Question for met. experts
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Join Date: Oct 2006
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Question for met. experts
I was in a window seat on a flight out of LHR last Tuesday morning. London was covered in fog. It was a lovely sight from the air, seeing all the tall tower tops - Telecom Tower, the "gherkin", Canary Wharf, peeking out of the tops of the clouds. One thing that puzzled me was that the entire Hyde Park, and the area 1/2 mile all around it, was completely clear. Everywhere else was 100% covered in fog. What was so special about Hyde Park that caused this hole in the fog layer?
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Non-expert reply!
My OS map says it not due to height, so I'd guess it's the result of all that greenery - different solar gain and humidity - plus it's farther from the river than Kew/Richmond. Any better thoughts?
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Its been puzzling me for a while this one too, as you would indeed expect the fog to form on the park first, due to lower air temperature. Something that has just entered my head is that in fog forming conditions, the air would be relatively still, and maybe as a result of relatively low circulation, the air quality is significantly better over the park in terms of particulate pollutants. Its often small airborne particles that cause cloud formation (condensation nucleii - silver nitrate IIRC being favoured by the 'rainmakers' of the Russian steppes & American prairies) so its possible the cleaner air in the park area could be less prone to fog formation...just a thought 
Another non-expert's opinion and/or guess, but the clue may be that you saw this in the morning. The urban areas overnight may have cooled off faster than the park dropping the air temperature below the dew point so fog formed there (this is called radiation fog). The park area, on the other hand, may have retained enough heat overnight to keep its air above the dew point and so no fog formed.
I think for the above to happen there would have had to have been no wind at all that morning to prevent any mixing between the two air masses. Do you know if that was the case?
I think for the above to happen there would have had to have been no wind at all that morning to prevent any mixing between the two air masses. Do you know if that was the case?
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Hygroscopic nuclei
First time I have come across a mention of the Hygroscopic nuclei that we were taught were one of the "Five factors of Fog" in ground school. Perhaps they are signficant and there just werent enough in the park!
I don't own this space under my name. I should have leased it while I still could
One of the conditions for fog forming is a disturbance of the air. In the parks, in wind free conditions we might expect the parks to remain clear whereas traffic moving in the city, coupled with thermal uplift, would be enough to generate fog.
Many years ago the airfield was covered in a very heavy hoar frost. It was calm and no cloud and a brilliant day. In the eastern corner of the airfield 3 aircraft ran up their engines for checks. When we deplanned a few minutes later it was in a complete pea-souper that persisted for hours.
It had simply been initiated by our engine runs.
Many years ago the airfield was covered in a very heavy hoar frost. It was calm and no cloud and a brilliant day. In the eastern corner of the airfield 3 aircraft ran up their engines for checks. When we deplanned a few minutes later it was in a complete pea-souper that persisted for hours.
It had simply been initiated by our engine runs.
Another idea for you-Underneath most of the esstern end of Hyde park is huge underground (but not far underground as its a single story) car park full of nice warm vehicles and I belive it is heated to a degree too. Other end of Hyde park is Serpentine ie large body of water normally warmer than land. So two large areas that could easily be plus 1 or 2 degrees above average ambient for 'tarmac' London which I think is typically colder than a grass surface as well so it could a sort of micro climate thing and as has been pointed out ehre fog is fickle beastie a degree or two up or down can make a lot of difference
Cool view though isnt it and one of those moments your are glad you had a window seat
PB
Cool view though isnt it and one of those moments your are glad you had a window seat
PB
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