During thunderstorms, the positive charges collect near the upper edges of the clouds and the negative charges accumulate near the lower edges.

The given statement is true.

 During a thunderstorm, the air currents move upwards while the water droplets move downward. These vigorous movements cause the separation of charges. This leads to the positive charges collecting near the clouds' upper edges and the negative charge accumulating near the clouds' lower edges. There is an accumulation of positive charges near the ground also. Negative and positive charges meet, producing bright light and sound streaks. We see streaks as lightning.