FOG
- GEOGRAPHY
News:
Why these winters in
Delhi are not wintry enough
What's
in the news?
●
The days have been unusually warm for
winter this December with the maximum
temperature remaining above normal.
Key
takeaways:
●
Delhi saw a warmer start to the winter
this year, with maximum temperatures remaining above normal till around
mid-December.
●
Scientists attributed this to a lower
number of western disturbances affecting the city.
●
This means that north westerly winds did
not bring much moisture and did not lead to any significant fog formation until
December 19.
●
An update from the SAFAR forecasting
system on Monday categorised the fog episode in Delhi on December 23 as “radiation fog”.
Fog:
●
Fog
forms like clouds do - when water vapour condenses.
●
The presence of moisture and a fall in the
temperature are key factors for the formation of fog.
●
With the land surface cooling down at
night, the air close to the surface also cools down.
●
Since
cooler air cannot hold as much moisture as warm air, the water vapour in the
air condenses to form fog.
●
There are different types of fogs such as
○
Radiation fog
○
Advection fog
○
Freezing fog
○
Valley fog
●
The radiation
and advection fog were seen on Indo-Gangetic plain areas whereas other two
were seen in temperate areas.
1.
Radiation fog:
●
Radiation fog (or ground fog) episodes
last for a few mornings on account of calm winds and western disturbances,
resulting in localised fog formation.
2.
Advection fog:
●
In contrast to radiation fog, “advection
fog” is larger in scale both in terms of
the area covered and duration.
●
Advection fog forms when warm, moist air
passes over a cool surface, causing water vapour to condense.
●
Advection fog mostly occurs where warm,
tropical air meets cooler ocean water. If the wind blows in the right
direction, sea fog can be transported over coastal land areas.
3.
Valley fog:
●
Valley fog is the result of mountains preventing dense air from escaping, and in
which the fog is trapped in the bowl of the valley and can last for several
days.
4.
Freezing fog:
Freezing fog is the result of liquid droplets freezing on solid surfaces. Cloud-covered
mountaintops often see freezing fog.