The optical properties of water droplets in the infrared

Abstract

Mie theory is used to calculate the extinction and scattering efficiencies and the mean cosine of scattering for spherical water droplets. Tables and graphs are presented of properties averaged over the black-body distribution at temperatures of 280K, 350K and 413K for a range of droplet radii between 0.5 and 20 mu m. The accuracy of using these wavelength-averaged quantities in the calculation of heat fluxes is found to be typically better than 5% for droplets with radius greater than 5 mu m. The effect of small systematic changes in the refractive index is examined: a change of a few per cent in the real part may produce changes of three to six times as much in scattering efficiencies. The values predicted by commonly used approximation formulae are compared with Mie theory values: agreement is found to be poor for small drops and around 10% for drops with radii greater than 7 mu m.