Mist or fog?

June 26th, 2010 | Uncategorized
It’s not so complicated really – the difference between mist and fog is about 1km….. Fog reduces visibility to less than 1 km, whereas mist reduces visibility to no less than 1 km but less than 2 km. For aviation purposes, a visibility of less than 2 km but greater than 999 m is considered to be mist if the relative humidity is 95% or greater – below 95% haze is reported. Are we all clear on that, or is it still all a bit foggy?
Fog usually forms when the difference between temperature and dew point is less than 2.5°C (4°F), and relative humidity is approaching 100%. The water vapour in the air condenses and the result is fog or mist.
The reason that I mention all this is because for the last few days our coastal region has been shrouded in sea fog – also known as sea fret or haar. Believe it or not coastal fog has a little more to do with the amount of salt in the air. All types of fog and/or mist need minute hydroscopic particles upon which the water vapour can condense, and the presence of sea salt in the air makes this much easier, allowing it to form where humidity is as low as 70%.
Of course all this sea mist hanging around is not good for grape growing – the sun cannot penetrate, the temperature stays lower, and the humidity remains high – ideal conditions for fungus in the form of oidium or mildew. Naturally we have to take precautionary measures to combat this….. 

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