FYI: Is that mist or fog obscuring the Peak?
Mist, fog, fret, haar, mo, bo mo - it's all the same; small droplets of liquid floating through the air, clouds that have formed at ground level. The difference is in the density and therefore degree of visibility. According to the Hong Kong Observatory, if visibility is less than 1,000 metres, it's referred to as fog; if it's between 1,000 and 5,000 metres, it's mist. Both usually form at night, when the air is too cold to hold all its moisture. The cool air causes condensation, which appears as mist. Condensation is most likely to form over areas where there is plenty of moisture, such as harbours.
Haar (as the Scots call it), fret (on the east coast of England) or bo mo (thin fog in the local vernacular) can be formed when warm air is cooled as it travels over a body of water. The bottom of the warmer air stream is cooled by the water below until it's bursting to release its moisture, which it does as fog.
The soupy haze obscuring the Peak is a little more complicated, being a product of humidity, wind direction, topography, season and, of course, pollution.
Typically, it's formed by the mixing of warm and cool air masses, but can be condensation blown in from the sea as humid moist air heralds the arrival of spring, or low rain clouds in the summer. One Peak resident remembers watching television in his lounge with the window open when all of a sudden, the picture went hazy. Putting on his glasses, he realised a cloud had entered through the open window. As a smug non-Peak resident pointed out: 'All that money and they can't even watch television.'
London was once well known for its 'pea-soup' fogs and, at the end of 1952, a combination of weather systems and pollution killed almost 4,000 people in four days in the city. An estimated 12,000 perished in the following years as a direct result of the four-day smog.