Clouds and Stability

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Clouds and Stability
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Fog, 800 am Friday, Sept. 14th
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No fog, 845 am Friday, Sept. 14th
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Fog, 630 am Monday, Sept. 17th
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No fog, 830 am Monday, Sept. 17th
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Four Basic Cloud Types

• High, above 6 km
• Cirrus (Ci), Cirrostratus (Cs), Cirrocumulis
  (Cc)
• Middle, 2-6 km
• Altostratus (As), Altocumulus (Ac)
• Low, 0-2 km
• Stratus (St), Stratocumulus (Sc), Fog
• Vertical Development, 1-10 km
• Cumulus (Cu), Cumulonimbus (Cb)
• Luke Howard (1803) Stratus layer, Nimbus
  rain, Alto mid-level, Cumulus heap, Cirrus
  wisp

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Cloud Identification Guides

• Cloud type information and photos
• Plymouth State University
• Meteorology Program
• Cloud Boutique
• http//vortex.plymouth.edu/clouds.html
• UK Met Office Cloud Guide
• http//www.metoffice.com/bookshelf/clouds/

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Clouds Are Formed by Lifting
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Important terms

• Divergence
• Convergence
• -------------- O
• Supersaturation
• RH 100 due to curvature of droplets
• Supercooled
• Cloud droplets can remain liquid at T

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Divergence and ConvergenceBays and Headlands
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Stability and Cloud Development

• Anthes 4 86-95

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Parcel of Air invisible bubble

• Rising cooling (parcel is buoyant when it is
  warmer than its environment).
• Parcel is unsaturated below the Lifting
  Condensation Level (LCL).
• Parcel is saturated at and above LCL so a cloud
  forms.
• Condensing water vapor releases latent heat in
  the cloud adding buoyancy.
• Sinking warming (also reduces RH of parcel)

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Lapse rates

• Rate of cooling with height ?T/?z
• The average environmental cooling rate is about
  6.5 oC / km
• When rising air is unsaturated, it cools at the
  dry adiabatic rate of - 10 oC / km
• When rising air is saturated, it cools at the
  moist adiabatic rate of - 6 oC / km
• Parcels rise and cool
• up dry to the LCL
• up moist in the cloud
• Parcels sink and warm
• down dry

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Conditional instability

• The environmental lapse rate of -6.5 oC/km is
  classified as "conditionally unstable."
• It is less than the dry adiabatic rate of -10
  oC/km (so dry parcels rising will find themselves
  always cooler than the environment and will sink
  or be stable)
• And it is greater than the moist adiabatic rate
  of about
• -6 oC/km (so moist parcels rising will find
  themselves always slightly warmer than the
  environment and will continue to rise or be
  unstable).
• Conditional instability, therefore, refers to
  whether the convection is dry (unsaturated) or
  moist (saturated).

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• a. superadiabatic -- absolutely unstable
• b. typical environmental -- conditionally
  unstable
• c. isothermal -- absolutely stable
• d. inversion -- absolutely stable

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Height of a cloud base

• Cloud base LCL lifting condensation level
  100 RH
• Dry cooling rate 10 oC / 1000 m
• Decrease in Td 2 oC / 1000 m
• So dry air rising and cooling approaches
  saturation (100 RH) at a rate of 8 oC / 1000 m
• Height of cloud base (LCL) (Ts - Td) 1000m /
  8 oC
• LCL 125 (Ts- Td)

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Windward rainforests Leeward rain shadowsFoehn
winds