Fog forecasting

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Fog forecasting

• Advection fog

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Chapter III ADVECTION FOG
The primary cause of advection fog is the
movement of air from a locality with one
temperature regime to another locality where the
temperature and moisture regimes are different.
Thus, in contrast to radiation fog, a significant
wind is essential for advection fog formation.
Warm advection fog
Warm advection fog forms when warm, moist air
flows over a colder sea or land surface.
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Over sea
CHECK
Have a chart of current sea T available Check
ships reports and Td gt sea T Synoptic
situation Sat imagery
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Over land
Formation Air mass dew point gt
Surface temperature Stable lapse rate
and only a slight hydrolapse Otherwise
turbulent mixing might disperse the fog
Moderate winds (10 15 kt) With stronger
winds, low stratus is more likely In calm
conditions fog tends to slowly disperse
Suitable wind direction
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Clearance

• Change of air mass
• Most common and reliable method
• Change of track of air mass to a drier source
• Over land insolation
• Radiation fog techniques may be used

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Climatology, sea advection fog

• Poleward airflow
• Trajectory over warm water, then rapid cooling
  SST
• Mostly during warm season (High pressure over
  oceans
• Frequency jun-jul-aug and dec-jan-feb 10-20-30
• May be longlasting and over vast areas

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Sea fog example

• Wind perpendicular to Sea-State (SST) isotherms

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Seafog trajectory
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06 1200Z

• Moist, well mixed

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07 0000Z

• Tsurf risen
• Td fallen
• Moist layer thicker

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07 1200Z

• Saturated layer
• Isothermal layer
• St

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08 0000Z

• Fog present
• Thinner BL due to High pressure

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08 1200

• Strong winds 30kt ? St clouds

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Critical factors

• Moist boundary layer
• Cooling of the surface temperature due to passage
  over colder water
• Large scale subsidence aloft

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Cold advection fog
Cold
Steam fog
Cu
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Steam fog over a lake
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Steam fog occurences
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Formation

• Very cold air over warmer water ?T13C
• Conduction, convection, turbulence ? heat
  moisture transfer
• Water condenses in much colder air (columns)
• Can become thick

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Air parcel trajectories

• At point 1 2 very cold and dry air
• Mixed boundary layer
• Inversion may exist

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Profile offshore

• Point 3 just offshore
• Strong hydrolapse
• BL unstable
• Point 4, further offshore
• Saturation in lower BL
• Steam fog
• BL still unstable

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Steam fog conclusion

• Key point rapid saturation, without significant
  air temperature change
• Primarily in winter in arctic air or autumn on
  lakes
• Short lived to upto one week
• Can cause considerable icing on ships

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Chapter IV UPSLOPE AND HILL FOG
1. Upslope fog
Requirements
- Flow of moist air over gently rising ground
over a wide area
- Stable lapse rate in the lowest levels
Very stable air may be deflected around the
edge of steeply rising ground if there are gaps
in the escarpment
On radiation nights, weak moist advection may
be combined with gentle upslope motion producing
multiple conditions favourable for fog the
likelihood of formation of upslope fog may be
determined from a representative T,Log P, the
height of the base being given by the lifting
condensation level of the upwind air
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Upslope fog formation
If turbulence is already causing stratus over
flat ground, the effect of upslope motion will be
to lower the cloud base on the wind ward side.
If turbulence is insufficient to form stratus,
the extra uplift generated by upslope motion may
cause patchy upslope fog to form. This can
sometimes be the first sign of stratus forming
more generally.
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2. Hill fog
- Hill fog does not necessarily require upslope
motion.
- It occurs when the cloud base happens to be
lower than the hill top
- Depending on the humidity profile below the
general cloud base, upslope motion may generate
cloud on hill at a lower level, either merging
with the general cloud layer or as a separate
layer beneath it.
- Radiation fog which has formed overnight in
valleys may lift into low stratus during the
morning, covering hills temporarily before
dissipating.
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Hill fog formation
Hill fog can occur in any air mass provided the
hills are high enough, or the cloud base low
enough. Here the base of the cumulus clouds is
low enough to give hill fog patches.
Radiation fog which has formed overnight in
valleys may lift into low stratus during morning
hours, covering hills temporarily before
dissipating.
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Chapter V Frontal fog
- Frontal rain falling into very cold stable air
beneath eventually saturates the layer,
frequently forming pannus, with fog confined to
higher ground.
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- Occasionally winds may be light enough to give
fog at low levels
- Visibility is often several hundred metres
forced ascent may reduce this.
Summary of factors favourable for frontal fog
development
- Ahead of active warm front or warm occlusion
- Very cold (or snow covered) ground
- Large temperature contrast between cold and
warm air masses
- Light surface winds.