Download Lecture 2 - De Anza College

Fog, dew and clouds
Meteorology 10
Prof. Jeff Gawrych
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Dew: air cooled to dewpoint
Frost: air cooled to dewpoint, but temp is at or below freezing
point
How can clouds form?
•
Air needs to be cooled to saturation. I.e. air
cooled to dew point. I.e air reaches 100% RH
1) As air rises, it cools
* How to force air to rise:
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–
–
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Topography
fronts
convection
Convergence ---> <----
2) Surface cools off quickly by emitting radiation
3) Air moves over colder surface/ into cooler area
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Three types of fog
1) Radiation fog
– When surface cools off rapidly and reaches saturation.
– A.k.a Valley fog/Tulle fog because it frequently occurs
in the wintertime in the valleys (Sacramento/San
Joaquin Valleys)
– A low, extremely dense fog
– Happens best on calm, clear days when radiation can
most easily escape back to space.
– Fog forms more easily on cold days because cold air
cannot hold as much moisture as warm air
– Why do valleys get colder than surrounding areas?
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Radiation fog
Three types of fog
2) Advection fog
– When warmer air moves over a colder surface, the air
may be cooled to saturation
• Advection = movement
– This is the SF/west coast fog that happens here, most
commonly in late spring/summer
– Why? Because sea surface temperatures (SST) close to
the shore are much cooler than SSTs just a little bit
offshore.
– Upslope fog: As air moves up a slope, the gradually
cooling air may cool to saturation and form fog
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Advection fog
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Three types of fog
3) Evaporation fog
– A.k.a steam fog.
– Explains why you can see your breath on cold
days.
– The warm, moist air from your mouth brings
the outside air to saturation, creating a cloud.
– Usually evaporates quickly.
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Evaporation/steam fog
# of days with fog
Cloud characteristics
• Clouds are identified by their height (low, medium
and high) and by their visual characteristics (flat or
puffy).
• The four basic cloud groups are:
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–
–
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Cirrus: _________________________
Cumulus: _______________________
Stratus: ________________________
Nimbus: ________________________
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Where do these terms come
from?
• The basic classification terminology has Latin
roots:
– Stratus (Latin for "layer") to describe sheet-like (stable)
clouds.
– Cumulus (Latin for "puffy") to denote a cloud with
some puffy vertical (unstable) extent.
– Cirrus (Latin for "curl of hair") denotes a wispy high
level cloud.
– Nimbus (Latin for "violent rain") means a rain cloud.
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Cloud Classification
As a first distinction, we classify
clouds by the height of the
cloud base.
LOW CLOUDS
• Low clouds have a base
below 5000ft.
• Stratocumulus (Sc),
Stratus (St) and are the
primary low-level clouds
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Cloud Classification
MIDDLE CLOUDS
• Middle Clouds have a cloud
base in the range of 2 - 7 km
(6000-23,000ft).
• Altostratus (As) and
altocumulus (Ac) are the
primary middle level clouds.
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Cloud Classification
HIGH CLOUDS
• High clouds typically refer
to clouds with a base
above 7 km (23,000ft)
altitude.
• Cirrus (Ci), Cirrostratus
(Cs), and Cirrocumulus
(Cc) are all high level
clouds.
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Cloud Classification
• High level clouds tend to be
fuzzy in comparison to low
level clouds.
• This is primarily because
high level clouds are made
of ice crystals instead of
droplets.
• Ice crystals usually
evaporate slowly and thus
have more time to diffuse.
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Cloud Classification
Three primary cloud classes overlap the height bands:
– Cumulus (Cu)
– Cumulonimbus (Cb)
– Nimbostratus (Ns)
“nimb-” defines that the cloud is precipitating.
“cumul-” defines that the clouds are a result of convection.
“strat-” defines that the clouds are layered clouds.
Various terms are commonly intermixed and aren’t limited
to any given altitude.
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