The Met Office has released the kind of weather warning known to set off a lively debate.
Thankfully, we can settle the debate with some guidance from the experts at the Met Office.
In basic terms, it boils down to how far you can see. Fog applies when you can’t see 1,000m away, while mist means you can see further.
The full definitions are slightly more nuanced than that, though.
What the Met Office says
As the Met Office puts it: “In our meteorological glossary fog is defined as ‘obscurity in the surface layers of the atmosphere, which is caused by a suspension of water droplets’.
“By international agreement (particularly for aviation purposes) fog is the name given to resulting visibility less than 1km. However, in forecasts for the public, this generally refers to visibility less than 180m.
“Mist is defined as ‘when there is such obscurity and the associated visibility is equal to or exceeds 1000m.’ Like fog, mist is still the result of the suspension of water droplets, but simply at a lower density.
“Mist typically is quicker to dissipate and can rapidly disappear with even slight winds, it’s also what you see when you can see your breath on a cold day.
And what of that third, less feted player in the furious squabbles which erupt on this subject – haze?
“This is a slightly different phenomenon which is a suspension of extremely small, dry particles in the air, not water droplets,” the Met Office says.
“These particles are invisible to the naked eye, but sufficient to give the air an opalescent appearance.
“These particles can also contribute to creating a red sky at sunrise or sunset.”
Bristol’s fog warning
The Met Office’s fog warning for Bristol is in place from 5pm to 11.59pm today (December 30).
According to the Highway Code, headlights should be used when visibility is below 100 metres. Before setting off on a foggy journey, drivers should make sure their lights – especially fog lights – are working properly.
The Met Office and Highways England say you should:
- Avoid travel if possible
- Drive very slowly with dipped headlights, full-beam lights reflect off the fog causing a ‘white wall’ effect
- Keep an eye on your speed, fog can give the illusion of moving in slow motion
- Use fog lights, but remember to turn them off when the visibility improves
- Don’t hang on the tail lights of the car in front, rear lights can give a false sense of security
- Watch out for freezing fog which is made of water droplets that freeze on contact with objects such as the pavement, road, car, etc. It can quickly form a layer of ice.
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