Fog, dew and clouds

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• Fog, dew and clouds
• Meteorology 10
• Prof. Jeff Gawrych

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Dew air cooled to dewpoint
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Frost air cooled to dewpoint, but temp is at or
below freezing point
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How can clouds form?

• Air needs to be cooled to saturation. I.e. air
  cooled to dew point. I.e air reaches 100 RH
• As air rises, it cools
• How to force air to rise
• Topography
• fronts
• convection
• Convergence ---gt lt----
• 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
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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
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of days with fog
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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
• 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
• Stratocumulus (Sc), Stratus (St) and are the
  primary low-level clouds

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Cloud Classification

• MIDDLE CLOUDS
• Altostratus (As) and altocumulus (Ac) are the
  primary middle level clouds.

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Cloud Classification

• HIGH CLOUDS
• 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 arent
  limited to any given altitude.

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