Why might Conan Doyle want to create this type of atmosphere?

t was a September evening, and not yet seven o'clock, but the day had been a dreary
one, and a dense drizzly fog lay low upon the great city. Mud-coloured clouds drooped
sadly over the muddy streets. Down the Strand the lamps were but misty splotches of
diffused light which threw a feeble circular glimmer upon the slimy pavement. The
yellow glare from the shop windows streamed out into the steamy, vaporous air, and
threw a murky, shifting radiance across the crowded thoroughfare. There was, to my
mind, something eerie and ghost-like in the endless procession of faces which flitted
across these narrow bars of light,—sad faces and glad, haggard and merry. Like all human
kind, they flitted from the gloom into the light, and so back into the gloom once more. I
am not subject to impressions, but the dull, heavy evening, with the strange business
upon which we were engaged, combined to make me nervous and depressed. I could see
from Miss Morstan's manner that she was suffering from the same feeling. Holmes alone
could rise superior to petty influences. He held his open note-book upon his knee, and

from time to time he jotted down figures and memoranda in the light of his pocket-lantern.

We had, indeed, reached a questionable and forbidding neighbourhood.
Long lines of dull brick houses were only relieved by the coarse glare and tawdry (flashy,
cheap) brilliancy of public houses at the corner. Then came rows of two-storied villas
each with a fronting of miniature garden, and then again interminable (seemingly
endless) lines of new staring brick buildings, -- the monster tentacles which the giant
city was throwing out into the country.