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Forecasting Visibility

This article discusses the major causes of visibility reductions, such as hydrometeors, dust and smoke, and air pollution. It also explains the methods used to observe visibility and the factors that affect particle size and visibility. Additionally, the article covers the impact of relative humidity on visibility and the challenges in forecasting fog and duststorms.

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Forecasting Visibility

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  1. Forecasting Visibility

  2. Reductions to Visibility Caused by the scattering and absorption of visible light. Major causes: • Hydrometeors: fog, drizzle, snow, rain…particularly heavy rain and snow • Dust and smoke: wildfires, dust raised by strong winds, volcanic eruptions • Other particles such as sand • Air pollution

  3. Observing Visibility • Old days: human observers would use local landmarks of known distances

  4. Today: automated sensors such as the ASOS Visibility Sensor • Uses Xenon flash light source and then measures how much light is scatttered into sensor

  5. Particles size and visibiliy • Large particles (compared to wavelength of light): Mie Scattering. Not wavelength dependent (whitish haze) • E.g., Large hygroscopic salt particles • Small particles (compared to light). Bluish haze. Rayleigh Scattering. Prop to 1/l4 • E.g., Combustion particles

  6. Rayleigh Scattering Mie Scattering

  7. Relative Humidity Variations Affect the Size of Particles, Particularly Hygroscopic Substances (e.g., salt)

  8. Thus Visibility Can Depend on Relative Humidity and Presence of Hygroscopic Nuclei

  9. Visibility Generally Improves During the Day • Temperatures increase, causing RH to decline • Less stable, mixes surface water vapor into BL, drier air down

  10. Offshore Flow/Downslope Flow and Enhanced Visibility

  11. Fog: The most important visibility issue

  12. Fog plays to all our deficiencies • Needs high resolution in horizontal and vertical • Depends critically on PBL schemes…and current ones have major problems..and other physics (e.g., micro, radiation) • Requires accurate simulation of surface properties • Often highly localized.

  13. FOG 101

  14. Radiation Fog

  15. Radiation Fog From Above

  16. 000 FXUS62 KGSP 131857 AFDGSP AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE GREENVILLE-SPARTANBURG SC 253 PM EDT THU APR 13 2006 .SHORT TERM /TONIGHT THROUGH SATURDAY/... NORTHWESTERLY FLOW ALOFT WILL REMAIN OVER THE AREA THRU FRIDAY. …. THERE IS THE POSSIBILITY THAT KAND/KCLT COULD HAVE A BRIEF PERIOD OF MVFR FOG AROUND SUNRISE FRI GIVEN THE FORECAST HYDROLAPSE. HOWEVER CROSSOVER TEMPS ARE WELL BELOW FORECAST LOW TEMPS...THEREFORE WILL LEAVE OUT FOR NOW. CANNOT RULE OUT SOME VFR CEILINGS FROM TIME TO TIME...BUT CHANCE NOT HIGH ENOUGH TO INCLUDE AT THIS TIME. && .GSP WATCHES/WARNINGS/ADVISORIES... GA...NONE. NC...NONE. SC...NONE. && $$ SHORT TERM...RWH/CSH LONG TERM...CSH AVIATION...RWH

  17. Advection Fog

  18. Upslope Fog

  19. Upslope Fog: Snoqualmie Pass

  20. Frontal Fog

  21. Steam Fog

  22. The annual variation can be different in other locations • Where advection fog is dominant often max in late spring/summer. Like coastal CA!

  23. Why is VERY Dense Fog often a good sign for later in the day?

  24. MOS is a powerful tool for fog

  25. Duststorms: An increasing forecasting problem

  26. Dust/sand storms • Often associated with strong cold fronts or the outflow from strong convection. • Minimum wind speed threshold depends on characteristics of surface (fine dust easier to loft than large sand particles).

  27. April 21, 1931: The Biggest Duststorm in NW History

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