Fog – in urban areas
Fog is defined as occurring when visibility is less than 1000m- irrespective of whether the decrease
invisibility is produced by water droplets or solid particles.( This gives one of the reasons why fog is
associated with urban areas.)
Thick fog---visibility less than 200m
Dense fog – visibility less than 50m
Freezing fog- super cooled fog when droplets are at a temperature which is equal to or less than
0C.
Fog is formed when the air is cooled to below its dew point i.e. the temperature at which water
vapour starts to condense.
Summary of fog types in urban areas.
Type of fog
Season
Radiation fog
October to
March
Smoke fog
(smog)
Winter
Photochemical Summer
smog
Areas
affected
Low lying
areas, moist
surfaces
Near
industrial
areas and
large
conurbations
Enclosed
streets, large
urban basins
Mode of
formation
Cooling due
to radiation
from the
ground on
clear nights
when the wind
is light
See above
Mode of
dispersal
Dispersed by
the sun’s
radiation or by
increased
wind.
Temperature
inversion and
light winds
trapping
pollution from
trafficnitrogen
oxides and
hydrocarbonsacted upon by
sunlight to
produce
ozone.
Wind ,
reduction in
intensity of
sun’s radiation
Dispersed by
wind increase
or by
convection
Fogs are most common from October to March. This is when the nights are longer so the cooling of
the air next to the ground goes on longer so that the air is more likely to be cooled to its dew point.
The air is slightly more humid in October and November as trees still have some leaves and there
are some plants still growingmore transpiration.
Urban areas have more fogs because:
i)
wind speeds are lower so that fogs are not so easily dispersed
ii)
presence of pollution provides hygroscopic nuclei for water droplets to form (and also
adds to reduced visibility)
iii)
presence of pollution prevents sun’s rays penetrating to the ground and warming up the
fogso fog not dispersed.
The frequency of thick fogs in the densely built up central parts of the cities is less than in the
suburbs. This is because in the centre the air is warmer so that it needs more cooling to reach its
dew point. Also the lack of vegetation etc. could mean that there is less water vapour in the air.
Overall the relative humidity is lower.
Widespread thick fog lasting a whole day is rare. Typically fog lasts only a few hours during the
early part of the day- although it may reform quickly after sunset.
Studies in London show that thick fogs of 12 hours or more happen about 3 times a year at
Heathrow and less in the centre.
In December 1952 visibility was less than 10 metres persisted for more than 114 hours and led to
the death of large numbers of people. This led directly to the first Clean Air Act in 1956 which set
up smokeless zones etc. There has been a decrease in the number of thick fogs since that date.
Costs /effects/solutions
Transport
Air – delays / need for automatic aids for aircraft ( there have been experiments with fog dispersal
such as wind machines)
Rail- safety signalling required
Road – warning signs about reducing speed/ best lighting is low pressure sodium lights-should be
low level and directed at right angles to drivers line of sight.
Highest mean annual frequencies of thick fog are recorded in areas of greatest density of
motorways. Some times the design of motorways can lead to the presence of the fog.
The recording of fogs/incidence of fogs depends on the location of the main meteorological stations
many of which are at airports. This means that statistics give unrepresentative or biased situation in
a region. Also the relative lack of fogs in summer may exaggerated because observations are
made at 9.00 am which is hours after sunrise so that the radiation fog formed over night will be
dispersed.
Problems of fogs- accidents/ illness/ cost of delays.
Dealing with fogs- educating drivers, artificial fog dispersal ( burning/ heat to evaporate droplets),
road design.
Main reason for reduction in number and intensity of fogs-- reduction of pollution-- ‘Clean Air Acts’,
better design of cars( catalytic converters, lead free petrol
[Fog is formed by the cooling of the air to ‘dew point’ and the resulting condensation of water
vapour in the atmosphere at ground level. Anticyclonic conditions with few clouds can produce
radiation and advection fog.
Radiation fog results when a body of moist air, in contact with the ground surface is cooled to its
dew point. This commonly occurs at night under cloudless anticyclonic conditions with only a light
breeze blowing. Because the sky is clear, the ground surface cools rapidly by radiation and in turn
cools the air immediately above it. Once the dew point has been reached, condensation occurs.
The cooled layer of air is stirred by the light winds so that it cools the air above it to its dew point,
and so the fog grows deeper. Radiation fogs occur under temperature inversions, which prevent
the air from rising. They may persist for several days in winter if the sun is too weak to disperse
them. They are more common in autumn/early winter because the air is more moist as more
vegetation around than later in the winter.]