Pea souper

Not really one per the actual definition below.
But it still was pretty thick this morning.

Data from Wikipedia.

Pea soup, or a pea souper, also known as a black fog or killer fog, is a very thick and often yellowish, greenish, or blackish smog, caused by air pollution that contains soot particulates and the poisonous gas sulfur dioxide. These very thick smogs occur in cities and are derived from the smoke given off by the burning of soft coal for home heating and in industrial processes. Smog of this intensity is often lethal to vulnerable people such as the elderly, the very young and those with respiratory problems.

Such fogs were prevalent in UK cities, especially London where the smoke from millions of chimneys combined with the mists and fogs of the Thames valley. The result was commonly known as a London particular or London fog, which then, in a reversal of the idiom, became the name for a thick pea and ham soup.