Why is the street light slowly lit up?

Street lamps are mostly low-pressure mercury lamps
Low pressure mercury lamp is the use of low pressure mercury vapor in the discharge process to radiate ultraviolet light, so that the phosphor emitted visible light principle

There are two filaments in a fluorescent lamp. The filament is coated with the electron emission materials terylene carbonate (barium carbonate, strontium carbonate and calcium carbonate), commonly known as electron powder.

Under AC voltage, the filament alternately acts as the cathode and anode.
The inner wall of the lamp is coated with phosphor. The tube is filled with argon gas at 400Pa to 500Pa pressure and a small amount of mercury.

After electricity is applied, the liquid mercury evaporates into mercury vapor with a pressure of 0.8 Pa. Under the action of electric field, mercury atoms are continuously excited from the original state into an excited state, and then spontaneously transition to the ground state, and radiate ultraviolet light with a wavelength of 253.7nm and 185nm (the main peak wavelength is 253.7nm, accounting for about 70-80% of the total radiant energy; The secondary peak wavelength is 185nm, about 10% of the total radiant energy) to release excess energy. The phosphor absorbs ultraviolet radiation and emits visible light.
That's why the street lights slowly lit up