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Was hoping to change the jpeg to illustrate the point. Ahh well :-(
When we can't see very far through the fog or mist, we say the fog/mist is like pea soup.
Historical speaking, this saying only goes back about 100 years & it's real use, faded in the 1950's. London & a few other English cities had a terrible problem with 'smog'.
Some definitions:
Vapour – water in your kettle or saucepan boils. The steam rises & is tiny droplets of water, becoming almost like a gas. This is vapour. Tiny droplets of water that are in the last stage of being a liquid, before become a gas. Not necessarily rising. From the kettle, steam rises because of the hot air carrying it up from the heated kettle/pan.
Dew – as the sun goes down, the water content in the atmosphere gets colder & settles on the cold exposed surfaces of plants, trees, houses, cars – everything not moving much. The atmospheric vapour condenses.
Condenses – think of the water droplets on your tiled walls in the bathroom, after a hot shower or bath. The hot water vapour has become colder because of the colder surfaces of the tiles & mirror too. Condensation. Also, your breathe condenses as it leaves your hot body & hits the cold air.
Evaporate – back to your kettle. Evaporation occurs as liquid turns into vapour.
Fog – naturally occurring 'banks of fog' = dense cloud of tiny water droplets that is kept near the ground due to a wet atmosphere, no wind & the sun heating up the dew from a previously cold night. The dew starts to evaporate with the heat of the morning sun, rises, but condenses again into fine air-bound droplets & forms this cloud, but is more earth-bound than high altitude clouds.
Mist – forms over water. It mainly occurs with a combination of suitable factors. As water runs in rivers & streams, the heating up of that surface water is mixed with flowing colder water underneath. Mist will not form over moving water, but water in a river does become still with the changing of the tides. This is when the similar fog process occurs.
Smog – was similar to fog & mist. London has a very wide river called 'The Thames' & flows from outside of London, passes through London & then continues to the South East coast of England. Smog is fog/mist that collects & carries the pollution within the cloud.
The particular pollution of that era, was from coal. Coal was used widely to generate electricity & also for heating & cooking in homes. Domestic chimneys would pump out the black & grey smoke & as houses then were not higher than 2 stories, the resulting smoke did not travel very far, but hung over the city.
Coal is not widely used now, with the advent of nuclear power, electric heaters & central heating. Natural gas was introduced for cooking & heating of water for washing.
Another phrase associated with this era, was "not being able to see 5 yards in front of your face". A yard is a measurement of distance. Equal to 3 feet (0.9144 metres). Therefore 5 yards was 15 feet or 4.572m.
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