Freezing Fog

[Atleastitwillbemild]

Hi I had a question about Freezing fog that I don't seem to be able to find the awnsers for by google searching. Its my favourite type of weather phenomenon but I don't get to see it very often in my part of the world (Thames Valley/Marlborough Downs)

We get plenty of fog here, autumn and winter, and there are many frost pockets and hollows but getting frost and fog at the same time always seems to be rare. I know freezing fog is brought about obviously by sub zero temperatures and areas of clear high pressure in winter, by why is that some clear some frosty highs produce freezing fog occaisonally while many more usually don't? 
What are the exact conditions needed for it to form other than clear highs and very low temps? 

During December 2010 we had almost 3 weeks of lying snow under both cloudy and clear conditions and freezing fog formed on only one of those nights. It was neither the mildest or the coldest night either.  I see a lot of people mention on forums when looking at certain charts that due to what they see, freezing fog could definitely be a risk. What is it about a particular chart that makes a cold frosty high more likely to produce FF than another?  It would be a great help if anyone could explain the partiuclar conditions that create the right sort of environment for FF to occur locally. Does it have to do with relative humidity? direction of source of cold air? How moist the ground is? 

Thank you for any helpful explanation.

Edited August 25, 2020 by Atleastitwillbemild

[MP-R]

4 hours ago, Atleastitwillbemild said:

Hi I had a question about Freezing fog that I don't seem to be able to find the awnsers for by google searching. Its my favourite type of weather phenomenon but I don't get to see it very often in my part of the world (Thames Valley/Marlborough Downs)

We get plenty of fog here, autumn and winter, and there are many frost pockets and hollows but getting frost and fog at the same time always seems to be rare. I know freezing fog is brought about obviously by sub zero temperatures and areas of clear high pressure in winter, by why is that some clear some frosty highs produce freezing fog occaisonally while many more usually don't? 
What are the exact conditions needed for it to form other than clear highs and very low temps? 

During December 2010 we had almost 3 weeks of lying snow under both cloudy and clear conditions and freezing fog formed on only one of those nights. It was neither the mildest or the coldest night either.  I see a lot of people mention on forums when looking at certain charts that due to what they see, freezing fog could definitely be a risk. What is it about a particular chart that makes a cold frosty high more likely to produce FF than another?  It would be a great help if anyone could explain the partiuclar conditions that create the right sort of environment for FF to occur locally. Does it have to do with relative humidity? direction of source of cold air? How moist the ground is? 

Thank you for any helpful explanation.

Interesting question and one I'd like to know the answer to as well, but looking back at occurrences of freezing fog in my locale, particularly in December 2007, December 2008/January 2009, January 2011 and December 2016/January 2017, they all occurred when there was an anticyclone overhead but pressure was moderately high, rather than really high. All occurred between 1020-1030mb. Perhaps pressure too high has an effect on moisture content in the air... can't be sure, but that's a general observation.

[Atleastitwillbemild]

9 hours ago, MP-R said:

Interesting question and one I'd like to know the answer to as well, but looking back at occurrences of freezing fog in my locale, particularly in December 2007, December 2008/January 2009, January 2011 and December 2016/January 2017, they all occurred when there was an anticyclone overhead but pressure was moderately high, rather than really high. All occurred between 1020-1030mb. Perhaps pressure too high has an effect on moisture content in the air... can't be sure, but that's a general observation.

Thank you and that's a really good start. Its such an elusive weather phenomenon but one I love more than snow! Interesting observation about the moderate pressure reading as opposed to intense highs. I suspected it would have something to do with the pressure. Thanks 

[johnholmes]

This from UK Met office

https://www.metoffice.gov.uk/weather/learn-about/weather/types-of-weather/fog/freezing-fog

Hope that helps,
personally I am not aware of any special conditions regarding actual pressure.

[damianslaw]

Freezing fog has been a rarity here in recent years, high pressures have often brought clear skies but little fog, I think moisture content may have something to do with it. Sometime we have freezing fog when a layer of cold dense air sits underneath saturated milder uppers higher up, this can occur at the boundary of a frontal feature usually warm front moving into high pressure, the cold layer is then mixed out from above, a slow thaw can happen.

 

[Atleastitwillbemild]

On 26/10/2020 at 21:49, damianslaw said:

Freezing fog has been a rarity here in recent years, high pressures have often brought clear skies but little fog, I think moisture content may have something to do with it. Sometime we have freezing fog when a layer of cold dense air sits underneath saturated milder uppers higher up, this can occur at the boundary of a frontal feature usually warm front moving into high pressure, the cold layer is then mixed out from above, a slow thaw can happen.

