Fog and Cloud formation:
How does
air become
saturated?
North Sea Fog – a coloured visible satellite image
Reading
• Ahrens 9th ed., Chapter 5 Condensation: Dew,
Fog, and Clouds, p110-121
(8th Ed: Chapter 5, p106-116)
• Ahrens, 9th ed., Chapter 6 Stability and Cloud
Development, p151-157
(8th Ed: Chapter 6: p149-155)
• My last lecture: condensation of cloud droplets
(micro-scale)
• This lecture: specific conditions and processes
that lead to condensation (macro-scale)
Dew, Frost, Haze and Fog
(i) Dew: Common on vegetation early morning after a clear
night.
(a) How? Air is cooled to Td so becomes saturated.
(b) Happens on clear nights because surface radiates heat
and this is not trapped by a cloud cover.
(c) Dew forms on vegetation because it cools more quickly
than (say) a road surface; vegetation has a lower heat capacity
per unit area.
Dew formation releases latent heat, and this reduces the
rate at which the surface cools. Therefore the dew point
temperature is often the minimum night-time
temperature.
(ii) Frost: Forms if dew point is below 0°C. This is now
called the frost point.
(a) White frost (or hoar frost) forms by ice deposition directly
from the vapour, in the form of 6-sided crystals.
(b) Black frost (or black ice) forms when the dew
point is just above 0°C so the dew forms first and
subsequently freezes as temperature drops below
0°C. A layer of ice is formed without the creation of
small crystallites.
(iii) Haze: Dry haze is caused by particulate nuclei,
which scatter light like the blue of the sky, and when
there is some water deposition, the haze thickens and
becomes wet haze.
(iv) FOG – defined when visibility < 1 km
Fog is equivalent to a cloud at the surface.
Fog forms in one of three ways:
1. by cooling the air
2. by evaporation of water into the air
3. by mixing of two air masses such that when
combined, the mixture is saturated
The types of fog are named with reference to the
method by which the air becomes saturated.
ADVECTION FOG - air is cooled to saturation by having
warm moist air moving over a cold surface..
San Francisco has a lot
of advection fog since
the surface water near
the coast is much colder
than the water farther off
shore. Warm moist air
from the Pacific Ocean is
advected over the cold
coastal waters, chilling
the advected air from
below.
UPSLOPE FOG
- forms as moist air flows up an elevated plain, hill or
mountain. As the air ascends, it expands, cools
adiabatically (same as any ascending air), and
saturates.
EVAPORATION (or MIXING) FOG
- formed by mixing two unsaturated air masses.
mixing ratio g kg-1
30
B
T = 30 °C
r = 20 g kg-1
25
C
T = 15 °C
r = 11.5 g kg-1
20
15
A
T = 0 °C
r = 3 g kg-1
10
5
But at 15 ° C
rs = 10.8 g kg-1
0
-20
-10
0
10
20
Temperature °C
30
STEAM FOG - forms when cold air moves over
warm water (e.g. heated outside swimming pool)
water evaporates from the pool into the air,
increasing the dew point and if mixing is sufficient
the air above becomes saturated. The colder air
directly above the water is heated from below and
rises forming what appears to be steam.
RADIATION (GROUND) FOG - produced by the radiative
cooling at the surface
- common over land in late autumn and winter.
- also form in low-lying areas.
Favourable conditions:
1. clear (cloud-free) nights (cold),
2. shallow layer of moist air near the ground (e.g.
recent rainfall, or near water body),
3. long nights,
4. light winds
Radiation cooling takes place
from the top of the fog layer;
mixing continues in the fog
layer, further cooling the ground
surface.
VALLEY FOG. Cold air and high moisture content in
river valleys make them susceptible to radiation fog.
Radiation fogs form at the ground and are deepest
around sunrise - sometimes an increase in thickness
at sunrise due to the evaporation of dew supplying
moisture to the fog
3 methods of cloud formation:
1. Cool air directly
2. Mix with colder air
3. Reduce pressure (adiabatic expansion and cooling)
The rapid drop in pressure above
an aircraft wing can result in sufficient
adiabatic cooling to cause saturation
Orographic uplift
Lee Wave clouds
or Lenticular clouds
rotor
As species: lenticularis
Ascending
thermals
-will form ‘heap’ clouds e.g. cumulus and cumulonimbus.
(may spread out if they reach an inversion, however to form
St, Sc)
Slantwise convection – forms most of the layered clouds
such as Stratus. Gradient of the slope is about 1 in 100 or
200; thus air may move horizontally 200 km to ascend 1 km.
Geostrophic Wind: vg
Friction near the surface causes crossisobar flow towards low pressure
Sub-geostrophic
wind, due to the
effects of friction
Cross-isobar
flow towards
low pressure
Cross-isobar
flow towards
low pressure
Clicker question
Air tends to __X__ above an area of __Y__
pressure, causing clouds to __Z__.
1. X=ascend;
Y=low;
2. X=descend; Y=low;
3. X=ascend;
Y=high;
4. X=descend; Y=high;
5. Both 2 and 3 are correct
6. Both 1 and 4 are correct
Z=form
Z=disperse
Z=form
Z=disperse
Midnight (last night)
Convergence occurs in areas of low surface pressure –
this is why cloud is usually associated with depressions and troughs.
Conversely, fine weather tends to occur when pressure is high, as
it is associated with subsidence and surface divergence.
Ahrens Figure 7.16
Rising air cools at the DALR but the dew point drops (at a
rate dependent on moisture content) but typically 2 °C/1000
m. Thus Ta and Td approach each other at about 8 ° C per
km.
Calculating
cloud base
height
general rule for convective clouds is:
cloud base  125 x (Tsurface-Td) + 200 m
Hand-in assignment
• It’s important – worth 10% of overall mark,
and 1/3 of coursework mark.
• Hand in by noon 9th November to Helen
McKeating, Crew 211.
• Include a signed ‘own work’ declaration.
• Penalties for late submission
Midnight (last night)
1200 Monday
0000 Tuesday
1200 Tuesday
0000 Thursday