Diurnal and Seasonal Variations in the

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Diurnal and Seasonal Variations in the Clarity
of Celestial Observations as Inferred from
Visibility Data Chip Konrad Associate
Professor Department of Geography UNC- Chapel
Hill
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Atmospheric Transmissivity
Transmissivity The ability of a medium to
transmit light. - Highly correlated with the
concentration of aerosols in the atmosphere.
Aerosols absorb and scatter light Primary Mineral
dustSea saltVolcanic dustOrganic
aerosols Industrial dustSoot (coal
burning)Biomass burning(smoke)
Secondary - conversion from gas to
solid Sulfates from biogenic gasesSulfates from
volcanic SO2Organic aerosols from VOCs Nitrates
from NOxSulfates from SO2Organic aerosols from
VOCsNitrates from NOx
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Atmospheric Visibility
Nocturnal Visibility - The greatest distance at
which it is just possible to see and identify
with the unaided eye a known moderately intense
light source.
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• Atmospheric Controls on Aerosol Concentrations
• Source region of air and speed of movement over
  various regions (urban, rural, maritime).

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• Methods
• Extract hourly time series of weather conditions
  at RDU for the period 1948- 1995.
• 2. Identify all hours in which the following
  conditions are met
• Mostly clear skies 20 or less cloud cover
• High visibility 15 miles or greater.
• 3. Determine seasonal and diurnal patterns in
  mostly clear skies and high visibility.
• 4. Compare Raleigh trends with those from
  Asheville.
• 5. Compute back trajectories of exceptional clear
  and hazy air (i.e. Determine its source region).

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Aerosol Size and Relative Humidity
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Examples of the Transport of Clear Air into the
Region
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Examples of Hazy Air Transport
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Mean Aerosol Concentrations for Urban, Rural, and
Maritime Air
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• Copies of this presentation will be available
  next week at
• http//www.unc.edu/depts/geog/people/faculty/konra
  d/
• Or Google konrad unc
• Web site for calculating backward air
  trajectories
• http//www.arl.noaa.gov/ready/hysplit4.html
• THANK YOU!