Fogs

• What is fog?


  Fog shows up when water vapour (WV) cools & condenses > during condensation, molecules of WV coalesce (come together) to make liquid water droplets > hang in the air > what we see as fog

  • Happens when humid > has to be loss of WV in air for the fog to form
  • For fog to form > dust or some particulate / chemical air pollution (aerosols) need to be in the air > WV condenses around it
• Difference between fog and mist:

  Fog - more dense, more water particles in same amount of space than mist

  Fog - can see about 1km

  Mist - less dense

  Mist - can see about 1-2km
• Types of fog

  1. Radiation fog
  2. Advection fog
  3. Freezing fog
  4. Valley fog
• 1. Radiation fog: 


  Forms in the evening:

  • Heat absorbed by surface during day > radiated into the air > as heat is transferred from ground to air > water droplets form > also known as 'ground fog' > fog that burns off in the morning sun
• 2. Advection fog:

  • Forms when warm, moist air passes over a cool surface > advection (movement of fluid) > when moist, warm air makes contact with cooler surface air > WV condenses to create fog
  • Shows up mostly in places where warm tropical air meets cooler ocean water
  • E.g. East Coast of Yorkshire > cold current is cooler than warm air along the coast > advection fog called 'sea fret'
• 3. Freezing fog:

  • Liquid fog droplets freeze to solid surfaces
  • Mountain tops covered by clouds are often covered by freezing fog|
  • As freezing fog lists > the ground, trees, objects e.g. spider webs are blanketed by a layer of frost
  • Common in Canada
• 4. Valley fog:

  • Forms in mountain vallyes, in winter
  • When mountains prevent the dense air form escaping > fog trapped in the bowl of the valley
  • E.g. Meuse Valley, Belgium - vapour condensed around particles of air pollution > killed 60 people
• Why is fog typically more dense, more frequent and longer lasting in urban areas? (London)

  • Kew in the middle suburbs of London has on average 80 hours of dense fog a year whereas London Heathrow Airport on the outer suburbs has only 45 hours a year
  • Further away from the urban areas of a city towards rural areas, fog density, frequency and length decreases
• Why do urban areas have more fog?

  • Urban areas generate large amounts of dust and pollution > these act as condensation nuclei and trigger cloud formation > condensation of water vapour onto aerosol particles
  • The higher the number of condensation nuclei in urban air the more fog that will form
  • E.g. combustion processes add particulates and moisture to the air > form sulphuric acid and solid aerosol ammonium sulphate > produces low visibilty urban fog
• However, UHI and 'fog holes':

  How do fog holes develop?

  • Aerosol particles from pollutioon serve as 'seeds' > make it easier for fog to form
  • At same time, opposing effect takes place. Fog formation requires stable layer of moist air, high relative humidity in lowe part of atmopshere near the ground.
  • Warm urban areas decrease relative humidity which decreases the moisture for fog to form > creates fog holes
• Particulate and photochemical smog - London: 1952 killer fog 'pea-souper':


  A 5 day smog killed 12,000, 1/1000 died

  • Burning of coal > produced sulfur dioxide and carbon monoxide > reacts with water > forms sulfuric acid > harm internal organs as it binds with hemoglobin > stops oxygen from going around the body > produces acid rain
  • Industrial revloution > increased coal burning > increase smog > irrated lungs
  • High pressure weather system at the time > kept smog > no wind to dissipate it or pollutants due to the calm
  • Planes had to ground and public transport cancelled