As you may know, pea soup is a thick, dense soup; ideal for a cold winter's day.
At this time of year, in the northern hemisphere, the combination of cold air and high pressure with little wind can create fog.
Sometimes, if there is no wind for days, it can form into dense, feezing fog - and that we can call pea soup.
In the early 1950s, London was famous for winter fog and none more so than the devastating December of 1952. For five days, a combination of fog and pollution from coal fires, shrouded the city in smog. (Fog = pollutants)
It's estimated that anywhere between 4,000 and 12,000 died of respitory illnesses. The good news is that it lead to The Clean Air Act and a much greater level of environmental awareness.
Here's the start of a fascinating documentary about it.
Watch it - this really is a pea soup!