单项选择题A cyclone in its final stage of development is called a(n)（）.

A.tornado
B.anticyclone
C.occluded cyclone or occluded front
D.polar cyclone

单项选择题A cloud sequence of cirrus，cirrostratus，and altostratus clouds followed by rain usually signifies the approach of a(n)（）.

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