Measurements of cloud ice crystal size distributions have been made by a backscatter cloud probe (BCP) mounted on five commercial airliners flying international routes that cross five continents. Bulk cloud parameters were also derived from the size distributions. As of 31 December 2014, a total of 4399 flights had accumulated data from 665 hours in more than 19 000 cirrus clouds larger than 5 km in length. The BCP measures the equivalent optical diameter (EOD) of individual crystals in the 5
90 mm range from which size distributions are derived and recorded every 4 seconds. The cirrus cloud property database, an ongoing development stemming from these measurements, registers the total crystal number and mass concentration, effective and median volume diameters and extinction coefficients derived from the size distribution. This information is accompanied by the environmental temperature, pressure, aircraft position, date and time of each sample. The seasonal variations of the cirrus cloud properties measured from 2012 to 2014 are determined for six geographic regions in the tropics and extratropics. Number concentrations range from a few per litre for thin cirrus to several hundreds of thousands for heavy cirrus. Temperatures range from 205 to 250 K and effective radii from 12 to 20 mm. A comparison of the regional and seasonal number and mass size distributions, and the bulk microphysical properties derived from them, demonstrates that cirrus properties cannot be easily parameterised by temperature or by latitude. The seasonal changes in the size distributions from the extratropical Atlantic and Eurasian air routes are distinctly different, showing shifts from monomodal to bi-modal spectra out of phase with one another. This phase difference may be linked to the timing of deep convection and cold fronts that lead to the cirrus formation. Likewise, the size spectra of cirrus over the tropical Atlantic and Eastern Brazil differ from each other although they were measured in adjoining regions. The cirrus crystals in the maritime continental tropical region over Malaysia form tri-modal spectra that are not found in any of the other regions measured by the IAGOS aircraft so far, a feature that is possibly linked to biomass burning or dust. Frequent measurements of ice crystal concentrations greater than 1105 L 1, often accompanied by anomalously warm temperature and erratic airspeed readings, suggest that aircraft often experience conditions that affect their sensors. This new instrument, if used operationally, has the potential of providing real-time and valuable information to assist in flight operations as well as providing real-time information for along-track nowcasting.