Fig. 4.

The change in expected observed radiance [W cm⁻² ster⁻¹ wavenumber⁻¹] for a given change in water ice optical depth as a function of local time for aphelion (top) and perihelion (bottom) conditions. A latitude-longitude grid of points taken between 30° N and 30° S was used for the computation. This quantity is directly related to the thermal contrast between the surface and the atmosphere where clouds reside. A larger absolute value of this quantity indicates that the observations are more sensitive to aerosol and a reliable retrieval can be performed. The red line at ∂I/∂τ_ice = −1 ×10⁻⁶ W cm⁻² ster⁻¹ wavenumber⁻¹ is our threshold for sufficient sensitivity for an acceptable retrieval. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)