1. Visibility

2. Initial Discussion • Assume Clear Air …no fog, particulate haze, or clouds • We’ll get to haze, fog and clouds later.

3. Definition (AMS) • The greatest distance in a given direction at which it is just possible to see and identify with the unaided eye • 1) in the daytime, a prominent dark object against the sky at the horizon, and • 2) at night, a known, preferably unfocused, moderately intense light source.

4. FAA Considerations • FAA relies on NWS (AMS definition) for visibility • Air traffic managers currently rely on the terminal aerodrome forecast (TAF) when making operational decisions impacted by low ceiling and visibility conditions. • We all rely on automated visibility sensors

5. ASOS Visibility Sensors • ASOS and AWOS have different sensors • ASOS uses scattering of light. • AWOS uses transmissivity

6. Visibility is perhaps better termed Visual Range

7. Recommended Visual Range Objects • Must subtend at least 0.5 degree at observers point • Daytime, should be black or nearly so • Night, unfocused light or mountains which can be seen against the sky

8. Physiological Factors • Brightness Contrast “C” of object is important • Defined as the Brightness “Bt”of the target and that of the background “Bo” • C = Bt - Bo Bo • Contrast has no dimensions

9. Daytime Contrast • An absolutely black target, B=0, in front of any not absolutely black target produces a contrast of -1 • A non-black target in front of an absolutely black background assumes a contrast of positive infinity • Dark targets in front of light backgrounds have contrasts ranging from 0 to -1 • Generally deal with absolute values

11. Threshold Contrast • Of order of C >= 0.003 under optimal conditions • Complicated by many conditions • Experiment during WW II • Thousands of Naval Observers • Job was to spot ship H. R. Blackwell, L. C. Tiffany Foundation Study 1946

12. Liminal Contrast • After each scanning, observer is forced to guess the direction to the ship • Some observers hit by chance (deducted) • As ship approaches the probability of a observation increases • When Probability of Detection reaches 50%, we have a standard threshold contrast.

13. Threshold Contrast as a function of probability of detection

14. Liminal Contrast • “It is remarkable that the observers are not conscious of the high performance of their eyes at this threshold value but will insist they are merely guessing. • It is only when the detection probability reaches 89% that the observers are confident that they actually see the target.”

15. Caviats • Condition of observer • General brightness level of the field around the observer (Adaptation Level) • Visual angle of target • Angle of responsibility of the observer, better if smaller

16. Nighttime Visibility • Moonlight is important • At full moon, a liminally visible target of one degree visual angle of one degree has a threshold constant of 0.02 • At new moon, the threshold constant is ~10 times as large. • Dark adaptation is important at least 30 minutes

17. Attenuation of Light by the Atmosphere • Assume no local smoke sources or thermal microturbulence affects the air • Flux density of light from a point source diminishes with distance Fx = B/x2 • Scattering is important • Absorbtion is important • Generalized extinction Coefficient b=a + s • In a pure atmosphere a = 0 • Complications – See Petty 11.5.2

18. Combined effect • F= B e-bx x2

19. Other effects • The attenuation coefficient is wavelength dependent, Rayleigh Scattering, giving • b proportional to l-4 (Petty p. 330) • Scattering is angle dependent

20. Next Reading in Petty • Chapter 3 • Chapter 4 (skip 4.1.4 unless curious)