Download Definitions and Types

Low cloud and poor visibility
Fog: Definitions and Types
● Definitions
● Fog types:
– Radiation
– Advection
– Upslope
– Steam
Mist is defined as ...
… a suspension in the air of microscopic
water droplets or wet hygroscopic
particles, reducing visibility at the Earth’s
surface.
 used when visibility is ≥ 1000m and
≤ 5000m and RH > 95%
Tdry-Tdew ≤ 0.5 C.
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Fog is defined as ...
… a suspension of microscopic water
droplets in the air reducing visibility at the
Earth’s surface to:
 < 1000m for met. observations,
aviation and shipping
 < 200m (dense fog) for public service
purposes
 Tdry ≈ Tdew.
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Freezing fog and ice fog
 Freezing fog
− Supercooled drops
which freeze on impact
with a surface (produce
rime)
 Ice fog
− Tiny suspended ice
particles
− Requires T≤ -30°C
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Fog types
There are three main types:
 Radiation fog
 Warm advection fog
 Upslope fog
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RADIATION FOG
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Radiation fog formation
 Overnight clear skies
 Moist air in lowest 100m
 Moist ground
 Light surface wind = fog
 > 5kt surface wind = low cloud (turbulent
stratus)
 Valleys or ‘bowls’
 Fog point
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Fog point
 Definition: The screen temperature at which
fog may be expected to form
 The fog point may be much lower than the air
mass dew point
 Why?
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Dew formation
 Fall in surface temperature with nocturnal
radiation
 Air cools to dew point
 Dew deposition
− Latent heat given out during condensation slows
temperature fall
− Air near the ground becomes drier
 Dew point falls
 So fog point < air mass dew point
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The effect of the hydrolapse
HM
R
Li n
es
7
5
3
2
Large positive hydrolapse
 Much drier air aloft
 Moisture losses at ground
not replaced by diffusion
 Large amount of cooling
for fog
 Fog point << airmass
dewpoint
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The effect of the hydrolapse
HM
RL
ines
7
5
3
2
Small positive hydrolapse
 Slightly drier air aloft
 Drying at surface soon
produces negative
hydrolapse
 Less cooling for fog
 Fog point slightly <
airmass dewpoint
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The effect of the hydrolapse
HM
RL
ines
7
5
3
2
Negative hydrolapse
 More moist air aloft
 Moisture losses at ground
readily replaced from
above
 No further cooling for fog
 Fog point = airmass
dewpoint
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WARM ADVECTION FOG
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Warm advection fog formation
 WARM MOIST air moving over cool land or sea
- Windward coasts
 Air cooled to dew point
 Light surface wind = fog
- overland heating lifts fog into low cloud during
the day, rapid in summer, slow in winter
 > 10kt surface wind = low cloud
 (Tdry - Tdew) x 350 = stratus base above ground
level
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UPSLOPE FOG
Wind direction
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Upslope fog (stratus) formation
 Air forced to rise over hills
 Warm, moist, moderate to strong winds
 Stable air
 Air cools on ascent
 Very common on windward coasts and hills
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Upslope fog or stratus
Airmass St/Sc
Low-level wind
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Upslope fog or stratus
Airmass St/Sc
LCL
Low-level wind
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LCL = Lifting condensation level
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Upslope fog or stratus
Airmass St/Sc
Upslope stratus
LCL
Low-level wind
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5
2
3
Forecasting upslope stratus
7
750
Too dry
9
LCL
1000
 Select a representative
ascent
 Then determine the
lowest LCL
 This is a process of trial
and error!
 Lowest LCL = upslope
stratus base
10
0
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STEAM FOG
 Cold advection fog
 ‘Arctic sea smoke’
 Cold air flowing over relatively warm sea
 Low layer near surface becomes very unstable
 Convective swirls
 Evaporation then condensation
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Steam fog
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Forward to
Mist and fog forecasting techniques (2): Saunders fog forecasting technique
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