Geoscience Reference
In-Depth Information
Fog
HUMAN IMPACT
Fog occurs when condensation of moisture near the ground surface leads to a reduction in visibility to less than
1 km. The meteorological processes leading to the formation of fog have been described earlier, but fogs can also
have a strong human impact through their effect on all forms of transport.
In the days of sailing ships, mariners had to use their ears for the sound of breaking surf as their ships approached
the shore. Most other forms of transport moved so slowly that fog did not have serious consequences. Rail transport
is closely regulated through signalling, so as long as the signals can be seen by the driver movement is possible.
With air transport, the use of radar has allowed automated landings even when visibility is very poor. Nevertheless
the reduction of visibility in fog does slow most methods of transport. In the dense fog affecting Heathrow Airport
before Christmas 2006 the distance between aircraft lining up to land and take-off was doubled, leading to a 50 per
cent reduction in the airport's capacity at a busy time. Many flights were cancelled altogether. Other parts of the
world can experience similar problems. In northern India in 2007 unusually thick fog formed in colder than normal
air caused air, rail and road traffic problems. Near Beijing in January 2006 visibility dropped to 3 m along the rail line
to Guangzhou that meant even electric signals were difficult to see.
Whilst meteorological factors can lead to dense fog, it is often the presence of pollution that worsens the situation.
This may be natural factors such as dust from the Gobi affecting Beijing but, more frequently, it is particles and gases
from industry and transport that cause most problems. Cars can release large quantities of hydrocarbons and carbon
monoxide, especially when old or not well serviced. Such pollutants may react with sunlight to produce photochemical
smogs, as often observed in large subtropical cities such as Los Angeles, Mexico and São Paulo. In these cases the
smog has a greater impact on health than traffic accidents.
precipitation process and leads to more intense rainfall.
Convectional systems may be embedded within the
cyclonic circulation to produce more complex patterns of
surface precipitation.
distance we can sometimes see these dense clouds
enveloping the mountains (
Plate 4.10
).
Orographic rain is also produced in another way, due
to changes in the stability of the air as it rises. If the air is
very moist near the ground surface but much drier above,
as it rises the rates of cooling between the top and bottom
of the layer will be different (
Figure 4.9
).
The upper part
will cool more quickly and so become colder, leading
to less stable air. The cloud development associated
with instability will increase and rain may fall over the
mountains. This situation is known as convective or
potential instability.
Hills as well as mountains act as favourable areas for
convectional showers. The slopes facing the sun will be
warmed more rapidly than flatter areas, because the slopes
act as thermal sources. The resulting cloud may produce
rainfall which is restricted to the upland area.
The orographic effect is most pronounced when it is
already raining upwind of the hills or mountains. Where
air is rising - associated with a depression, for example -
the rate of uplift is increased by the extra ascent forced by
the hills. This leads to a greater rate of condensation on
the windward side, larger drops of rain being formed, and
so a higher rainfall at the surface. There may be a carry-
Orographic precipitation
Almost all mountain areas are wetter than the surround-
ing lowlands. To take two examples, Hokitika on the west
coast of New Zealand receives an average of 2,950 mm per
year. At Arthur's Pass, 740 m higher in the New Zealand
Alps, the annual average has risen to 3,980 mm, compared
with less than 670 mm for Christchurch, on the more
sheltered lowlands to the east. Even the Ahaggar and
Tibesti mountains in the centre of the Sahara receive
more rain than do the surrounding lowlands - Asekrem,
at 2,700 m, has an annual average of about 125 mm,
compared with only 13 mm at Silet, 720 m above sea level.
Why should this be so?
Where air meets an extensive barrier it is forced to rise.
Rising, as we know, leads to cooling of the air, and cooling
encourages condensation. On the mountain slopes and
above the mountain summits the clouds start to pile up,
reflecting the forced ascent of air. Often they reach
thicknesses sufficient to give drizzle and rain. From a
