In situ measurements of total and gas phase water as well as ice crystals have been obtained during several airborne field experiments in the Arctic, at midlatitudes and in the tropics.
From the data set obtained in these experiments, the ice water content (IWC) in cirrus clouds is derived as a function of temperature for the range 183–250 K, thus extending previous climatologies to much lower temperatures and lower detectable IWC. For each temperature, IWC covers a broad band, decreasing with temperature over the whole temperature range. In the tropics, several events of enhanced ice water content are observed which are related to recent impact of convection.
The relative humidity with respect to ice (RHi) in clear air and in ice clouds is also investigated. RHi above water saturation are not detected in the entire data set, but, super- or subsaturations are frequently observed inside of cirrus at temperatures <205K. From a combined analysis of supersaturations and ice crystal numbers, we show that the in-cloud high, persistent supersaturations can possibly be explained by unexpected, frequent very low ice crystal numbers that could scarcely be caused by homogeneous ice nucleation.