Weather processes such as wind, frost, rain and snow occur in () A. Troposphere B. Stratosphere C. Upper atmosphere D. Ozone.

A, the troposphere temperature decreases with the increase of height, the air convection movement is remarkable, and the weather phenomenon is complex and changeable, so it is correct;

B, stratospheric temperature increases with altitude, the atmosphere is dominated by advection, and the weather is clear, which does not meet the meaning of the question;

C, the air density is very small, the air pressure is very low, and there is an ionosphere at high altitude, which is conducive to radio communication and does not meet the meaning of the question;

D, the ozone layer is located in the stratosphere, so it does not meet the question.

So choose: a.