
Thunder is a discharge phenomenon in the thundercloud. In spring and summer, affected by the warm and humid air streams from the south, most of areas in China have moist air. Intense solar radiation makes the near-surface air heated and rising and the upper cold air sinking constantly so that a strong convection is formed, therefore, there are many thunderstorms.
While in winter, controlled by the cold air mass control of the mainland, the air is cold and dry, coupled with weak solar radiation, the air is not easy to form intense convection, therefore, there are few thundershowers. But sometimes the weather is warmer in winter, the warm and moist air has a strong force, so when stronger cold air goes down to the south sometimes, the warm and moist air is forced to uplift, and the convection is intensified, there will be thunderstorms.
Formation of thunderstorms is not dependent on the temperature itself, instead, dependent on the upper and lower distribution of the temperature, which is to say that although the temperature in winter is not high, strong convection will form and therefore a thunderstorm will form when the temperature diffidence between the upper air and the lower air reaches a certain value. Thunder in the winter of China is rare, but it often occurs in the winter of Toronto, Canada.