There are many factors that contribute to acne. Weather might be one of them.
Sunlight contains ultraviolet rays that damage hair follicles and induces sebaceous glands to produce excessive sebum. Hair follicles are very vulnerable to damage by external and internal stimuli.
This might explain in part why some patients’ acne is worse in the summer, when ultraviolet exposure is highest, and better in the winter when the sun’s rays are much less intense.