The fog of a heather pipe refers to the mist exhaled from a pipe made of heather.

This is a lyric from the song "Chapter Seven of the Night" sung by Jay Chou and Pan Er. The "briar pipe" in this lyric comes from "Charlotte".

The relevant lyrics are: The typewriter continues to push, the next line that is close to the truth, the mist of the heather pipe, drifts towards the withered tree, silently crying to me.

In Sherlock Holmes, Watson often uses his pipe when he is arguing about issues rather than thinking about them. Heather (briar root) pipes appear in cases of men with crooked lips. As for the case of the man with crooked lips, the key word is: disguise.

The following is a description of Sherlock Holmes's pipe. The clay pipe was used by Sherlock Holmes when he was thinking about problems.

Holmes was silent for a few minutes, still holding the tips of his fingers, his legs stretched forward, and his eyes staring upward at the ceiling. Then he took down from the shelf the old, greasy clay pipe that seemed to him like a counselor. After lighting the tobacco, he leaned back on his chair, the thick blue smoke lingering, and his face showed an expression of infinite contemplation.

In other words, the fog here refers to blue smoke. Photochemical pollution, also known as photochemical smog, is a light blue smog. As long as it is a clear day, there will be a light blue smog that fills the sky over the city. In other words, clear weather is a necessary condition for the formation of smoke.