Fig. 5.

For an evening local time of 19:00 h representative of current THEMIS observations, the red boxes show those regions where there the condition ∂I/ ∂τ_ice < −1 × 10⁻⁶ W cm⁻² ster⁻¹ wavenumber⁻¹ is met allowing a reliable retrieval of water ice cloud optical depth. The green boxes indicate those areas where consistently good retrievals are available for all seasons. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)