 

That's the most helpful answer yet! Learned a lot from that so thank you.

[KeegansPerm]

I'm also a fan of freezing fog, especially when it is long duration and allows a good rime to build up on objects (I've attached a picture I took in Lincolnshire in January 2013!). The fog you are most likely talking about is radiation fog, which forms due to radiation processes overnight.

How does radiation fog form?

Radiation fog (whether freezing or not) forms when the air at the surface cools to its dew point. Dew point is the temperature that air must be cooled to become saturated with respect to water vapour. Once you reach this temperature water vapour can begin to condense out as cloud /fog droplets, which would be as fog is the temperature above zero, or freezing fog with supercooled water droplets if the temperature is below zero. However the point where the air becomes saturated and fog forms is difficult to assess, as processes that lead to decreasing air temperatures overnight, often lead to a decreasing dewpoints too.

So overnight when skies are clear the ground radiates heat to space and cools, this cools the air immediately above resulting in a shallow cold stable layer of air forming close to the surface. However in this routine scenario the ground is cooler than the air, meaning that the ground normally cools below the dewpoint of the overlying air first and this means that as the air comes into contact with the ground is deposited as either dew (or frost when the ground is below zero), and these processes will remove water vapour hence the dewpoint of the overlaying air.

You often get into this evolution, where as the air is cooled towards it dewpoint (the point at which fog can form) by the colder underlying ground  at the same underlying ground is reducing the dewpoint of the air by capturing the water vapour in the air and locking it up as either dew or frost deposits. See attached graph.

Hence you need to achieve conditions where the air is able to cool to its dewpoint, faster than the ground is able to capture the moisture.....often very finely balanced!

What factors help fog form?

Some additional factor can aid the formation of fog if the conditions described above are met, these include abundant aerosol/pollutants which encourage water vapour to condense out into fog, being near wet ground especially with standing water (and being downwind from a lake or sea) which remains warmer than the surrounding ground and continues to provide additional moisture, light winds to continue to mix moisture down close to the cool surface (to replace that lost as dew or frost), and then the advection of  more moisture rich air to your location overnight, and being in a hollow where the coldest air from all around drains into (via density currents).

In the UK low lyying river valleys, vales and marshlands often have the most favourable for radiation fog to form for these reasons.

What about once fog forms?

When fog initially forms it is generally very shallow, meaning although you cannot see very far horizontally (indeed shallow fog often gives the poorest horizontal visibilities), you can often see the stars and moon. This means that the ground is able to radiate to space still and continue cooling. However through turbulent mixing and other processes the depth of the fog will often increase into a mature fog (with the sky no longer visible through it). This completely changes the radiation process with the ground  (due to warmer temperatures at depth) now warming the surface radiating into the fog layer and slightly warming this, with the top of the fog layer now being the surface that radiates to space and and cools....this leads to the fog been warmer at the base, and cooler at the top and leads to gradual convective overturning further increasing the fog depth. A mature fog is significantly less likely to clear or be slower to clear during the following day. See diagram.

How does fog clear?

Put simply the temperatures of the air has to exceed the dewpoint, leading to the fog droplets to evaporate and become water vapour once more. However in this case the processes which work against fog formation overnight, work against fog clearance during the morning.

The ground heated by the faint rays of the sun warms first, and then the air overlaying this is warmed by the ground. This means that as air temperature rises, so does dew point as moisture is released from melting frost and evaporating dew. This means that fog often clears from the bottom up, lifting into low cloud as the warmer lower layers become unsaturated first before finally clearing . See attached graph (above).

Other things can clear fog very effectively, including increasing wind speed which turbulently mixes warmer and drier air down to the surface, and the advection of drier air into a region.

What other types of fog exist?

Fog can form anywhere that air is cooled to it's dewpoint, so warm air over cold sea or snow covered ground is an effective way of generating fog (advection fog common in the spring and early summer), and a similar scenario can occur in the mixing area of frontal zones (frontal fog). But if you live inland radiation fog will dominate for you.

Hope that helps give you a good baseline understanding of the processes around radiation fog.  If you want anymore info feel free to get in touch. 

 

[Atleastitwillbemild]

This is an incedibly helpful post and thank you so much for taking the time to post it. Really answers one of the main things that have been bugging me about freezing fog - which is to say why it forms in some subzero temperatures and not others - really well. Thank you x

[SortingHat]

Do you think it could be where on the high pressure your placed? If your on the north or eastern end of the high would that affect things vs being on the south and west quadrant of the same high?>