Last week we were looking at clouds and you may have been puzzling over how to tell your cirro- from your alto- as it is difficult to judge height. This week is easier as we are looking at clouds at ground level.
Or is it? What is the difference between mist and fog? And what is smog?
Both mist and fog are basically cloud at ground level. The difference between them is how thick the cloud is: how far you can see through it. The boundary is usually taken as 1 km. So if you can’t see something 1 km away, then you are in fog.
If you can see through the cloudiness for more than 1 km, then it’s mist. So you aren’t judging height now but distance, which tends to be easier as there are more things around for scale.
The word smog comes from the two words smoke and fog. It was originally used to describe just that: smoke particles mixed with fog. This was common in cities, when open fires were the main form of household heating.
The most famous example of smog in Britain occurred in 1952, when London was wrapped up in it for 5 days. Breathing the polluted air resulted in many deaths.
Today the word smog is used to refer to the hazy effect that is caused by air pollution. Again it is usually found in cities, where there are lots of vehicles, releasing exhaust fumes, as well as gases from industries.
Smog still indicates poor air quality. Breathing it in can result in sickness. So since 2008, London has operated a low emission zone to try to reduce this type of smog. Vehicles that don’t meet the required standards have to pay to drive in London. The minimum is £100 per day. At that price you would avoid London if you could, thus reducing the pollution in the city.