Extinction of visible and infrared beams by falling snow

Abstract

Classical optics holds that the extinction cross of particles should be equal to twice their geometric cross section, in the limit where the particles are much larger than the wavelength. It follows that the extinction coefficient of such large scatterers should be independent of wavelength. Snowflakes are much larger than the wavelengths of visible and infrared radiation, yet many investigators have found that the visible and infrared extinction coefficient of falling snow measured with transmissometers is wavelength dependent. This dependency is known to be a result of the scattering contribution to the transmissometer signal. Furthermore, many measurements in the visible and infrared show that extinction values measured simultaneously with two transmissometers are linearly related up to at least 12 km(-1). The slope depends on the wavelengths and optical characteristics of the transmissometers. We show that for small values of extinction, the observations can be explained by taking into account single-scattering contributions to transmissometer signals. For high values of extinction, a multiplescattering model gives good agreement with measurements.

Extinction efficiency in the infrared (2–18 μm) of laboratory ice clouds: observations of scattering minima in the Christiansen bands of ice

“…The fractional error in optical-depth measurements because of light scattered into the finite collection angle of the detector 7 was estimated to be less than 1% for the 0.685-µm wavelength and less yet for the IR. Although snow is well known to scatter by diffraction a nonnegligible amount of energy into typical detectors, [7][8][9] the fractional amount of light scattered into typical detectors is much less for the comparatively much smaller particle laboratory icecloud scatter. 15 The fractional error in underestimation of extinction measurements scales as the product 1single-scatter albedo D 2 l 22 2 so that IR measurements are less error prone than are visible measurements for comparable source and detector dimensions.…”

“…Single-scattering contributions were sufficient to explain the main features of the extinction measurements in snow for optical depths less than unity. 8 Reference 8 also shows the calculated effect of multiple scattering on a 0.633-µm laser beam captured completely by the detector. Figure 13 of Ref.…”

“…However, snow crystals and snowflakes are at the very large end of the particle size spectrum, where D < 100-10000 µm, so that extinction is determined mainly by the average projected area of the crystal. [7][8][9] Theoretical results give us clues to the numbers of small ice crystals that are necessary to influence the radiative properties of cirrus clouds. 10 In situ cirrus sampling with research aircraft equipped with a Formvar icecrystal replicator indicates that the number of naturally occurring small ice crystals is sufficiently high that these crystals can contribute significantly to cloud radiative properties 11 such as solar extinction and IR emission.…”

Identifying Equipment Factors Associated with Snowplow Operator Fatigue

“…Auxiliary lights mounted away from the operator's direct line of sight (i.e., on the passenger side) reduced back-reflected light and eye discomfort [37][38][39]. Additionally, narrow beam light (i.e., spot lights) produced less back-reflected light compared to wide beams of light [40,41].…”

“…Auxiliary lights should be placed outside of the snowplow operator's line of sight (i.e., locate lights on the passenger side of the vehicle) [37][38][39]. Exterior lights should have narrow-beam spread bulbs (i.e., spot lights) [39][40][41]. Some research suggested that longer wavelength light (e.g., amber and red) reduced the amount of reflected light from warning lights; however, the effects from light color were limited compared to placement and beam spread [36].…”