Why is the sky in many cities green?

It may be lightning (if the weather is bad at high altitude), aurora (if it is close to the polar region), or light from buildings in surrounding cities (if it is not far from the city).

Aurora is a colorful luminous phenomenon, which appears over the high magnetic latitudes of the earth. Aurora is a brilliant and beautiful glow that appears at night in high altitude areas near the north and south poles of the earth because the charged particle flow of the sun (solar wind) enters the earth's magnetic field.

It is called the Southern Lights in Antarctica and the Northern Lights in the Arctic. Aurora is produced by the high-energy charged particle flow (solar wind) from the earth's magnetosphere or the sun exciting (or ionizing) molecules or atoms in the upper atmosphere.

Aurora often appears in the sky near the earth's magnetic pole in latitude, generally in the form of strips, arcs, curtains and radiations, and these shapes are sometimes stable and sometimes constantly changing. Aurora has three conditions: atmosphere, magnetic field and high-energy charged particles. These three are indispensable.

Aurora not only appears on the earth, but also on other planets with magnetic fields in the solar system.