This note describes an improved method for the measurement of the ice water content (IWC) of cirrus cloud using a total water content probe. A previous version of this technique assumed that the air in cloud-containing regions was saturated with respect to ice. This assumption has now been replaced with measurements of the water vapor content from a fast-response Lyman-α fluorescence water vapor sensor. The improved measurement of the vapor phase resolves anomalies in the earlier measurements that were due to the assumption of saturation with respect to ice everywhere within cloud. The comparison of IWC measurements made by this new method with those from a 2D optical array probe is greatly improved. The new measurements may now be used to provide much more stringent tests of the algorithms used for the derivation of crystal mass from measured size in 2D probe data. Abstract This note describes an improved method for the measurement of the ice water content (IWC) of cirrus cloud using a total water content probe. A previous version of this technique assumed that the air in cloud-containing regions was saturated with respect to ice. This assumption has now been replaced with measurements of the water vapor content from a fast-response Lyman-α fluorescence water vapor sensor. The improved measurement of the vapor phase resolves anomalies in the earlier measurements that were due to the assumption of saturation with respect to ice everywhere within cloud. The comparison of IWC measurements made by this new method with those from a 2D optical array probe is greatly improved. The new measurements may now be used to provide much more stringent tests of the algorithms used for the derivation of crystal mass from measured size in 2D probe data.

The physical and optical properties of Saharan dust aerosol measured by the Met Office C‐130 during the Saharan Dust Experiment (SHADE) are presented. Additional radiation measurements enable the determination of the aerosol optical depth, τ_aerλ, and the direct radiative effect (DRE) of the mineral dust. The results suggest that the absorption by Saharan dust is significantly overestimated in the solar spectrum if standard refractive indices are used. Our measurements suggest an imaginary part of the refractive index of 0.0015i is appropriate at a wavelength λ of 0.55 μm. Different methods for determining τ_aerλ=0.55 are presented, and the accuracy of each retrieval method is assessed. The value τ_aerλ=0.55 is estimated as 1.48 ± 0.05 during the period of heaviest dust loading, which is derived from an instantaneous DRE of approximately −129 ± 5 Wm⁻² or an enhancement of the local planetary albedo over ocean of a factor of 2.7 ± 0.1. A comparison of the DRE derived from the C‐130 instrumentation and from the Clouds and the Earth's Radiant Energy System (CERES) instrument on the Tropical Rainfall Measuring Mission (TRMM) satellite is presented; the results generally showing agreement to within a factor of 1.2. The results suggest that Saharan dust aerosol exerts the largest local and global DRE of all aerosol species and should be considered explicitly in global radiation budget studies.